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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="boost_contract.tutorial"></a><a class="link" href="tutorial.html" title="Tutorial">Tutorial</a>
</h2></div></div></div>
<div class="toc"><dl class="toc">
<dt><span class="section"><a href="tutorial.html#boost_contract.tutorial.non_member_functions">Non-Member
      Functions</a></span></dt>
<dt><span class="section"><a href="tutorial.html#boost_contract.tutorial.preconditions">Preconditions</a></span></dt>
<dt><span class="section"><a href="tutorial.html#boost_contract.tutorial.postconditions">Postconditions</a></span></dt>
<dt><span class="section"><a href="tutorial.html#boost_contract.tutorial.return_values">Return Values</a></span></dt>
<dt><span class="section"><a href="tutorial.html#boost_contract.tutorial.old_values">Old Values</a></span></dt>
<dt><span class="section"><a href="tutorial.html#boost_contract.tutorial.exception_guarantees">Exception
      Guarantees</a></span></dt>
<dt><span class="section"><a href="tutorial.html#boost_contract.tutorial.class_invariants">Class Invariants</a></span></dt>
<dt><span class="section"><a href="tutorial.html#boost_contract.tutorial.constructors">Constructors</a></span></dt>
<dt><span class="section"><a href="tutorial.html#boost_contract.tutorial.destructors">Destructors</a></span></dt>
<dt><span class="section"><a href="tutorial.html#boost_contract.tutorial.public_functions">Public Functions</a></span></dt>
<dt><span class="section"><a href="tutorial.html#boost_contract.tutorial.virtual_public_functions">Virtual
      Public Functions</a></span></dt>
<dt><span class="section"><a href="tutorial.html#boost_contract.tutorial.public_function_overrides__subcontracting_">Public
      Function Overrides (Subcontracting)</a></span></dt>
<dt><span class="section"><a href="tutorial.html#boost_contract.tutorial.base_classes__subcontracting_">Base
      Classes (Subcontracting)</a></span></dt>
<dt><span class="section"><a href="tutorial.html#boost_contract.tutorial.static_public_functions">Static
      Public Functions</a></span></dt>
</dl></div>
<p>
      This section is a guide to basic usage of this library.
    </p>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_contract.tutorial.non_member_functions"></a><a class="link" href="tutorial.html#boost_contract.tutorial.non_member_functions" title="Non-Member Functions">Non-Member
      Functions</a>
</h3></div></div></div>
<p>
        Contracts for non-member functions are programmed using <code class="computeroutput"><a class="link" href="../boost/contract/function.html" title="Function function">boost::contract::function</a></code>.
        For example (see <a href="../../../example/features/non_member.cpp" target="_top"><code class="literal">non_member.cpp</code></a>):
      </p>
<p>
</p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">contract</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>

<span class="comment">// Contract for a non-member function.</span>
<span class="keyword">int</span> <span class="identifier">inc</span><span class="special">(</span><span class="keyword">int</span><span class="special">&amp;</span> <span class="identifier">x</span><span class="special">)</span> <span class="special">{</span>
    <span class="keyword">int</span> <span class="identifier">result</span><span class="special">;</span>
    <span class="identifier">boost</span><span class="special">::</span><span class="identifier">contract</span><span class="special">::</span><span class="identifier">old_ptr</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span> <span class="identifier">old_x</span> <span class="special">=</span> <span class="identifier">BOOST_CONTRACT_OLDOF</span><span class="special">(</span><span class="identifier">x</span><span class="special">);</span>
    <span class="identifier">boost</span><span class="special">::</span><span class="identifier">contract</span><span class="special">::</span><span class="identifier">check</span> <span class="identifier">c</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">contract</span><span class="special">::</span><span class="identifier">function</span><span class="special">()</span>
        <span class="special">.</span><span class="identifier">precondition</span><span class="special">([&amp;]</span> <span class="special">{</span>
            <span class="identifier">BOOST_CONTRACT_ASSERT</span><span class="special">(</span><span class="identifier">x</span> <span class="special">&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;::</span><span class="identifier">max</span><span class="special">());</span>
        <span class="special">})</span>
        <span class="special">.</span><span class="identifier">postcondition</span><span class="special">([&amp;]</span> <span class="special">{</span>
            <span class="identifier">BOOST_CONTRACT_ASSERT</span><span class="special">(</span><span class="identifier">x</span> <span class="special">==</span> <span class="special">*</span><span class="identifier">old_x</span> <span class="special">+</span> <span class="number">1</span><span class="special">);</span>
            <span class="identifier">BOOST_CONTRACT_ASSERT</span><span class="special">(</span><span class="identifier">result</span> <span class="special">==</span> <span class="special">*</span><span class="identifier">old_x</span><span class="special">);</span>
        <span class="special">})</span>
        <span class="special">.</span><span class="identifier">except</span><span class="special">([&amp;]</span> <span class="special">{</span>
            <span class="identifier">BOOST_CONTRACT_ASSERT</span><span class="special">(</span><span class="identifier">x</span> <span class="special">==</span> <span class="special">*</span><span class="identifier">old_x</span><span class="special">);</span>
        <span class="special">})</span>
    <span class="special">;</span>

    <span class="keyword">return</span> <span class="identifier">result</span> <span class="special">=</span> <span class="identifier">x</span><span class="special">++;</span> <span class="comment">// Function body.</span>
<span class="special">}</span>
</pre>
<p>
      </p>
<p>
        All necessary header files of this library are included by <code class="computeroutput"><span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">contract</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span></code>.
        Alternatively, programmers can selectively include only the header files
        they actually need among <code class="literal">boost/contract/*.hpp</code> (see <a class="link" href="getting_started.html" title="Getting Started">Getting Started</a>).
      </p>
<p>
        It is possible to specify preconditions, postconditions, and exception guarantees
        for non-member functions (see <a class="link" href="tutorial.html#boost_contract.tutorial.preconditions" title="Preconditions">Preconditions</a>,
        <a class="link" href="tutorial.html#boost_contract.tutorial.postconditions" title="Postconditions">Postconditions</a>,
        and <a class="link" href="tutorial.html#boost_contract.tutorial.exception_guarantees" title="Exception Guarantees">Exception
        Guarantees</a>).
      </p>
<p>
        The <code class="computeroutput"><a class="link" href="../boost/contract/function.html" title="Function function">boost::contract::function</a></code>
        function returns an RAII object that must always be assigned to a local variable
        of type <code class="computeroutput"><a class="link" href="../boost/contract/check.html" title="Class check">boost::contract::check</a></code>
        (otherwise this library will generate a run-time error, see <code class="computeroutput"><a class="link" href="../BOOST_CO_idm45394999694064.html" title="Macro BOOST_CONTRACT_ON_MISSING_CHECK_DECL">BOOST_CONTRACT_ON_MISSING_CHECK_DECL</a></code>).
        <a href="#ftn.boost_contract.tutorial.non_member_functions.f0" class="footnote" name="boost_contract.tutorial.non_member_functions.f0"><sup class="footnote">[19]</sup></a> Furthermore, C++11 <code class="computeroutput"><span class="keyword">auto</span></code>
        declarations cannot be used here and the <code class="computeroutput"><a class="link" href="../boost/contract/check.html" title="Class check">boost::contract::check</a></code>
        type must be explicitly specified (otherwise this library will generate a
        compile-time error prior C++17 and a run-time error post C++17). <a href="#ftn.boost_contract.tutorial.non_member_functions.f1" class="footnote" name="boost_contract.tutorial.non_member_functions.f1"><sup class="footnote">[20]</sup></a> The function body is programmed right after the declaration of
        this RAII object.
      </p>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../doc/src/images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
          In some cases, it might be necessary to program some code before the contract.
          For example for acquiring resources that will be used while checking the
          contract like old values, but also to lock mutexes (or other synchronization
          mechanisms) in multi-threaded programs.
        </p></td></tr>
</table></div>
<p>
        At construction, the <code class="computeroutput"><a class="link" href="../boost/contract/check.html" title="Class check">boost::contract::check</a></code>
        RAII object for non-member functions does the following (enclosing function
        entry):
      </p>
<div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem">
            Check preconditions, by calling the nullary functor <code class="literal"><span class="emphasis"><em>r</em></span></code><code class="computeroutput"><span class="special">()</span></code> passed to <code class="computeroutput"><span class="special">.</span><span class="identifier">precondition</span><span class="special">(</span></code><code class="literal"><span class="emphasis"><em>r</em></span></code><code class="computeroutput"><span class="special">)</span></code>.
          </li></ol></div>
<p>
        At destruction instead (enclosing function exit):
      </p>
<div class="orderedlist"><ol class="orderedlist" type="1">
<li class="listitem">
            If the function body did not throw an exception:
            <div class="orderedlist"><ol class="orderedlist" type="a"><li class="listitem">
                  Check postconditions, by calling the nullary functor <code class="literal"><span class="emphasis"><em>s</em></span></code><code class="computeroutput"><span class="special">()</span></code> passed to <code class="computeroutput"><span class="special">.</span><span class="identifier">postcondition</span><span class="special">(</span></code><code class="literal"><span class="emphasis"><em>s</em></span></code><code class="computeroutput"><span class="special">)</span></code>.
                </li></ol></div>
          </li>
<li class="listitem">
            Else:
            <div class="orderedlist"><ol class="orderedlist" type="a"><li class="listitem">
                  Check exception guarantees, by calling the nullary functor <code class="literal"><span class="emphasis"><em>e</em></span></code><code class="computeroutput"><span class="special">()</span></code> passed to <code class="computeroutput"><span class="special">.</span><span class="identifier">except</span><span class="special">(</span></code><code class="literal"><span class="emphasis"><em>e</em></span></code><code class="computeroutput"><span class="special">)</span></code>.
                </li></ol></div>
          </li>
</ol></div>
<p>
        This ensures that non-member function contracts are correctly checked at
        run-time (see <a class="link" href="contract_programming_overview.html#boost_contract.contract_programming_overview.function_calls" title="Function Calls">Function
        Calls</a>). (Also note that functions will correctly check their contracts
        even when they are called via function pointers, function objects, etc.)
      </p>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../doc/src/images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
          A non-member function can avoid calling <code class="computeroutput"><a class="link" href="../boost/contract/function.html" title="Function function">boost::contract::function</a></code>
          for efficiency but only when it has no preconditions, no postconditions,
          and no exception guarantees.
        </p></td></tr>
</table></div>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_contract.tutorial.preconditions"></a><a class="link" href="tutorial.html#boost_contract.tutorial.preconditions" title="Preconditions">Preconditions</a>
</h3></div></div></div>
<p>
        When preconditions are specified, they are programmed using a functor <code class="literal"><span class="emphasis"><em>r</em></span></code>
        passed to <code class="computeroutput"><span class="special">.</span><span class="identifier">precondition</span><span class="special">(</span></code><code class="literal"><span class="emphasis"><em>r</em></span></code><code class="computeroutput"><span class="special">)</span></code> that can be called with no parameters as
        in <code class="literal"><span class="emphasis"><em>r</em></span></code><code class="computeroutput"><span class="special">()</span></code>.
        Contracts that do not have preconditions simply do not call <code class="computeroutput"><span class="special">.</span><span class="identifier">precondition</span><span class="special">(...)</span></code>. Preconditions must appear before postconditions
        and exception guarantees when these are all present (see <a class="link" href="tutorial.html#boost_contract.tutorial.postconditions" title="Postconditions">Postconditions</a>
        and <a class="link" href="tutorial.html#boost_contract.tutorial.exception_guarantees" title="Exception Guarantees">Exception
        Guarantees</a>).
      </p>
<p>
        C++11 lambda functions are convenient to program preconditions, but any other
        nullary functor can be used (see <a class="link" href="extras.html#boost_contract.extras.no_lambda_functions__no_c__11_" title="No Lambda Functions (No C++11)">No
        Lambda Functions</a>). <a href="#ftn.boost_contract.tutorial.preconditions.f0" class="footnote" name="boost_contract.tutorial.preconditions.f0"><sup class="footnote">[21]</sup></a> For example, for <code class="computeroutput"><a class="link" href="../boost/contract/function.html" title="Function function">boost::contract::function</a></code>
        (similarly for public functions, instead destructors do not have preconditions
        and constructors use <code class="computeroutput"><a class="link" href="../boost/contract/constructor_precondition.html" title="Class template constructor_precondition">boost::contract::constructor_precondition</a></code>,
        see <a class="link" href="tutorial.html#boost_contract.tutorial.public_functions" title="Public Functions">Public Functions</a>,
        <a class="link" href="tutorial.html#boost_contract.tutorial.destructors" title="Destructors">Destructors</a>, and
        <a class="link" href="tutorial.html#boost_contract.tutorial.constructors" title="Constructors">Constructors</a>):
      </p>
<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">f</span><span class="special">(...)</span> <span class="special">{</span>
    <span class="identifier">boost</span><span class="special">::</span><span class="identifier">contract</span><span class="special">::</span><span class="identifier">check</span> <span class="identifier">c</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">contract</span><span class="special">::</span><span class="identifier">function</span><span class="special">()</span>  <span class="comment">// Same for all other contracts.</span>
        <span class="special">.</span><span class="identifier">precondition</span><span class="special">([&amp;]</span> <span class="special">{</span>                                 <span class="comment">// Capture by reference or value...</span>
            <span class="identifier">BOOST_CONTRACT_ASSERT</span><span class="special">(...);</span>                     <span class="comment">// ...and should not modify captures.</span>
            <span class="special">...</span>
        <span class="special">})</span>
        <span class="special">...</span>
    <span class="special">;</span>

    <span class="special">...</span>
<span class="special">}</span>
</pre>
<p>
        The precondition functor should capture all the variables that it needs to
        assert the preconditions. These variables can be captured by value when the
        overhead of copying such variables is acceptable. <a href="#ftn.boost_contract.tutorial.preconditions.f1" class="footnote" name="boost_contract.tutorial.preconditions.f1"><sup class="footnote">[22]</sup></a> In any case, programmers should not write precondition assertions
        that modify the value of the captured variables, even when those are captured
        by reference (see <a class="link" href="contract_programming_overview.html#boost_contract.contract_programming_overview.constant_correctness" title="Constant-Correctness">Constant-Correctness</a>).
      </p>
<p>
        Any code can be programmed in the precondition functor, but it is recommended
        to keep this code simple using mainly assertions and if-statements (to avoid
        programming complex preconditions that might be buggy and also slow to check
        at run-time). It is also recommended to use <code class="computeroutput"><a class="link" href="../BOOST_CONTRACT_ASSERT.html" title="Macro BOOST_CONTRACT_ASSERT">BOOST_CONTRACT_ASSERT</a></code>
        to program precondition assertions because that enables this library to print
        informative error messages when the asserted conditions are evaluated to
        be false (note that this is not a variadic macro, see <a class="link" href="extras.html#boost_contract.extras.no_macros__and_no_variadic_macros_" title="No Macros (and No Variadic Macros)">No
        Macros</a>):
      </p>
<pre class="programlisting"><span class="identifier">BOOST_CONTRACT_ASSERT</span><span class="special">(</span><code class="literal"><span class="emphasis"><em>boolean-condition</em></span></code><span class="special">)</span>
<span class="comment">// Or, if `boolean-condition` contains commas `,` not already within parenthesis `()`...</span>
<span class="identifier">BOOST_CONTRACT_ASSERT</span><span class="special">((</span><code class="literal"><span class="emphasis"><em>boolean-condition</em></span></code><span class="special">))</span> <span class="comment">// ...use extra parenthesis (not a variadic macro).</span>
</pre>
<p>
        This library will automatically call the failure handler <code class="computeroutput"><a class="link" href="../boost/contract/precondition_failure.html" title="Function precondition_failure">boost::contract::precondition_failure</a></code>
        if any of the <code class="computeroutput"><a class="link" href="../BOOST_CONTRACT_ASSERT.html" title="Macro BOOST_CONTRACT_ASSERT">BOOST_CONTRACT_ASSERT</a></code>
        conditions are false or, more in general, if calling the functor specified
        via <code class="computeroutput"><span class="special">.</span><span class="identifier">precondition</span><span class="special">(...)</span></code> throws any exception. By default, this
        failure handler prints an error message to <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">cerr</span></code>
        and terminates the program calling <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">terminate</span></code>
        (see <a class="link" href="advanced.html#boost_contract.advanced.throw_on_failures__and__noexcept__" title="Throw on Failures (and noexcept)">Throw
        on Failures</a> to change the failure handler to throw exceptions, exit
        the program with an error code, etc.).
      </p>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../doc/src/images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
          Contracts are most useful when their assertions only use public members
          that are accessible to the caller so the caller can properly check and
          use the contract. In particular, preconditions of a public function or
          constructor that use non-public members are essentially incorrect because
          they cannot be fully checked by the caller (in fact, Eiffel generates a
          compile-time error in this case). However, this library does not enforce
          such a constraint and it leaves it up to programmers to only use public
          members when programming contracts, especially when asserting preconditions
          (see <a class="link" href="contract_programming_overview.html#boost_contract.contract_programming_overview.specifications_vs__implementation" title="Specifications vs. Implementation">Specifications
          vs. Implementation</a>).
        </p></td></tr>
</table></div>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_contract.tutorial.postconditions"></a><a class="link" href="tutorial.html#boost_contract.tutorial.postconditions" title="Postconditions">Postconditions</a>
</h3></div></div></div>
<p>
        When postconditions are specified, they are programmed using a functor <code class="literal"><span class="emphasis"><em>s</em></span></code>
        passed to <code class="computeroutput"><span class="special">.</span><span class="identifier">postcondition</span><span class="special">(</span></code><code class="literal"><span class="emphasis"><em>s</em></span></code><code class="computeroutput"><span class="special">)</span></code> that can be called with no parameters as
        in <code class="literal"><span class="emphasis"><em>s</em></span></code><code class="computeroutput"><span class="special">()</span></code>.
        Contracts that do not have postconditions simply do not call <code class="computeroutput"><span class="special">.</span><span class="identifier">postcondition</span><span class="special">(...)</span></code>. Postconditions must appear after preconditions
        but before exception guarantees when these are all present (see <a class="link" href="tutorial.html#boost_contract.tutorial.preconditions" title="Preconditions">Preconditions</a>
        and <a class="link" href="tutorial.html#boost_contract.tutorial.exception_guarantees" title="Exception Guarantees">Exception
        Guarantees</a>).
      </p>
<p>
        C++11 lambda functions are convenient to program postconditions, but any
        other nullary functor can be used (see <a class="link" href="extras.html#boost_contract.extras.no_lambda_functions__no_c__11_" title="No Lambda Functions (No C++11)">No
        Lambda Functions</a>). For example, for <code class="computeroutput"><a class="link" href="../boost/contract/function.html" title="Function function">boost::contract::function</a></code>
        (similarly for all other contracts):
      </p>
<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">f</span><span class="special">(...)</span> <span class="special">{</span>
    <span class="identifier">boost</span><span class="special">::</span><span class="identifier">contract</span><span class="special">::</span><span class="identifier">check</span> <span class="identifier">c</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">contract</span><span class="special">::</span><span class="identifier">function</span><span class="special">()</span>  <span class="comment">// Same for all other contracts.</span>
        <span class="special">...</span>
        <span class="special">.</span><span class="identifier">postcondition</span><span class="special">([&amp;]</span> <span class="special">{</span>                                <span class="comment">// Capture by reference...</span>
            <span class="identifier">BOOST_CONTRACT_ASSERT</span><span class="special">(...);</span>                     <span class="comment">// ...but should not modify captures.</span>
            <span class="special">...</span>
        <span class="special">})</span>
        <span class="special">...</span>
    <span class="special">;</span>

    <span class="special">...</span>
<span class="special">}</span>
</pre>
<p>
        The postcondition functor should capture all the variables that it needs
        to assert the postconditions. In general, these variables should be captured
        by reference and not by value (because postconditions need to access the
        value that these variables will have at function exit, and not the value
        these variables had when the postcondition functor was first declared). Postconditions
        can also capture return and old values (see <a class="link" href="tutorial.html#boost_contract.tutorial.return_values" title="Return Values">Return
        Values</a> and <a class="link" href="tutorial.html#boost_contract.tutorial.old_values" title="Old Values">Old
        Values</a>). In any case, programmers should not write postcondition assertions
        that modify the value of the captured variables, even when those are captured
        by reference (see <a class="link" href="contract_programming_overview.html#boost_contract.contract_programming_overview.constant_correctness" title="Constant-Correctness">Constant-Correctness</a>).
      </p>
<p>
        Any code can be programmed in the postcondition functor, but it is recommended
        to keep this code simple using mainly assertions and if-statements (to avoid
        programming complex postconditions that might be buggy and slow to check
        at run-time). It is also recommended to use <code class="computeroutput"><a class="link" href="../BOOST_CONTRACT_ASSERT.html" title="Macro BOOST_CONTRACT_ASSERT">BOOST_CONTRACT_ASSERT</a></code>
        to program postcondition assertions because that enables this library to
        print informative error messages when the asserted conditions are evaluated
        to be false (note that this is not a variadic macro, see <a class="link" href="extras.html#boost_contract.extras.no_macros__and_no_variadic_macros_" title="No Macros (and No Variadic Macros)">No
        Macros</a>):
      </p>
<pre class="programlisting"><span class="identifier">BOOST_CONTRACT_ASSERT</span><span class="special">(</span><code class="literal"><span class="emphasis"><em>boolean-condition</em></span></code><span class="special">)</span>
<span class="comment">// Or, if `boolean-condition` has commas `,` not already within parenthesis `()`...</span>
<span class="identifier">BOOST_CONTRACT_ASSERT</span><span class="special">((</span><code class="literal"><span class="emphasis"><em>boolean-condition</em></span></code><span class="special">))</span> <span class="comment">// ...use extra parenthesis (not a variadic macro).</span>
</pre>
<p>
        This library will automatically call the failure handler <code class="computeroutput"><a class="link" href="../boost/contract/postcondition_failure.html" title="Function postcondition_failure">boost::contract::postcondition_failure</a></code>
        if any of the <code class="computeroutput"><a class="link" href="../BOOST_CONTRACT_ASSERT.html" title="Macro BOOST_CONTRACT_ASSERT">BOOST_CONTRACT_ASSERT</a></code>
        conditions are false or, more in general, if calling the functor specified
        via <code class="computeroutput"><span class="special">.</span><span class="identifier">postcondition</span><span class="special">(...)</span></code> throws any exception. By default, this
        failure handler prints an error message to <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">cerr</span></code>
        and terminates the program calling <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">terminate</span></code>
        (see <a class="link" href="advanced.html#boost_contract.advanced.throw_on_failures__and__noexcept__" title="Throw on Failures (and noexcept)">Throw
        on Failures</a> to change the failure handler to throw exceptions, exit
        the program with an error code, etc.).
      </p>
<p>
        For non-void virtual public functions and non-void public function overrides,
        the functor <code class="literal"><span class="emphasis"><em>s</em></span></code> passed to <code class="computeroutput"><span class="special">.</span><span class="identifier">postcondition</span><span class="special">(</span></code><code class="literal"><span class="emphasis"><em>s</em></span></code><code class="computeroutput"><span class="special">)</span></code> is not a nullary functor, instead it is
        a unary functor taking a variable holding the return value as its one parameter
        <code class="literal"><span class="emphasis"><em>s</em></span></code><code class="computeroutput"><span class="special">(</span></code><code class="literal"><span class="emphasis"><em>result</em></span></code><code class="computeroutput"><span class="special">)</span></code> (this is to properly support subcontracting,
        see <a class="link" href="tutorial.html#boost_contract.tutorial.virtual_public_functions" title="Virtual Public Functions">Virtual
        Public Functions</a> and <a class="link" href="tutorial.html#boost_contract.tutorial.public_function_overrides__subcontracting_" title="Public Function Overrides (Subcontracting)">Public
        Function Overrides</a>).
      </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_contract.tutorial.return_values"></a><a class="link" href="tutorial.html#boost_contract.tutorial.return_values" title="Return Values">Return Values</a>
</h3></div></div></div>
<p>
        In non-void functions, postconditions might need to access the function return
        value to program assertions. In these cases, programmers are responsible
        to declare a local variable before the contract and to assign it to the return
        value at function exit (when the function does not throw an exception).
        <a href="#ftn.boost_contract.tutorial.return_values.f0" class="footnote" name="boost_contract.tutorial.return_values.f0"><sup class="footnote">[23]</sup></a> For example, for <code class="computeroutput"><a class="link" href="../boost/contract/function.html" title="Function function">boost::contract::function</a></code>
        (similarly for all other contracts):
      </p>
<pre class="programlisting"><span class="identifier">return_type</span> <span class="identifier">f</span><span class="special">(...)</span> <span class="special">{</span>
    <span class="identifier">return_type</span> <span class="identifier">result</span><span class="special">;</span>                                     <span class="comment">// Must be later assigned to return value.</span>
    <span class="identifier">boost</span><span class="special">::</span><span class="identifier">contract</span><span class="special">::</span><span class="identifier">check</span> <span class="identifier">c</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">contract</span><span class="special">::</span><span class="identifier">function</span><span class="special">()</span>  <span class="comment">// Same for all other contracts.</span>
        <span class="special">...</span>
        <span class="special">.</span><span class="identifier">postcondition</span><span class="special">([&amp;]</span> <span class="special">{</span>                                <span class="comment">// Also capture `result` reference...</span>
            <span class="identifier">BOOST_CONTRACT_ASSERT</span><span class="special">(</span><span class="identifier">result</span> <span class="special">==</span> <span class="special">...);</span>           <span class="comment">// ...but should not modify captures.</span>
            <span class="special">...</span>
        <span class="special">})</span>
        <span class="special">...</span>
    <span class="special">;</span>

    <span class="special">...</span>
    <span class="keyword">return</span> <span class="identifier">result</span> <span class="special">=</span> <span class="special">...;</span>                                    <span class="comment">// Assign `result` at each return.</span>
<span class="special">}</span>
</pre>
<p>
        At any point where the enclosing function returns, programmers are responsible
        to assign the result variable to the expression being returned. This can
        be done ensuring that <span class="emphasis"><em>all</em></span> <code class="computeroutput"><span class="keyword">return</span></code>
        statements in the function are of the form:
      </p>
<pre class="programlisting"><code class="literal"><span class="emphasis"><em>return-type</em></span></code> <span class="identifier">result</span><span class="special">;</span>
<span class="special">...</span>
<span class="keyword">return</span> <span class="identifier">result</span> <span class="special">=</span> <code class="literal"><span class="emphasis"><em>return-expression</em></span></code><span class="special">;</span>                           <span class="comment">// Assign `result` at each return.</span>
</pre>
<p>
        The functor used to program postconditions should capture the result variable
        by reference and not by value (because postconditions must access the value
        the result variable will have at function exit, and not the value the result
        variable had when the postcondition functor was first declared). The return
        value should never be used in preconditions, old value copies, or exception
        guarantees (because the return value is not yet correctly evaluated and set
        when preconditions are checked, old values are copied, or if the function
        throws an exception). In any case, programmers should not modify the result
        variable in the contract assertions (see <a class="link" href="contract_programming_overview.html#boost_contract.contract_programming_overview.constant_correctness" title="Constant-Correctness">Constant-Correctness</a>).
      </p>
<p>
        It is also possible to declared the result variable using <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span></code>
        when the function return type does not have a default constructor, or if
        the default constructor is too expensive or undesirable to execute when first
        declaring the result variable (see <a class="link" href="advanced.html#boost_contract.advanced.optional_return_values" title="Optional Return Values">Optional
        Return Values</a>).
      </p>
<p>
        Non-void virtual public functions and non-void public function overrides
        must always declare and use a result variable even when postconditions do
        not directly use the function return value (this is to properly support subcontracting,
        see <a class="link" href="tutorial.html#boost_contract.tutorial.virtual_public_functions" title="Virtual Public Functions">Virtual
        Public Functions</a> and <a class="link" href="tutorial.html#boost_contract.tutorial.public_function_overrides__subcontracting_" title="Public Function Overrides (Subcontracting)">Public
        Function Overrides</a>).
      </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_contract.tutorial.old_values"></a><a class="link" href="tutorial.html#boost_contract.tutorial.old_values" title="Old Values">Old Values</a>
</h3></div></div></div>
<p>
        When old values are used in postconditions or in exception guarantees, programmes
        are responsible to declare local variables before the contract and to assign
        them to related old value expressions using <code class="computeroutput"><a class="link" href="../BOOST_CONTRACT_OLDOF.html" title="Macro BOOST_CONTRACT_OLDOF">BOOST_CONTRACT_OLDOF</a></code>.
        <a href="#ftn.boost_contract.tutorial.old_values.f0" class="footnote" name="boost_contract.tutorial.old_values.f0"><sup class="footnote">[24]</sup></a> For example, for <code class="computeroutput"><a class="link" href="../boost/contract/function.html" title="Function function">boost::contract::function</a></code>
        (similarly for all other contracts):
      </p>
<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">f</span><span class="special">(...)</span> <span class="special">{</span>
    <span class="identifier">boost</span><span class="special">::</span><span class="identifier">contract</span><span class="special">::</span><span class="identifier">old_ptr</span><span class="special">&lt;</span><span class="identifier">old_type</span><span class="special">&gt;</span> <span class="identifier">old_var</span> <span class="special">=</span> <span class="identifier">BOOST_CONTRACT_OLDOF</span><span class="special">(</span><span class="identifier">old_expr</span><span class="special">);</span>
    <span class="special">...</span>                                                     <span class="comment">// More old value declarations here if needed.</span>
    <span class="identifier">boost</span><span class="special">::</span><span class="identifier">contract</span><span class="special">::</span><span class="identifier">check</span> <span class="identifier">c</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">contract</span><span class="special">::</span><span class="identifier">function</span><span class="special">()</span>  <span class="comment">// Same for all other contracts.</span>
        <span class="special">...</span>                                                 <span class="comment">// Preconditions shall not use old values.</span>
        <span class="special">.</span><span class="identifier">postcondition</span><span class="special">([&amp;]</span> <span class="special">{</span>                                <span class="comment">// Capture by reference...</span>
            <span class="identifier">BOOST_CONTRACT_ASSERT</span><span class="special">(*</span><span class="identifier">old_var</span> <span class="special">==</span> <span class="special">...);</span>         <span class="comment">// ...but should not modify captures.</span>
            <span class="special">...</span>
        <span class="special">})</span>
        <span class="special">.</span><span class="identifier">except</span><span class="special">([&amp;]</span> <span class="special">{</span>                                       <span class="comment">// Capture by reference...</span>
            <span class="identifier">BOOST_CONTRACT_ASSERT</span><span class="special">(</span><span class="identifier">old_var</span><span class="special">-&gt;...);</span>            <span class="comment">// ...but should not modify captures.</span>
            <span class="special">...</span>
        <span class="special">})</span>
    <span class="special">;</span>

    <span class="special">...</span>
<span class="special">}</span>
</pre>
<p>
        Old values are handled by this library using the smart pointer class template
        <code class="computeroutput"><a class="link" href="../boost/contract/old_ptr.html" title="Class template old_ptr">boost::contract::old_ptr</a></code>
        (so programmers do not directly manage allocation and deallocation of the
        pointed memory). <a href="#ftn.boost_contract.tutorial.old_values.f1" class="footnote" name="boost_contract.tutorial.old_values.f1"><sup class="footnote">[25]</sup></a> The pointed old value type is automatically qualified as <code class="computeroutput"><span class="keyword">const</span></code> (so old values cannot be mistakenly
        changed by contract assertions, see <a class="link" href="contract_programming_overview.html#boost_contract.contract_programming_overview.constant_correctness" title="Constant-Correctness">Constant-Correctness</a>).
        This library ensures that old value pointers are always not null by the time
        postconditions and exception guarantees are checked (so programmers can safely
        dereference and use these pointers in postcondition and exception guarantee
        assertions using <code class="computeroutput"><span class="keyword">operator</span><span class="special">*</span></code>
        and <code class="computeroutput"><span class="keyword">operator</span><span class="special">-&gt;</span></code>
        without having to check if old value pointers are not null first).
      </p>
<p>
        Old values should not be used in preconditions and this library does not
        guarantee that old value pointers are always not null when preconditions
        are checked. <a href="#ftn.boost_contract.tutorial.old_values.f2" class="footnote" name="boost_contract.tutorial.old_values.f2"><sup class="footnote">[26]</sup></a> See <a class="link" href="advanced.html#boost_contract.advanced.old_values_copied_at_body" title="Old Values Copied at Body">Old
        Values Copied at Body</a> for delaying the copy of old values until after
        class invariants (for constructors, destructors, and public functions) and
        preconditions are checked (when necessary, this allows to program old value
        expressions under the simplifying assumption that class invariant and precondition
        assertions are satisfied already).
      </p>
<p>
        <code class="computeroutput"><a class="link" href="../BOOST_CONTRACT_OLDOF.html" title="Macro BOOST_CONTRACT_OLDOF">BOOST_CONTRACT_OLDOF</a></code> is
        a variadic macro and it takes an extra parameter when used in virtual public
        functions or public function overrides (see <a class="link" href="tutorial.html#boost_contract.tutorial.virtual_public_functions" title="Virtual Public Functions">Virtual
        Public Functions</a> and <a class="link" href="tutorial.html#boost_contract.tutorial.public_function_overrides__subcontracting_" title="Public Function Overrides (Subcontracting)">Public
        Function Overrides</a>). C++11 auto declarations can be used with <code class="computeroutput"><a class="link" href="../BOOST_CONTRACT_OLDOF.html" title="Macro BOOST_CONTRACT_OLDOF">BOOST_CONTRACT_OLDOF</a></code> for brevity
        <code class="computeroutput"><span class="keyword">auto</span> </code><code class="literal">old_<span class="emphasis"><em>variable-name</em></span>
        = BOOST_CONTRACT_OLDOF(<span class="emphasis"><em>expression</em></span>)</code> (but see
        also <a class="link" href="extras.html#boost_contract.extras.old_value_requirements__templates_" title="Old Value Requirements (Templates)">Old
        Value Requirements</a>). See <a class="link" href="extras.html#boost_contract.extras.no_macros__and_no_variadic_macros_" title="No Macros (and No Variadic Macros)">No
        Macros</a> to program old values without using <code class="computeroutput"><a class="link" href="../BOOST_CONTRACT_OLDOF.html" title="Macro BOOST_CONTRACT_OLDOF">BOOST_CONTRACT_OLDOF</a></code>
        (e.g., on compilers that do not support variadic macros).
      </p>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../doc/src/images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
          This library ensures that old values are copied only once. This library
          also ensures that old values are never copied when postconditions and exception
          guarantees are disabled defining both <code class="computeroutput"><a class="link" href="../BOOST_CO_idm45394999656032.html" title="Macro BOOST_CONTRACT_NO_POSTCONDITIONS">BOOST_CONTRACT_NO_POSTCONDITIONS</a></code>
          and <code class="computeroutput"><a class="link" href="../BOOST_CONTRACT_NO_EXCEPTS.html" title="Macro BOOST_CONTRACT_NO_EXCEPTS">BOOST_CONTRACT_NO_EXCEPTS</a></code>
          (note that both these two macros must be defined, defining only <code class="computeroutput"><a class="link" href="../BOOST_CO_idm45394999656032.html" title="Macro BOOST_CONTRACT_NO_POSTCONDITIONS">BOOST_CONTRACT_NO_POSTCONDITIONS</a></code>
          or only <code class="computeroutput"><a class="link" href="../BOOST_CONTRACT_NO_EXCEPTS.html" title="Macro BOOST_CONTRACT_NO_EXCEPTS">BOOST_CONTRACT_NO_EXCEPTS</a></code>
          is not sufficient to prevent the run-time cost of old value copies).
        </p></td></tr>
</table></div>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_contract.tutorial.exception_guarantees"></a><a class="link" href="tutorial.html#boost_contract.tutorial.exception_guarantees" title="Exception Guarantees">Exception
      Guarantees</a>
</h3></div></div></div>
<p>
        When exception guarantees are specified, they are programmed using a functor
        <code class="literal"><span class="emphasis"><em>e</em></span></code> passed to <code class="computeroutput"><span class="special">.</span><span class="identifier">except</span><span class="special">(</span></code><code class="literal"><span class="emphasis"><em>e</em></span></code><code class="computeroutput"><span class="special">)</span></code> that can be called with no parameters as
        in <code class="literal"><span class="emphasis"><em>e</em></span></code><code class="computeroutput"><span class="special">()</span></code>.
        Contracts that do not have exception guarantees simply do not call <code class="computeroutput"><span class="special">.</span><span class="identifier">except</span><span class="special">(...)</span></code>. Exception guarantees must appear after
        both preconditions and postconditions when these are all present (see <a class="link" href="tutorial.html#boost_contract.tutorial.preconditions" title="Preconditions">Preconditions</a> and
        <a class="link" href="tutorial.html#boost_contract.tutorial.postconditions" title="Postconditions">Postconditions</a>).
      </p>
<p>
        C++11 lambda functions are convenient to program exception guarantees, but
        any other nullary functor can be used (see <a class="link" href="extras.html#boost_contract.extras.no_lambda_functions__no_c__11_" title="No Lambda Functions (No C++11)">No
        Lambda Functions</a>). For example, for <code class="computeroutput"><a class="link" href="../boost/contract/function.html" title="Function function">boost::contract::function</a></code>
        (similarly for all other contracts):
      </p>
<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">f</span><span class="special">(...)</span> <span class="special">{</span>
    <span class="identifier">boost</span><span class="special">::</span><span class="identifier">contract</span><span class="special">::</span><span class="identifier">check</span> <span class="identifier">c</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">contract</span><span class="special">::</span><span class="identifier">function</span><span class="special">()</span>  <span class="comment">// Same for all other contracts.</span>
        <span class="special">...</span>
        <span class="special">.</span><span class="identifier">except</span><span class="special">([&amp;]</span> <span class="special">{</span>                                       <span class="comment">// Capture by reference...</span>
            <span class="identifier">BOOST_CONTRACT_ASSERT</span><span class="special">(...);</span>                     <span class="comment">// ...but should not modify captures.</span>
            <span class="special">...</span>
        <span class="special">})</span>
    <span class="special">;</span>

    <span class="special">...</span>
<span class="special">}</span>
</pre>
<p>
        The exception guarantee functor should capture all the variables that it
        needs to assert the exception guarantees. In general, these variables should
        be captured by reference and not by value (because exception guarantees need
        to access the value that these variables will have when the function throws,
        and not the value these variables had when the exception guarantee functor
        was first declared). Exception guarantees can also capture old values (see
        <a class="link" href="tutorial.html#boost_contract.tutorial.old_values" title="Old Values">Old Values</a>) but
        they should not access the function return value instead (because the return
        value is not be properly set when the function throws an exception). In any
        case, programmers should not write exception guarantee assertions that modify
        the value of the captured variables, even when those are captured by reference
        (see <a class="link" href="contract_programming_overview.html#boost_contract.contract_programming_overview.constant_correctness" title="Constant-Correctness">Constant-Correctness</a>).
      </p>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../doc/src/images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
          In real production code, it might be difficult to program meaningful exception
          guarantees without resorting to expensive old value copies that will slow
          down execution. Therefore, the authors recognize that exception guarantees,
          even if supported by this library, might not be used often in practice
          (and they are not used in most of the examples listed in the rest of this
          documentation). In any case, these performance considerations are ultimately
          left to programmers and their specific application domains.
        </p></td></tr>
</table></div>
<p>
        Any code can be programmed in the exception guarantee functor, but it is
        recommended to keep this code simple using mainly assertions and if-statements
        (to avoid programming complex exception guarantees that might be buggy and
        slow to check at run-time). It is also recommended to use <code class="computeroutput"><a class="link" href="../BOOST_CONTRACT_ASSERT.html" title="Macro BOOST_CONTRACT_ASSERT">BOOST_CONTRACT_ASSERT</a></code>
        to program exception guarantee assertions because that enables this library
        to print informative error messages when the asserted conditions are evaluated
        to be false (note that this is not a variadic macro, see <a class="link" href="extras.html#boost_contract.extras.no_macros__and_no_variadic_macros_" title="No Macros (and No Variadic Macros)">No
        Macros</a>):
      </p>
<pre class="programlisting"><span class="identifier">BOOST_CONTRACT_ASSERT</span><span class="special">(</span><code class="literal"><span class="emphasis"><em>boolean-condition</em></span></code><span class="special">)</span>
<span class="comment">// Or, if `boolean-condition` has commas `,` not already within parenthesis `()`...</span>
<span class="identifier">BOOST_CONTRACT_ASSERT</span><span class="special">((</span><code class="literal"><span class="emphasis"><em>boolean-condition</em></span></code><span class="special">))</span> <span class="comment">// ...use extra parenthesis (not a variadic macro).</span>
</pre>
<p>
        This library will automatically call the failure handler <code class="computeroutput"><a class="link" href="../boost/contract/except_failure.html" title="Function except_failure">boost::contract::except_failure</a></code>
        if any of the <code class="computeroutput"><a class="link" href="../BOOST_CONTRACT_ASSERT.html" title="Macro BOOST_CONTRACT_ASSERT">BOOST_CONTRACT_ASSERT</a></code>
        conditions are false or, more in general, if calling the functor specified
        via <code class="computeroutput"><span class="special">.</span><span class="identifier">except</span><span class="special">(...)</span></code> throws any exception. By default, this
        failure handler prints an error message to <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">cerr</span></code>
        and terminates the program calling <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">terminate</span></code>
        (see <a class="link" href="advanced.html#boost_contract.advanced.throw_on_failures__and__noexcept__" title="Throw on Failures (and noexcept)">Throw
        on Failures</a> to change the failure handler to exit the program with
        an error code or to take some other custom action).
      </p>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../doc/src/images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
          While it is technically possible for programmers to specify an exception
          guarantee handler that throws an exception in case of an exception guarantee
          failure, this will force C++ to terminate the program. That is because
          the handler will throw an exception while there is already an active exception
          on the stack (the exception thrown by the function body that caused the
          exception guarantees to be checked in the first place). Therefore, programmers
          should not change the exception guarantee failure handler to throw exceptions.
        </p></td></tr>
</table></div>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_contract.tutorial.class_invariants"></a><a class="link" href="tutorial.html#boost_contract.tutorial.class_invariants" title="Class Invariants">Class Invariants</a>
</h3></div></div></div>
<p>
        Public member functions, constructors, and destructors can be programmed
        to also check class invariants. When class invariants are specified, they
        are programmed in a public <code class="computeroutput"><span class="keyword">const</span></code>
        function named <code class="computeroutput"><span class="identifier">invariant</span></code>
        taking no argument and returning <code class="computeroutput"><span class="keyword">void</span></code>.
        Classes that do not have invariants, simply do not declare the <code class="computeroutput"><span class="identifier">invariant</span></code> function. <a href="#ftn.boost_contract.tutorial.class_invariants.f0" class="footnote" name="boost_contract.tutorial.class_invariants.f0"><sup class="footnote">[27]</sup></a> For example:
      </p>
<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">u</span> <span class="special">{</span>
<span class="keyword">public</span><span class="special">:</span>                                 <span class="comment">// Must be public.</span>
    <span class="keyword">void</span> <span class="identifier">invariant</span><span class="special">()</span> <span class="keyword">const</span> <span class="special">{</span>            <span class="comment">// Must be const.</span>
        <span class="identifier">BOOST_CONTRACT_ASSERT</span><span class="special">(...);</span>
        <span class="special">...</span>
    <span class="special">}</span>

    <span class="special">...</span>
<span class="special">};</span>
</pre>
<p>
        This member function must be <code class="computeroutput"><span class="keyword">const</span></code>
        because contracts should not modify the object state (see <a class="link" href="contract_programming_overview.html#boost_contract.contract_programming_overview.constant_correctness" title="Constant-Correctness">Constant-Correctness</a>).
        This library will generate a compile-time error if the <code class="computeroutput"><span class="keyword">const</span></code>
        qualifier is missing (unless <code class="computeroutput"><a class="link" href="../BOOST_CONTRACT_PERMISSIVE.html" title="Macro BOOST_CONTRACT_PERMISSIVE">BOOST_CONTRACT_PERMISSIVE</a></code>
        is defined).
      </p>
<p>
        Any code can be programmed in the <code class="computeroutput"><span class="identifier">invariant</span></code>
        function, but it is recommended to keep this code simple using mainly assertions
        and if-statements (to avoid programming complex invariants that might be
        buggy and slow to check at run-time). It is also recommended to use <code class="computeroutput"><a class="link" href="../BOOST_CONTRACT_ASSERT.html" title="Macro BOOST_CONTRACT_ASSERT">BOOST_CONTRACT_ASSERT</a></code> to program
        class invariant assertions because that enables this library to print informative
        error messages when the asserted conditions are evaluated to be false (note
        that this is not a variadic macro, see <a class="link" href="extras.html#boost_contract.extras.no_macros__and_no_variadic_macros_" title="No Macros (and No Variadic Macros)">No
        Macros</a>):
      </p>
<pre class="programlisting"><span class="identifier">BOOST_CONTRACT_ASSERT</span><span class="special">(</span><code class="literal"><span class="emphasis"><em>boolean-condition</em></span></code><span class="special">)</span>
<span class="comment">// Or, if `boolean-condition` has commas `,` not already within parenthesis `()`...</span>
<span class="identifier">BOOST_CONTRACT_ASSERT</span><span class="special">((</span><code class="literal"><span class="emphasis"><em>boolean-condition</em></span></code><span class="special">))</span> <span class="comment">// ...use extra parenthesis (not a variadic macro).</span>
</pre>
<p>
        This library will automatically call failure handlers <code class="computeroutput"><a class="link" href="../boost/contract/entry_invariant_failure.html" title="Function entry_invariant_failure">boost::contract::entry_invariant_failure</a></code>
        or <code class="computeroutput"><a class="link" href="../boost/contract/exit_invariant_failure.html" title="Function exit_invariant_failure">boost::contract::exit_invariant_failure</a></code>
        if any of the <code class="computeroutput"><a class="link" href="../BOOST_CONTRACT_ASSERT.html" title="Macro BOOST_CONTRACT_ASSERT">BOOST_CONTRACT_ASSERT</a></code>
        conditions are false or, more in general, if the <code class="computeroutput"><span class="identifier">invariant</span></code>
        function throws an exception when invariants are checked at function entry
        or exit respectively. By default, these handlers print an error message to
        <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">cerr</span></code> and terminate the program calling
        <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">terminate</span></code> (see <a class="link" href="advanced.html#boost_contract.advanced.throw_on_failures__and__noexcept__" title="Throw on Failures (and noexcept)">Throw
        on Failures</a> to change these failure handlers to throw exceptions,
        exit the program with an error code, etc.).
      </p>
<p>
        See <a class="link" href="advanced.html#boost_contract.advanced.access_specifiers" title="Access Specifiers">Access Specifiers</a>
        to avoid making the <code class="computeroutput"><span class="identifier">invariant</span></code>
        member function <code class="computeroutput"><span class="keyword">public</span></code>. <a href="#ftn.boost_contract.tutorial.class_invariants.f1" class="footnote" name="boost_contract.tutorial.class_invariants.f1"><sup class="footnote">[28]</sup></a> See <code class="computeroutput"><a class="link" href="../BOOST_CO_idm45394999724496.html" title="Macro BOOST_CONTRACT_INVARIANT_FUNC">BOOST_CONTRACT_INVARIANT_FUNC</a></code>
        to use a name different from <code class="computeroutput"><span class="identifier">invariant</span></code>
        (e.g., because <code class="computeroutput"><span class="identifier">invariant</span></code>
        clashes with other names in user-defined classes).
      </p>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../doc/src/images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
          Contract assertions are not checked (not even class invariants) when data
          members are accessed directly (this is different from Eiffel where even
          accessing public data members checks class invariants). Therefore, it might
          be best for both <code class="computeroutput"><span class="keyword">class</span></code>es and
          <code class="computeroutput"><span class="keyword">struct</span></code>s (and also <code class="computeroutput"><span class="keyword">union</span></code>s, see <a class="link" href="extras.html#boost_contract.extras.unions" title="Unions">Unions</a>)
          that have invariants to have no mutable public data members and to access
          data members publicly only via appropriate public functions (e.g., setters
          and getters) that can be programmed to check the class invariants using
          this library.
        </p></td></tr>
</table></div>
<p>
        See <a class="link" href="extras.html#boost_contract.extras.volatile_public_functions" title="Volatile Public Functions">Volatile
        Public Functions</a> to program invariants for classes with <code class="computeroutput"><span class="keyword">volatile</span></code> public functions.
      </p>
<h5>
<a name="boost_contract.tutorial.class_invariants.h0"></a>
        <span class="phrase"><a name="boost_contract.tutorial.class_invariants.static_class_invariants"></a></span><a class="link" href="tutorial.html#boost_contract.tutorial.class_invariants.static_class_invariants">Static
        Class Invariants</a>
      </h5>
<p>
        Static public functions can be programmed to check static class invariants.
        When static class invariants are specified, they are programmed in a public
        <code class="computeroutput"><span class="keyword">static</span></code> function named <code class="computeroutput"><span class="identifier">static_invariant</span></code> taking no argument and
        returning <code class="computeroutput"><span class="keyword">void</span></code>. Classes that
        do not have static class invariants, simply do not declare the <code class="computeroutput"><span class="identifier">static_invariant</span></code> function. <a href="#ftn.boost_contract.tutorial.class_invariants.f2" class="footnote" name="boost_contract.tutorial.class_invariants.f2"><sup class="footnote">[29]</sup></a> For example:
      </p>
<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">u</span> <span class="special">{</span>
<span class="keyword">public</span><span class="special">:</span>                                 <span class="comment">// Must be public.</span>
    <span class="keyword">static</span> <span class="keyword">void</span> <span class="identifier">static_invariant</span><span class="special">()</span> <span class="special">{</span>    <span class="comment">// Must be static.</span>
        <span class="identifier">BOOST_CONTRACT_ASSERT</span><span class="special">(...);</span>
        <span class="special">...</span>
    <span class="special">}</span>

    <span class="special">...</span>
<span class="special">};</span>
</pre>
<p>
        This member function must be <code class="computeroutput"><span class="keyword">static</span></code>
        (and it correctly cannot access the object <code class="computeroutput"><span class="keyword">this</span></code>).
        This library will generate a compile-time error if the <code class="computeroutput"><span class="keyword">static</span></code>
        classifier is missing (unless the <code class="computeroutput"><a class="link" href="../BOOST_CONTRACT_PERMISSIVE.html" title="Macro BOOST_CONTRACT_PERMISSIVE">BOOST_CONTRACT_PERMISSIVE</a></code>
        macro is defined).
      </p>
<p>
        Any code can be programmed in the <code class="computeroutput"><span class="identifier">static_invariant</span></code>
        function, but it is recommended to keep this code simple using mainly assertions
        and if-statements (to avoid programming complex static invariants that might
        be buggy and slow to check at run-time). It is also recommended to use <code class="computeroutput"><a class="link" href="../BOOST_CONTRACT_ASSERT.html" title="Macro BOOST_CONTRACT_ASSERT">BOOST_CONTRACT_ASSERT</a></code> to program
        the assertions because that enables this library to print informative error
        messages when the asserted conditions are evaluated to be false (note that
        this is not a variadic macro, see <a class="link" href="extras.html#boost_contract.extras.no_macros__and_no_variadic_macros_" title="No Macros (and No Variadic Macros)">No
        Macros</a>):
      </p>
<pre class="programlisting"><span class="identifier">BOOST_CONTRACT_ASSERT</span><span class="special">(</span><code class="literal"><span class="emphasis"><em>boolean-condition</em></span></code><span class="special">)</span>
<span class="comment">// Or, if `boolean-condition` has commas `,` not already within parenthesis `()`...</span>
<span class="identifier">BOOST_CONTRACT_ASSERT</span><span class="special">((</span><code class="literal"><span class="emphasis"><em>boolean-condition</em></span></code><span class="special">))</span> <span class="comment">// ...use extra parenthesis (not a variadic macro).</span>
</pre>
<p>
        This library will automatically call failure handlers <code class="computeroutput"><a class="link" href="../boost/contract/entry_invariant_failure.html" title="Function entry_invariant_failure">boost::contract::entry_invariant_failure</a></code>
        or <code class="computeroutput"><a class="link" href="../boost/contract/exit_invariant_failure.html" title="Function exit_invariant_failure">boost::contract::exit_invariant_failure</a></code>
        if any of the <code class="computeroutput"><a class="link" href="../BOOST_CONTRACT_ASSERT.html" title="Macro BOOST_CONTRACT_ASSERT">BOOST_CONTRACT_ASSERT</a></code>
        conditions are false or, more in general, if the <code class="computeroutput"><span class="identifier">static_invariant</span></code>
        function throws an exception when invariants are checked at function entry
        or exit respectively. By default, these handlers print an error message to
        <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">cerr</span></code> and terminate the program calling
        <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">terminate</span></code> (see <a class="link" href="advanced.html#boost_contract.advanced.throw_on_failures__and__noexcept__" title="Throw on Failures (and noexcept)">Throw
        on Failures</a> to change these failure handlers to throw exceptions,
        exit the program with an error code, etc.).
      </p>
<p>
        See <a class="link" href="advanced.html#boost_contract.advanced.access_specifiers" title="Access Specifiers">Access Specifiers</a>
        to avoid making <code class="computeroutput"><span class="identifier">static_invariant</span></code>
        member function <code class="computeroutput"><span class="keyword">public</span></code>. <a href="#ftn.boost_contract.tutorial.class_invariants.f3" class="footnote" name="boost_contract.tutorial.class_invariants.f3"><sup class="footnote">[30]</sup></a> See <code class="computeroutput"><a class="link" href="../BOOST_CO_idm45394999714768.html" title="Macro BOOST_CONTRACT_STATIC_INVARIANT_FUNC">BOOST_CONTRACT_STATIC_INVARIANT_FUNC</a></code>
        to use a name different from <code class="computeroutput"><span class="identifier">static_invariant</span></code>
        (e.g., because <code class="computeroutput"><span class="identifier">static_invariant</span></code>
        clashes with other names in user-defined classes). <a href="#ftn.boost_contract.tutorial.class_invariants.f4" class="footnote" name="boost_contract.tutorial.class_invariants.f4"><sup class="footnote">[31]</sup></a>
      </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_contract.tutorial.constructors"></a><a class="link" href="tutorial.html#boost_contract.tutorial.constructors" title="Constructors">Constructors</a>
</h3></div></div></div>
<p>
        Contracts for constructors are programmed using the <code class="computeroutput"><a class="link" href="../boost/contract/constructor.html" title="Function template constructor">boost::contract::constructor</a></code>
        function and the <code class="computeroutput"><a class="link" href="../boost/contract/constructor_precondition.html" title="Class template constructor_precondition">boost::contract::constructor_precondition</a></code>
        base class. For example (see <a href="../../../example/features/public.cpp" target="_top"><code class="literal">public.cpp</code></a>):
      </p>
<p>
</p>
<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">unique_identifiers</span> <span class="special">:</span>
    <span class="keyword">private</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">contract</span><span class="special">::</span><span class="identifier">constructor_precondition</span><span class="special">&lt;</span><span class="identifier">unique_identifiers</span><span class="special">&gt;</span>
<span class="special">{</span>
<span class="keyword">public</span><span class="special">:</span>
    <span class="keyword">void</span> <span class="identifier">invariant</span><span class="special">()</span> <span class="keyword">const</span> <span class="special">{</span>
        <span class="identifier">BOOST_CONTRACT_ASSERT</span><span class="special">(</span><span class="identifier">size</span><span class="special">()</span> <span class="special">&gt;=</span> <span class="number">0</span><span class="special">);</span>
    <span class="special">}</span>
</pre>
<p>
      </p>
<p>
</p>
<pre class="programlisting"><span class="keyword">public</span><span class="special">:</span>
    <span class="comment">// Contract for a constructor.</span>
    <span class="identifier">unique_identifiers</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">from</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">to</span><span class="special">)</span> <span class="special">:</span>
        <span class="identifier">boost</span><span class="special">::</span><span class="identifier">contract</span><span class="special">::</span><span class="identifier">constructor_precondition</span><span class="special">&lt;</span><span class="identifier">unique_identifiers</span><span class="special">&gt;([&amp;]</span> <span class="special">{</span>
            <span class="identifier">BOOST_CONTRACT_ASSERT</span><span class="special">(</span><span class="identifier">from</span> <span class="special">&gt;=</span> <span class="number">0</span><span class="special">);</span>
            <span class="identifier">BOOST_CONTRACT_ASSERT</span><span class="special">(</span><span class="identifier">to</span> <span class="special">&gt;=</span> <span class="identifier">from</span><span class="special">);</span>
        <span class="special">})</span>
    <span class="special">{</span>
        <span class="identifier">boost</span><span class="special">::</span><span class="identifier">contract</span><span class="special">::</span><span class="identifier">check</span> <span class="identifier">c</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">contract</span><span class="special">::</span><span class="identifier">constructor</span><span class="special">(</span><span class="keyword">this</span><span class="special">)</span>
            <span class="special">.</span><span class="identifier">postcondition</span><span class="special">([&amp;]</span> <span class="special">{</span>
                <span class="identifier">BOOST_CONTRACT_ASSERT</span><span class="special">(</span><span class="identifier">size</span><span class="special">()</span> <span class="special">==</span> <span class="special">(</span><span class="identifier">to</span> <span class="special">-</span> <span class="identifier">from</span> <span class="special">+</span> <span class="number">1</span><span class="special">));</span>
            <span class="special">})</span>
        <span class="special">;</span>

        <span class="comment">// Constructor body.</span>
        <span class="keyword">for</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">id</span> <span class="special">=</span> <span class="identifier">from</span><span class="special">;</span> <span class="identifier">id</span> <span class="special">&lt;=</span> <span class="identifier">to</span><span class="special">;</span> <span class="special">++</span><span class="identifier">id</span><span class="special">)</span> <span class="identifier">vect_</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="identifier">id</span><span class="special">);</span>
    <span class="special">}</span>
</pre>
<p>
      </p>
<p>
</p>
<pre class="programlisting">    <span class="comment">/* ... */</span>
<span class="special">};</span>
</pre>
<p>
      </p>
<p>
        It is not possible to specify preconditions using <code class="computeroutput"><span class="special">.</span><span class="identifier">precondition</span><span class="special">(...)</span></code>
        for constructors (this library will generate a compile-time error if <code class="computeroutput"><span class="special">.</span><span class="identifier">precondition</span><span class="special">(...)</span></code> is used on the object returned by <code class="computeroutput"><a class="link" href="../boost/contract/constructor.html" title="Function template constructor">boost::contract::constructor</a></code>).
        Constructor preconditions are specified using the <code class="computeroutput"><a class="link" href="../boost/contract/constructor_precondition.html" title="Class template constructor_precondition">boost::contract::constructor_precondition</a></code>
        base class instead (same considerations as the ones made in <a class="link" href="tutorial.html#boost_contract.tutorial.preconditions" title="Preconditions">Preconditions</a>
        apply also to the precondition functor passed to <code class="computeroutput"><a class="link" href="../boost/contract/constructor_precondition.html" title="Class template constructor_precondition">boost::contract::constructor_precondition</a></code>).
        Programmes should not access the object <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> from constructor preconditions (because
        the object does not exists yet before the constructor body is executed).
        <a href="#ftn.boost_contract.tutorial.constructors.f0" class="footnote" name="boost_contract.tutorial.constructors.f0"><sup class="footnote">[32]</sup></a> Constructors without preconditions simply do not explicitly initialize
        the <code class="computeroutput"><a class="link" href="../boost/contract/constructor_precondition.html" title="Class template constructor_precondition">boost::contract::constructor_precondition</a></code>
        base (because <code class="computeroutput"><a class="link" href="../boost/contract/constructor_precondition.html" title="Class template constructor_precondition">boost::contract::constructor_precondition</a></code>
        default constructor checks no contract). When the <code class="computeroutput"><a class="link" href="../boost/contract/constructor_precondition.html" title="Class template constructor_precondition">boost::contract::constructor_precondition</a></code>
        base class is used: <a href="#ftn.boost_contract.tutorial.constructors.f1" class="footnote" name="boost_contract.tutorial.constructors.f1"><sup class="footnote">[33]</sup></a>
      </p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
            It should be specified as the <span class="emphasis"><em>first</em></span> class in the
            inheritance list (so constructor preconditions are checked before initializing
            any other base class or data member).
          </li>
<li class="listitem">
            Its inheritance access specifier should always be <code class="computeroutput"><span class="keyword">private</span></code>
            (so this extra base class does not alter the public inheritance tree
            of its derived classes).
          </li>
<li class="listitem">
            It should never be declared as a <code class="computeroutput"><span class="keyword">virtual</span></code>
            base (because virtual bases are initialized only once across the entire
            inheritance hierarchy preventing preconditions of other base classes
            from being checked).
          </li>
<li class="listitem">
            It takes the derived class as template parameter. <a href="#ftn.boost_contract.tutorial.constructors.f2" class="footnote" name="boost_contract.tutorial.constructors.f2"><sup class="footnote">[34]</sup></a>
          </li>
</ul></div>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../doc/src/images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
          A class can avoid inheriting from <code class="computeroutput"><a class="link" href="../boost/contract/constructor_precondition.html" title="Class template constructor_precondition">boost::contract::constructor_precondition</a></code>
          for efficiency but only when all its constructors have no preconditions.
        </p></td></tr>
</table></div>
<p>
        It is possible to specify postconditions for constructors (see <a class="link" href="tutorial.html#boost_contract.tutorial.postconditions" title="Postconditions">Postconditions</a>),
        but programmers should not access the old value of the object <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> in constructor
        postconditions (because the object did not exist yet before the constructor
        body was executed). <a href="#ftn.boost_contract.tutorial.constructors.f3" class="footnote" name="boost_contract.tutorial.constructors.f3"><sup class="footnote">[35]</sup></a> It is also possible to specify exceptions guarantees for constructors
        (see <a class="link" href="tutorial.html#boost_contract.tutorial.exception_guarantees" title="Exception Guarantees">Exception
        Guarantees</a>), but programmers should not access the object <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> or its
        old value in constructor exception guarantees (because the object did not
        exist before executing the constructor body and it was not properly constructed
        given the constructor body threw an exception). <a href="#ftn.boost_contract.tutorial.constructors.f4" class="footnote" name="boost_contract.tutorial.constructors.f4"><sup class="footnote">[36]</sup></a> The <code class="computeroutput"><a class="link" href="../boost/contract/constructor.html" title="Function template constructor">boost::contract::constructor</a></code>
        function takes <code class="computeroutput"><span class="keyword">this</span></code> as a parameter
        (because constructors check class invariants, see <a class="link" href="tutorial.html#boost_contract.tutorial.class_invariants" title="Class Invariants">Class
        Invariants</a>).
      </p>
<p>
        The <code class="computeroutput"><a class="link" href="../boost/contract/constructor.html" title="Function template constructor">boost::contract::constructor</a></code>
        function returns an RAII object that must always be assigned to a local variable
        of type <code class="computeroutput"><a class="link" href="../boost/contract/check.html" title="Class check">boost::contract::check</a></code>
        (otherwise this library will generate a run-time error, see <code class="computeroutput"><a class="link" href="../BOOST_CO_idm45394999694064.html" title="Macro BOOST_CONTRACT_ON_MISSING_CHECK_DECL">BOOST_CONTRACT_ON_MISSING_CHECK_DECL</a></code>).
        Furthermore, C++11 <code class="computeroutput"><span class="keyword">auto</span></code> declarations
        cannot be used here and the <code class="computeroutput"><a class="link" href="../boost/contract/check.html" title="Class check">boost::contract::check</a></code>
        type must be explicitly specified (otherwise this library will generate a
        compile-time error prior C++17 and a run-time error post C++17). The constructor
        body is programmed right after the declaration of this RAII object.
      </p>
<p>
        At construction, the <code class="computeroutput"><a class="link" href="../boost/contract/check.html" title="Class check">boost::contract::check</a></code>
        RAII object for constructors does the following (enclosing constructor entry):
      </p>
<div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem">
            Check static class invariants, by calling <code class="literal"><span class="emphasis"><em>type-of</em></span></code><code class="computeroutput"><span class="special">(*</span><span class="keyword">this</span><span class="special">)::</span><span class="identifier">static_invariant</span><span class="special">()</span></code> (but not non-static class invariants
            because the object does not exist yet).
          </li></ol></div>
<p>
        At destruction instead (enclosing constructor exit):
      </p>
<div class="orderedlist"><ol class="orderedlist" type="1">
<li class="listitem">
            Check static class invariants, by calling <code class="literal"><span class="emphasis"><em>type-of</em></span></code><code class="computeroutput"><span class="special">(*</span><span class="keyword">this</span><span class="special">)::</span><span class="identifier">static_invariant</span><span class="special">()</span></code>.
          </li>
<li class="listitem">
            If the constructor body did not throw an exception:
            <div class="orderedlist"><ol class="orderedlist" type="a">
<li class="listitem">
                  Check non-static class invariants, by calling <code class="computeroutput"><span class="keyword">this</span><span class="special">-&gt;</span><span class="identifier">invariant</span><span class="special">()</span></code>.
                </li>
<li class="listitem">
                  Check postconditions, by calling the nullary functor <code class="literal"><span class="emphasis"><em>s</em></span></code><code class="computeroutput"><span class="special">()</span></code> passed to <code class="computeroutput"><span class="special">.</span><span class="identifier">postcondition</span><span class="special">(</span></code><code class="literal"><span class="emphasis"><em>s</em></span></code><code class="computeroutput"><span class="special">)</span></code>.
                </li>
</ol></div>
          </li>
<li class="listitem">
            Else:
            <div class="orderedlist"><ol class="orderedlist" type="a"><li class="listitem">
                  Check exception guarantees, by calling the nullary functor <code class="literal"><span class="emphasis"><em>e</em></span></code><code class="computeroutput"><span class="special">()</span></code> passed to <code class="computeroutput"><span class="special">.</span><span class="identifier">except</span><span class="special">(</span></code><code class="literal"><span class="emphasis"><em>e</em></span></code><code class="computeroutput"><span class="special">)</span></code>.
                </li></ol></div>
          </li>
</ol></div>
<p>
        This together with C++ object construction mechanism of base classes and
        the use of <code class="computeroutput"><a class="link" href="../boost/contract/constructor_precondition.html" title="Class template constructor_precondition">boost::contract::constructor_precondition</a></code>
        ensures that the constructor contracts are correctly checked at run-time
        (see <a class="link" href="contract_programming_overview.html#boost_contract.contract_programming_overview.constructor_calls" title="Constructor Calls">Constructor
        Calls</a>).
      </p>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../doc/src/images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top">
<p>
          A constructor can avoid calling <code class="computeroutput"><a class="link" href="../boost/contract/constructor.html" title="Function template constructor">boost::contract::constructor</a></code>
          for efficiency but only when it has no postconditions, no exception guarantees,
          and its class has no invariants (even if <code class="computeroutput"><a class="link" href="../boost/contract/constructor.html" title="Function template constructor">boost::contract::constructor</a></code>
          is not used by a derived class, contracts of base class constructors will
          still be correctly checked by C++ object construction mechanism).
        </p>
<p>
          The default constructor and copy constructor automatically generated by
          C++ will not check contracts. Therefore, unless these constructors are
          not public or they have no preconditions, no postconditions, no exception
          guarantees, and their class has no invariants, programmers should manually
          define them using <code class="computeroutput"><a class="link" href="../boost/contract/constructor.html" title="Function template constructor">boost::contract::constructor</a></code>
          and <code class="computeroutput"><a class="link" href="../boost/contract/constructor_precondition.html" title="Class template constructor_precondition">boost::contract::constructor_precondition</a></code>.
          Similar considerations apply to all other constructors automatically generated
          by C++ (e.g., the move constructor).
        </p>
</td></tr>
</table></div>
<h5>
<a name="boost_contract.tutorial.constructors.h0"></a>
        <span class="phrase"><a name="boost_contract.tutorial.constructors.private_and_protected_constructors"></a></span><a class="link" href="tutorial.html#boost_contract.tutorial.constructors.private_and_protected_constructors">Private
        and Protected Constructors</a>
      </h5>
<p>
        Private and protected constructors can omit <code class="computeroutput"><a class="link" href="../boost/contract/constructor.html" title="Function template constructor">boost::contract::constructor</a></code>
        (because they are not part of the public interface of the class so they are
        not required to check class invariants, see <a class="link" href="contract_programming_overview.html#boost_contract.contract_programming_overview.constructor_calls" title="Constructor Calls">Constructor
        Calls</a>). They could still use <code class="computeroutput"><a class="link" href="../boost/contract/constructor_precondition.html" title="Class template constructor_precondition">boost::contract::constructor_precondition</a></code>
        to check preconditions before member initializations, and even use <code class="computeroutput"><a class="link" href="../boost/contract/function.html" title="Function function">boost::contract::function</a></code>
        (but not <code class="computeroutput"><a class="link" href="../boost/contract/constructor.html" title="Function template constructor">boost::contract::constructor</a></code>)
        to only check postconditions and exception guarantees without checking class
        invariants and without calling <code class="computeroutput"><span class="special">.</span><span class="identifier">precondition</span><span class="special">(...)</span></code>
        (see <a class="link" href="advanced.html#boost_contract.advanced.private_and_protected_functions" title="Private and Protected Functions">Private
        and Protected Functions</a>). For example:
      </p>
<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">u</span> <span class="special">:</span> <span class="keyword">private</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">contract</span><span class="special">::</span><span class="identifier">constructor_precondition</span><span class="special">&lt;</span><span class="identifier">u</span><span class="special">&gt;</span> <span class="special">{</span>
<span class="keyword">protected</span><span class="special">:</span>
    <span class="comment">// Contract for a protected constructor (same for private constructors).</span>
    <span class="identifier">u</span><span class="special">()</span> <span class="special">:</span> <span class="comment">// Still use this base class to check constructor preconditions.</span>
        <span class="identifier">boost</span><span class="special">::</span><span class="identifier">contract</span><span class="special">::</span><span class="identifier">constructor_precondition</span><span class="special">&lt;</span><span class="identifier">u</span><span class="special">&gt;([&amp;]</span> <span class="special">{</span>
            <span class="identifier">BOOST_CONTRACT_ASSERT</span><span class="special">(...);</span>
            <span class="special">...</span>
        <span class="special">})</span>
    <span class="special">{</span>
        <span class="comment">// Following will correctly not check class invariants.</span>
        <span class="identifier">boost</span><span class="special">::</span><span class="identifier">contract</span><span class="special">::</span><span class="identifier">check</span> <span class="identifier">c</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">contract</span><span class="special">::</span><span class="identifier">function</span><span class="special">()</span>
            <span class="comment">// Do not use `.precondition(...)` here.</span>
            <span class="special">.</span><span class="identifier">postcondition</span><span class="special">([&amp;]</span> <span class="special">{</span>
                <span class="identifier">BOOST_CONTRACT_ASSERT</span><span class="special">(...);</span>
                <span class="special">...</span>
            <span class="special">})</span>
            <span class="special">.</span><span class="identifier">except</span><span class="special">([&amp;]</span> <span class="special">{</span>
                <span class="identifier">BOOST_CONTRACT_ASSERT</span><span class="special">(...);</span>
                <span class="special">...</span>
            <span class="special">})</span>
        <span class="special">;</span>

        <span class="special">...</span> <span class="comment">// Constructor body.</span>
    <span class="special">}</span>

    <span class="special">...</span>
<span class="special">};</span>
</pre>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_contract.tutorial.destructors"></a><a class="link" href="tutorial.html#boost_contract.tutorial.destructors" title="Destructors">Destructors</a>
</h3></div></div></div>
<p>
        Contracts for destructors are programmed using <code class="computeroutput"><a class="link" href="../boost/contract/destructor.html" title="Function template destructor">boost::contract::destructor</a></code>.
        For example (see <a href="../../../example/features/public.cpp" target="_top"><code class="literal">public.cpp</code></a>):
      </p>
<p>
</p>
<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">unique_identifiers</span> <span class="special">:</span>
    <span class="keyword">private</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">contract</span><span class="special">::</span><span class="identifier">constructor_precondition</span><span class="special">&lt;</span><span class="identifier">unique_identifiers</span><span class="special">&gt;</span>
<span class="special">{</span>
<span class="keyword">public</span><span class="special">:</span>
    <span class="keyword">void</span> <span class="identifier">invariant</span><span class="special">()</span> <span class="keyword">const</span> <span class="special">{</span>
        <span class="identifier">BOOST_CONTRACT_ASSERT</span><span class="special">(</span><span class="identifier">size</span><span class="special">()</span> <span class="special">&gt;=</span> <span class="number">0</span><span class="special">);</span>
    <span class="special">}</span>
</pre>
<p>
      </p>
<p>
</p>
<pre class="programlisting"><span class="keyword">public</span><span class="special">:</span>
    <span class="comment">// Contract for a destructor.</span>
    <span class="keyword">virtual</span> <span class="special">~</span><span class="identifier">unique_identifiers</span><span class="special">()</span> <span class="special">{</span>
        <span class="comment">// Following contract checks class invariants.</span>
        <span class="identifier">boost</span><span class="special">::</span><span class="identifier">contract</span><span class="special">::</span><span class="identifier">check</span> <span class="identifier">c</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">contract</span><span class="special">::</span><span class="identifier">destructor</span><span class="special">(</span><span class="keyword">this</span><span class="special">);</span>

        <span class="comment">// Destructor body here... (do nothing in this example).</span>
    <span class="special">}</span>
</pre>
<p>
      </p>
<p>
</p>
<pre class="programlisting">    <span class="comment">/* ... */</span>
<span class="special">};</span>
</pre>
<p>
      </p>
<p>
        It is not possible to specify preconditions for destructors (this library
        will generate a compile-time error if <code class="computeroutput"><span class="special">.</span><span class="identifier">precondition</span><span class="special">(...)</span></code>
        is used here and that is because destructors can be called at any time after
        construction so they have no precondition). It is possible to specify postconditions
        for destructors (see <a class="link" href="tutorial.html#boost_contract.tutorial.postconditions" title="Postconditions">Postconditions</a>,
        and also <a class="link" href="tutorial.html#boost_contract.tutorial.static_public_functions" title="Static Public Functions">Static
        Public Functions</a> for an example), but programmers should not access
        the object <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
        in destructor postconditions (because the object no longer exists after the
        destructor body has been executed). <a href="#ftn.boost_contract.tutorial.destructors.f0" class="footnote" name="boost_contract.tutorial.destructors.f0"><sup class="footnote">[37]</sup></a> It is also possible to specify exceptions guarantees for destructors
        (see <a class="link" href="tutorial.html#boost_contract.tutorial.exception_guarantees" title="Exception Guarantees">Exception
        Guarantees</a>, even if destructors should usually be programmed to not
        throw exceptions in C++, in fact destructors are implicitly declared <code class="computeroutput"><span class="keyword">noexcept</span></code> since C++11). <a href="#ftn.boost_contract.tutorial.destructors.f1" class="footnote" name="boost_contract.tutorial.destructors.f1"><sup class="footnote">[38]</sup></a> The <code class="computeroutput"><a class="link" href="../boost/contract/destructor.html" title="Function template destructor">boost::contract::destructor</a></code>
        function takes <code class="computeroutput"><span class="keyword">this</span></code> as a parameter
        (because destructors check class invariants, see <a class="link" href="tutorial.html#boost_contract.tutorial.class_invariants" title="Class Invariants">Class
        Invariants</a>).
      </p>
<p>
        The <code class="computeroutput"><a class="link" href="../boost/contract/destructor.html" title="Function template destructor">boost::contract::destructor</a></code>
        function returns an RAII object that must always be assigned to a local variable
        of type <code class="computeroutput"><a class="link" href="../boost/contract/check.html" title="Class check">boost::contract::check</a></code>
        (otherwise this library will generate a run-time error, see <code class="computeroutput"><a class="link" href="../BOOST_CO_idm45394999694064.html" title="Macro BOOST_CONTRACT_ON_MISSING_CHECK_DECL">BOOST_CONTRACT_ON_MISSING_CHECK_DECL</a></code>).
        Furthermore, C++11 <code class="computeroutput"><span class="keyword">auto</span></code> declarations
        cannot be used here and the <code class="computeroutput"><a class="link" href="../boost/contract/check.html" title="Class check">boost::contract::check</a></code>
        type must be explicitly specified (otherwise this library will generate a
        compile-time error prior C++17 and a run-time error post C++17). The destructor
        body is programmed right after the declaration of this RAII object.
      </p>
<p>
        At construction, the <code class="computeroutput"><a class="link" href="../boost/contract/check.html" title="Class check">boost::contract::check</a></code>
        RAII object for destructors does the following (enclosing destructor entry):
      </p>
<div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem">
            Check static and non-static class invariants, by calling <span class="emphasis"><em><code class="literal">type-of</code></em></span><code class="computeroutput"><span class="special">(*</span><span class="keyword">this</span><span class="special">)::</span><span class="identifier">static_invariant</span><span class="special">()</span></code> <a class="link" href="contract_programming_overview.html#and_anchor"><code class="literal"><span class="emphasis"><em>AND</em></span></code></a>
            <code class="computeroutput"><span class="keyword">this</span><span class="special">-&gt;</span><span class="identifier">invariant</span><span class="special">()</span></code>.
          </li></ol></div>
<p>
        At destruction instead (enclosing destructor exit):
      </p>
<div class="orderedlist"><ol class="orderedlist" type="1">
<li class="listitem">
            Check static class invariants, by calling <code class="literal"><span class="emphasis"><em>type-of</em></span></code><code class="computeroutput"><span class="special">(*</span><span class="keyword">this</span><span class="special">)::</span><span class="identifier">static_invariant</span><span class="special">()</span></code>.
          </li>
<li class="listitem">
            If the destructor body did not throw an exception:
            <div class="orderedlist"><ol class="orderedlist" type="a"><li class="listitem">
                  Check postconditions, by calling the nullay functor <code class="literal"><span class="emphasis"><em>s</em></span></code><code class="computeroutput"><span class="special">()</span></code> passed to <code class="computeroutput"><span class="special">.</span><span class="identifier">postcondition</span><span class="special">(</span></code><code class="literal"><span class="emphasis"><em>s</em></span></code><code class="computeroutput"><span class="special">)</span></code>.
                </li></ol></div>
          </li>
<li class="listitem">
            Else (even if destructors should generally be programmed not to throw
            in C++):
            <div class="orderedlist"><ol class="orderedlist" type="a">
<li class="listitem">
                  Check non-static class invariants, by calling <code class="computeroutput"><span class="keyword">this</span><span class="special">-&gt;</span><span class="identifier">invariant</span><span class="special">()</span></code> (because the object was not successfully
                  destructed).
                </li>
<li class="listitem">
                  Check exception guarantees, by calling the nullary functor <code class="literal"><span class="emphasis"><em>e</em></span></code><code class="computeroutput"><span class="special">()</span></code> passed to <code class="computeroutput"><span class="special">.</span><span class="identifier">except</span><span class="special">(</span></code><code class="literal"><span class="emphasis"><em>e</em></span></code><code class="computeroutput"><span class="special">)</span></code>.
                </li>
</ol></div>
          </li>
</ol></div>
<p>
        This together with C++ object destruction mechanism of base classes ensures
        that destructor contracts are correctly checked at run-time (see <a class="link" href="contract_programming_overview.html#boost_contract.contract_programming_overview.destructor_calls" title="Destructor Calls">Destructor
        Calls</a>).
      </p>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../doc/src/images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top">
<p>
          A destructor can avoid calling <code class="computeroutput"><a class="link" href="../boost/contract/destructor.html" title="Function template destructor">boost::contract::destructor</a></code>
          for efficiency but only when it has no postconditions, no exception guarantees,
          and its class has no invariants (even if <code class="computeroutput"><a class="link" href="../boost/contract/destructor.html" title="Function template destructor">boost::contract::destructor</a></code>
          is not used by a derived class, contracts of base class destructors will
          still be correctly checked by C++ object destruction mechanism).
        </p>
<p>
          The default destructor automatically generated by C++ will not check contracts.
          Therefore, unless the destructor is not public or it has no postconditions,
          no exception guarantees, and its class has no invariants, programmers should
          manually define it using <code class="computeroutput"><a class="link" href="../boost/contract/destructor.html" title="Function template destructor">boost::contract::destructor</a></code>.
        </p>
</td></tr>
</table></div>
<h5>
<a name="boost_contract.tutorial.destructors.h0"></a>
        <span class="phrase"><a name="boost_contract.tutorial.destructors.private_and_protected_destructors"></a></span><a class="link" href="tutorial.html#boost_contract.tutorial.destructors.private_and_protected_destructors">Private
        and Protected Destructors</a>
      </h5>
<p>
        Private and protected destructors can omit <code class="computeroutput"><a class="link" href="../boost/contract/destructor.html" title="Function template destructor">boost::contract::destructor</a></code>
        (because they are not part of the public interface of the class so they are
        not required to check class invariants, see <a class="link" href="contract_programming_overview.html#boost_contract.contract_programming_overview.destructor_calls" title="Destructor Calls">Destructor
        Calls</a>). They could use <code class="computeroutput"><a class="link" href="../boost/contract/function.html" title="Function function">boost::contract::function</a></code>
        (but not <code class="computeroutput"><a class="link" href="../boost/contract/destructor.html" title="Function template destructor">boost::contract::destructor</a></code>)
        to only check postconditions and exception guarantees without checking class
        invariants and without calling <code class="computeroutput"><span class="special">.</span><span class="identifier">precondition</span><span class="special">(...)</span></code>
        (see <a class="link" href="advanced.html#boost_contract.advanced.private_and_protected_functions" title="Private and Protected Functions">Private
        and Protected Functions</a>). For example:
      </p>
<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">u</span> <span class="special">{</span>
<span class="keyword">protected</span><span class="special">:</span>
    <span class="comment">// Contract for a protected destructor (same for private destructors).</span>
    <span class="keyword">virtual</span> <span class="special">~</span><span class="identifier">u</span><span class="special">()</span> <span class="special">{</span>
        <span class="comment">// Following will correctly not check class invariants.</span>
        <span class="identifier">boost</span><span class="special">::</span><span class="identifier">contract</span><span class="special">::</span><span class="identifier">check</span> <span class="identifier">c</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">contract</span><span class="special">::</span><span class="identifier">function</span><span class="special">()</span>
            <span class="comment">// Do not use `.precondition(...)` here.</span>
            <span class="special">.</span><span class="identifier">postcondition</span><span class="special">([&amp;]</span> <span class="special">{</span>
                <span class="identifier">BOOST_CONTRACT_ASSERT</span><span class="special">(...);</span>
                <span class="special">...</span>
            <span class="special">})</span>
            <span class="comment">// Could use `.except(...)` here in rare cases of destructors declared to throw.</span>
        <span class="special">;</span>

        <span class="special">...</span> <span class="comment">// Destructor body.</span>
    <span class="special">}</span>

    <span class="special">...</span>
<span class="special">};</span>
</pre>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_contract.tutorial.public_functions"></a><a class="link" href="tutorial.html#boost_contract.tutorial.public_functions" title="Public Functions">Public Functions</a>
</h3></div></div></div>
<p>
        Contracts for public functions are programmed using <code class="computeroutput"><a class="link" href="../boost/contract/public_f_idm45394998885120.html" title="Function template public_function">boost::contract::public_function</a></code>.
        In this section, let's consider public functions that are not static, not
        virtual, and do not override any function from base classes. For example
        (see <a href="../../../example/features/public.cpp" target="_top"><code class="literal">public.cpp</code></a>):
      </p>
<p>
</p>
<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">unique_identifiers</span> <span class="special">:</span>
    <span class="keyword">private</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">contract</span><span class="special">::</span><span class="identifier">constructor_precondition</span><span class="special">&lt;</span><span class="identifier">unique_identifiers</span><span class="special">&gt;</span>
<span class="special">{</span>
<span class="keyword">public</span><span class="special">:</span>
    <span class="keyword">void</span> <span class="identifier">invariant</span><span class="special">()</span> <span class="keyword">const</span> <span class="special">{</span>
        <span class="identifier">BOOST_CONTRACT_ASSERT</span><span class="special">(</span><span class="identifier">size</span><span class="special">()</span> <span class="special">&gt;=</span> <span class="number">0</span><span class="special">);</span>
    <span class="special">}</span>
</pre>
<p>
      </p>
<p>
</p>
<pre class="programlisting"><span class="keyword">public</span><span class="special">:</span>
    <span class="comment">// Contract for a public function (but no static, virtual, or override).</span>
    <span class="keyword">bool</span> <span class="identifier">find</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">id</span><span class="special">)</span> <span class="keyword">const</span> <span class="special">{</span>
        <span class="keyword">bool</span> <span class="identifier">result</span><span class="special">;</span>
        <span class="identifier">boost</span><span class="special">::</span><span class="identifier">contract</span><span class="special">::</span><span class="identifier">check</span> <span class="identifier">c</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">contract</span><span class="special">::</span><span class="identifier">public_function</span><span class="special">(</span><span class="keyword">this</span><span class="special">)</span>
            <span class="special">.</span><span class="identifier">precondition</span><span class="special">([&amp;]</span> <span class="special">{</span>
                <span class="identifier">BOOST_CONTRACT_ASSERT</span><span class="special">(</span><span class="identifier">id</span> <span class="special">&gt;=</span> <span class="number">0</span><span class="special">);</span>
            <span class="special">})</span>
            <span class="special">.</span><span class="identifier">postcondition</span><span class="special">([&amp;]</span> <span class="special">{</span>
                <span class="keyword">if</span><span class="special">(</span><span class="identifier">size</span><span class="special">()</span> <span class="special">==</span> <span class="number">0</span><span class="special">)</span> <span class="identifier">BOOST_CONTRACT_ASSERT</span><span class="special">(!</span><span class="identifier">result</span><span class="special">);</span>
            <span class="special">})</span>
        <span class="special">;</span>

        <span class="comment">// Function body.</span>
        <span class="keyword">return</span> <span class="identifier">result</span> <span class="special">=</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">find</span><span class="special">(</span><span class="identifier">vect_</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">vect_</span><span class="special">.</span><span class="identifier">end</span><span class="special">(),</span> <span class="identifier">id</span><span class="special">)</span> <span class="special">!=</span>
                <span class="identifier">vect_</span><span class="special">.</span><span class="identifier">end</span><span class="special">();</span>
    <span class="special">}</span>
</pre>
<p>
      </p>
<p>
</p>
<pre class="programlisting">    <span class="comment">/* ... */</span>
<span class="special">};</span>
</pre>
<p>
      </p>
<p>
        It is possible to specify preconditions, postconditions, and exception guarantees
        for public functions (see <a class="link" href="tutorial.html#boost_contract.tutorial.preconditions" title="Preconditions">Preconditions</a>,
        <a class="link" href="tutorial.html#boost_contract.tutorial.postconditions" title="Postconditions">Postconditions</a>,
        and <a class="link" href="tutorial.html#boost_contract.tutorial.exception_guarantees" title="Exception Guarantees">Exception
        Guarantees</a>). When called from non-static public functions, the <code class="computeroutput"><a class="link" href="../boost/contract/public_f_idm45394998885120.html" title="Function template public_function">boost::contract::public_function</a></code>
        function takes <code class="computeroutput"><span class="keyword">this</span></code> as a parameter
        (because public functions check class invariants, see <a class="link" href="tutorial.html#boost_contract.tutorial.class_invariants" title="Class Invariants">Class
        Invariants</a>).
      </p>
<p>
        The <code class="computeroutput"><a class="link" href="../boost/contract/public_f_idm45394998885120.html" title="Function template public_function">boost::contract::public_function</a></code>
        function returns an RAII object that must always be assigned to a local variable
        of type <code class="computeroutput"><a class="link" href="../boost/contract/check.html" title="Class check">boost::contract::check</a></code>
        (otherwise this library will generate a run-time error, see <code class="computeroutput"><a class="link" href="../BOOST_CO_idm45394999694064.html" title="Macro BOOST_CONTRACT_ON_MISSING_CHECK_DECL">BOOST_CONTRACT_ON_MISSING_CHECK_DECL</a></code>).
        Furthermore, C++11 <code class="computeroutput"><span class="keyword">auto</span></code> declarations
        cannot be used here and the <code class="computeroutput"><a class="link" href="../boost/contract/check.html" title="Class check">boost::contract::check</a></code>
        type must be explicitly specified (otherwise this library will generate a
        compile-time error prior C++17 and a run-time error post C++17). The public
        function body is programmed right after the declaration of this RAII object.
      </p>
<p>
        At construction, the <code class="computeroutput"><a class="link" href="../boost/contract/check.html" title="Class check">boost::contract::check</a></code>
        RAII object for public functions does the following (enclosing public function
        entry):
      </p>
<div class="orderedlist"><ol class="orderedlist" type="1">
<li class="listitem">
            Check static and non-static class invariants, by calling <code class="literal"><span class="emphasis"><em>type-of</em></span></code><code class="computeroutput"><span class="special">(*</span><span class="keyword">this</span><span class="special">)::</span><span class="identifier">static_invariant</span><span class="special">()</span></code> <a class="link" href="contract_programming_overview.html#and_anchor"><code class="literal"><span class="emphasis"><em>AND</em></span></code></a>
            <code class="computeroutput"><span class="keyword">this</span><span class="special">-&gt;</span><span class="identifier">invariant</span><span class="special">()</span></code>.
          </li>
<li class="listitem">
            Check preconditions, by calling the nullary functor <code class="literal"><span class="emphasis"><em>r</em></span></code><code class="computeroutput"><span class="special">()</span></code> passed to <code class="computeroutput"><span class="special">.</span><span class="identifier">precondition</span><span class="special">(</span></code><code class="literal"><span class="emphasis"><em>r</em></span></code><code class="computeroutput"><span class="special">)</span></code>.
          </li>
</ol></div>
<p>
        At destruction instead (enclosing public function exit):
      </p>
<div class="orderedlist"><ol class="orderedlist" type="1">
<li class="listitem">
            Check static and non-static class invariants, by calling <code class="literal"><span class="emphasis"><em>type-of</em></span></code><code class="computeroutput"><span class="special">(*</span><span class="keyword">this</span><span class="special">)::</span><span class="identifier">static_invariant</span><span class="special">()</span></code> <a class="link" href="contract_programming_overview.html#and_anchor"><code class="literal"><span class="emphasis"><em>AND</em></span></code></a>
            <code class="computeroutput"><span class="keyword">this</span><span class="special">-&gt;</span><span class="identifier">invariant</span><span class="special">()</span></code>
            (even if the function body threw an exception).
          </li>
<li class="listitem">
            If the function body did not throw an exception:
            <div class="orderedlist"><ol class="orderedlist" type="a"><li class="listitem">
                  Check postconditions, by calling the nullary functor <code class="literal"><span class="emphasis"><em>s</em></span></code><code class="computeroutput"><span class="special">()</span></code> passed to <code class="computeroutput"><span class="special">.</span><span class="identifier">postcondition</span><span class="special">(</span></code><code class="literal"><span class="emphasis"><em>s</em></span></code><code class="computeroutput"><span class="special">)</span></code>.
                </li></ol></div>
          </li>
<li class="listitem">
            Else:
            <div class="orderedlist"><ol class="orderedlist" type="a"><li class="listitem">
                  Check exception guarantees, by calling the nullary functor <code class="literal"><span class="emphasis"><em>e</em></span></code><code class="computeroutput"><span class="special">()</span></code> passed to <code class="computeroutput"><span class="special">.</span><span class="identifier">except</span><span class="special">(</span></code><code class="literal"><span class="emphasis"><em>e</em></span></code><code class="computeroutput"><span class="special">)</span></code>.
                </li></ol></div>
          </li>
</ol></div>
<p>
        This ensures that public function contracts are correctly checked at run-time
        (see <a class="link" href="contract_programming_overview.html#boost_contract.contract_programming_overview.public_function_calls" title="Public Function Calls">Public
        Function Calls</a>).
      </p>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../doc/src/images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top">
<p>
          A public function can avoid calling <code class="computeroutput"><a class="link" href="../boost/contract/public_f_idm45394998885120.html" title="Function template public_function">boost::contract::public_function</a></code>
          for efficiency but only when it has no preconditions, no postconditions,
          no exception guarantees, it is not virtual, it does not override any virtual
          function, and its class has no invariants.
        </p>
<p>
          The default copy assignment operator automatically generated by C++ will
          not check contracts. Therefore, unless this operator is not public or it
          has no preconditions, no postconditions, no exception guarantees, and its
          class has no invariants, programmers should manually define it using <code class="computeroutput"><a class="link" href="../boost/contract/public_f_idm45394998885120.html" title="Function template public_function">boost::contract::public_function</a></code>.
          Similar considerations apply to all other operators automatically generated
          by C++ (e.g., the move operator).
        </p>
</td></tr>
</table></div>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_contract.tutorial.virtual_public_functions"></a><a class="link" href="tutorial.html#boost_contract.tutorial.virtual_public_functions" title="Virtual Public Functions">Virtual
      Public Functions</a>
</h3></div></div></div>
<p>
        Contracts for public functions are programmed using <code class="computeroutput"><a class="link" href="../boost/contract/public_f_idm45394998885120.html" title="Function template public_function">boost::contract::public_function</a></code>.
        In this section, let's consider public functions that are virtual but that
        do not override any function from base classes. For example (see <a href="../../../example/features/public.cpp" target="_top"><code class="literal">public.cpp</code></a>):
      </p>
<p>
</p>
<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">unique_identifiers</span> <span class="special">:</span>
    <span class="keyword">private</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">contract</span><span class="special">::</span><span class="identifier">constructor_precondition</span><span class="special">&lt;</span><span class="identifier">unique_identifiers</span><span class="special">&gt;</span>
<span class="special">{</span>
<span class="keyword">public</span><span class="special">:</span>
    <span class="keyword">void</span> <span class="identifier">invariant</span><span class="special">()</span> <span class="keyword">const</span> <span class="special">{</span>
        <span class="identifier">BOOST_CONTRACT_ASSERT</span><span class="special">(</span><span class="identifier">size</span><span class="special">()</span> <span class="special">&gt;=</span> <span class="number">0</span><span class="special">);</span>
    <span class="special">}</span>
</pre>
<p>
      </p>
<p>
</p>
<pre class="programlisting"><span class="keyword">public</span><span class="special">:</span>
    <span class="comment">// Contract for a public virtual function (but no override).</span>
    <span class="keyword">virtual</span> <span class="keyword">int</span> <span class="identifier">push_back</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">id</span><span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">contract</span><span class="special">::</span><span class="identifier">virtual_</span><span class="special">*</span> <span class="identifier">v</span> <span class="special">=</span> <span class="number">0</span><span class="special">)</span> <span class="special">{</span> <span class="comment">// Extra `v`.</span>
        <span class="keyword">int</span> <span class="identifier">result</span><span class="special">;</span>
        <span class="identifier">boost</span><span class="special">::</span><span class="identifier">contract</span><span class="special">::</span><span class="identifier">old_ptr</span><span class="special">&lt;</span><span class="keyword">bool</span><span class="special">&gt;</span> <span class="identifier">old_find</span> <span class="special">=</span>
                <span class="identifier">BOOST_CONTRACT_OLDOF</span><span class="special">(</span><span class="identifier">v</span><span class="special">,</span> <span class="identifier">find</span><span class="special">(</span><span class="identifier">id</span><span class="special">));</span> <span class="comment">// Pass `v`.</span>
        <span class="identifier">boost</span><span class="special">::</span><span class="identifier">contract</span><span class="special">::</span><span class="identifier">old_ptr</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span> <span class="identifier">old_size</span> <span class="special">=</span>
                <span class="identifier">BOOST_CONTRACT_OLDOF</span><span class="special">(</span><span class="identifier">v</span><span class="special">,</span> <span class="identifier">size</span><span class="special">());</span> <span class="comment">// Pass `v`.</span>
        <span class="identifier">boost</span><span class="special">::</span><span class="identifier">contract</span><span class="special">::</span><span class="identifier">check</span> <span class="identifier">c</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">contract</span><span class="special">::</span><span class="identifier">public_function</span><span class="special">(</span>
                <span class="identifier">v</span><span class="special">,</span> <span class="identifier">result</span><span class="special">,</span> <span class="keyword">this</span><span class="special">)</span> <span class="comment">// Pass `v` and `result`.</span>
            <span class="special">.</span><span class="identifier">precondition</span><span class="special">([&amp;]</span> <span class="special">{</span>
                <span class="identifier">BOOST_CONTRACT_ASSERT</span><span class="special">(</span><span class="identifier">id</span> <span class="special">&gt;=</span> <span class="number">0</span><span class="special">);</span>
                <span class="identifier">BOOST_CONTRACT_ASSERT</span><span class="special">(!</span><span class="identifier">find</span><span class="special">(</span><span class="identifier">id</span><span class="special">));</span> <span class="comment">// ID cannot be already present.</span>
            <span class="special">})</span>
            <span class="special">.</span><span class="identifier">postcondition</span><span class="special">([&amp;]</span> <span class="special">(</span><span class="keyword">int</span> <span class="keyword">const</span> <span class="identifier">result</span><span class="special">)</span> <span class="special">{</span>
                <span class="keyword">if</span><span class="special">(!*</span><span class="identifier">old_find</span><span class="special">)</span> <span class="special">{</span>
                    <span class="identifier">BOOST_CONTRACT_ASSERT</span><span class="special">(</span><span class="identifier">find</span><span class="special">(</span><span class="identifier">id</span><span class="special">));</span>
                    <span class="identifier">BOOST_CONTRACT_ASSERT</span><span class="special">(</span><span class="identifier">size</span><span class="special">()</span> <span class="special">==</span> <span class="special">*</span><span class="identifier">old_size</span> <span class="special">+</span> <span class="number">1</span><span class="special">);</span>
                <span class="special">}</span>
                <span class="identifier">BOOST_CONTRACT_ASSERT</span><span class="special">(</span><span class="identifier">result</span> <span class="special">==</span> <span class="identifier">id</span><span class="special">);</span>
            <span class="special">})</span>
        <span class="special">;</span>

        <span class="comment">// Function body.</span>
        <span class="identifier">vect_</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="identifier">id</span><span class="special">);</span>
        <span class="keyword">return</span> <span class="identifier">result</span> <span class="special">=</span> <span class="identifier">id</span><span class="special">;</span>
    <span class="special">}</span>
</pre>
<p>
      </p>
<p>
</p>
<pre class="programlisting">    <span class="comment">/* ... */</span>
<span class="special">};</span>
</pre>
<p>
      </p>
<p>
        Virtual public functions must declare an extra trailing parameter of type
        <code class="computeroutput"><a class="link" href="../boost/contract/virtual_.html" title="Class virtual_">boost::contract::virtual_</a></code><code class="computeroutput"><span class="special">*</span></code> with default value <code class="computeroutput"><span class="number">0</span></code>
        (i.e., <code class="computeroutput"><span class="keyword">nullptr</span></code>). <a href="#ftn.boost_contract.tutorial.virtual_public_functions.f0" class="footnote" name="boost_contract.tutorial.virtual_public_functions.f0"><sup class="footnote">[39]</sup></a> This extra parameter is the last parameter and it has a default
        value so it does not alter the calling interface of the virtual function
        (callers will rarely, if ever, have to explicitly deal with this extra parameter
        a part from when manipulating the virtual function type directly for function
        pointer type-casting, etc.). Programmers must pass the extra virtual parameter
        as the very first argument to all <code class="computeroutput"><a class="link" href="../BOOST_CONTRACT_OLDOF.html" title="Macro BOOST_CONTRACT_OLDOF">BOOST_CONTRACT_OLDOF</a></code>
        and <code class="computeroutput"><a class="link" href="../boost/contract/public_f_idm45394998885120.html" title="Function template public_function">boost::contract::public_function</a></code>
        calls in the virtual public function definition. <a href="#ftn.boost_contract.tutorial.virtual_public_functions.f1" class="footnote" name="boost_contract.tutorial.virtual_public_functions.f1"><sup class="footnote">[40]</sup></a>
      </p>
<p>
        When called from virtual public functions, the <code class="computeroutput"><a class="link" href="../boost/contract/public_f_idm45394998885120.html" title="Function template public_function">boost::contract::public_function</a></code>
        function takes <code class="computeroutput"><span class="keyword">this</span></code> as a parameter
        (because public functions check class invariants, see <a class="link" href="tutorial.html#boost_contract.tutorial.class_invariants" title="Class Invariants">Class
        Invariants</a>). For virtual public functions returning <code class="computeroutput"><span class="keyword">void</span></code>:
      </p>
<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">u</span> <span class="special">{</span>
<span class="keyword">public</span><span class="special">:</span>
    <span class="comment">// A void virtual public function (that does not override).</span>
    <span class="keyword">virtual</span> <span class="keyword">void</span> <span class="identifier">f</span><span class="special">(</span><span class="identifier">t_1</span> <span class="identifier">a_1</span><span class="special">,</span> <span class="special">...,</span> <span class="identifier">t_n</span> <span class="identifier">a_n</span><span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">contract</span><span class="special">::</span><span class="identifier">virtual_</span><span class="special">*</span> <span class="identifier">v</span> <span class="special">=</span> <span class="number">0</span><span class="special">)</span> <span class="special">{</span>
        <span class="identifier">boost</span><span class="special">::</span><span class="identifier">contract</span><span class="special">::</span><span class="identifier">check</span> <span class="identifier">c</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">contract</span><span class="special">::</span><span class="identifier">public_function</span><span class="special">(</span>
                <span class="identifier">v</span><span class="special">,</span> <span class="keyword">this</span><span class="special">)</span>                                    <span class="comment">// No result parameter...</span>
            <span class="special">.</span><span class="identifier">precondition</span><span class="special">([&amp;]</span> <span class="special">{</span> <span class="special">...</span> <span class="special">})</span>
            <span class="special">.</span><span class="identifier">postcondition</span><span class="special">([&amp;]</span> <span class="special">{</span> <span class="special">...</span> <span class="special">})</span>                     <span class="comment">// ...so nullary functor.</span>
            <span class="special">.</span><span class="identifier">except</span><span class="special">([&amp;]</span> <span class="special">{</span> <span class="special">...</span> <span class="special">})</span>
        <span class="special">;</span>

        <span class="special">...</span>
    <span class="special">}</span>

    <span class="special">...</span>
<span class="special">}</span>
</pre>
<p>
        For virtual public functions not returning <code class="computeroutput"><span class="keyword">void</span></code>,
        programmers must also pass a reference to the function return value as the
        second argument to <code class="computeroutput"><a class="link" href="../boost/contract/public_f_idm45394998885120.html" title="Function template public_function">boost::contract::public_function</a></code>.
        In this case, the library will pass this return value reference to the postcondition
        functor that must therefore take one single argument matching the return
        type, otherwise this library will generate a compile-time error (the functor
        parameter can be a constant reference <code class="computeroutput"><span class="keyword">const</span><span class="special">&amp;</span></code> to avoid extra copies of the return
        value): <a href="#ftn.boost_contract.tutorial.virtual_public_functions.f2" class="footnote" name="boost_contract.tutorial.virtual_public_functions.f2"><sup class="footnote">[41]</sup></a>
      </p>
<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">u</span> <span class="special">{</span>
<span class="keyword">public</span><span class="special">:</span>
    <span class="comment">// A void virtual public function (that does not override).</span>
    <span class="keyword">virtual</span> <span class="identifier">t</span> <span class="identifier">f</span><span class="special">(</span><span class="identifier">t_1</span> <span class="identifier">a_1</span><span class="special">,</span> <span class="special">...,</span> <span class="identifier">t_n</span> <span class="identifier">a_n</span><span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">contract</span><span class="special">::</span><span class="identifier">virtual_</span><span class="special">*</span> <span class="identifier">v</span> <span class="special">=</span> <span class="number">0</span><span class="special">)</span> <span class="special">{</span>
        <span class="identifier">t</span> <span class="identifier">result</span><span class="special">;</span>
        <span class="identifier">boost</span><span class="special">::</span><span class="identifier">contract</span><span class="special">::</span><span class="identifier">check</span> <span class="identifier">c</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">contract</span><span class="special">::</span><span class="identifier">public_function</span><span class="special">(</span>
                <span class="identifier">v</span><span class="special">,</span> <span class="identifier">result</span><span class="special">,</span> <span class="keyword">this</span><span class="special">)</span>                            <span class="comment">// Result parameter...</span>
            <span class="special">.</span><span class="identifier">precondition</span><span class="special">([&amp;]</span> <span class="special">{</span> <span class="special">...</span> <span class="special">})</span>
            <span class="special">.</span><span class="identifier">postcondition</span><span class="special">([&amp;]</span> <span class="special">(</span><span class="identifier">t</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">result</span><span class="special">)</span> <span class="special">{</span> <span class="special">...</span> <span class="special">})</span>   <span class="comment">// ...so unary functor.</span>
            <span class="special">.</span><span class="identifier">except</span><span class="special">([&amp;]</span> <span class="special">{</span> <span class="special">...</span> <span class="special">})</span>
        <span class="special">;</span>

        <span class="special">...</span>                                                 <span class="comment">// Assign `result` at each return.</span>
    <span class="special">}</span>

    <span class="special">...</span>
<span class="special">}</span>
</pre>
<div class="important"><table border="0" summary="Important">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Important]" src="../../../../../doc/src/images/important.png"></td>
<th align="left">Important</th>
</tr>
<tr><td align="left" valign="top">
<p>
          It is the responsibility of the programmers to pass the extra virtual parameter
          <code class="computeroutput"><span class="identifier">v</span></code> to all <code class="computeroutput"><a class="link" href="../BOOST_CONTRACT_OLDOF.html" title="Macro BOOST_CONTRACT_OLDOF">BOOST_CONTRACT_OLDOF</a></code>
          and <code class="computeroutput"><a class="link" href="../boost/contract/public_f_idm45394998885120.html" title="Function template public_function">boost::contract::public_function</a></code>
          calls within virtual public functions, and also to pass the return value
          reference after <code class="computeroutput"><span class="identifier">v</span></code> to <code class="computeroutput"><a class="link" href="../boost/contract/public_f_idm45394998885120.html" title="Function template public_function">boost::contract::public_function</a></code>
          for non-void virtual public functions. This library cannot automatically
          generate compile-time errors if programmers fail to do so (but in general
          this will prevent the library from correctly checking contracts at run-time).
          <a href="#ftn.boost_contract.tutorial.virtual_public_functions.f3" class="footnote" name="boost_contract.tutorial.virtual_public_functions.f3"><sup class="footnote">[42]</sup></a>
        </p>
<p>
          <span class="bold"><strong>Mnemonics:</strong></span>
        </p>
<div class="blockquote"><blockquote class="blockquote"><p>
            When <code class="computeroutput"><span class="identifier">v</span></code> is present, always
            pass it as the first argument to <code class="computeroutput"><a class="link" href="../boost/contract/public_f_idm45394998885120.html" title="Function template public_function">boost::contract::public_function</a></code>
            and <code class="computeroutput"><a class="link" href="../BOOST_CONTRACT_OLDOF.html" title="Macro BOOST_CONTRACT_OLDOF">BOOST_CONTRACT_OLDOF</a></code>.
          </p></blockquote></div>
<div class="blockquote"><blockquote class="blockquote"><p>
            Always pass <code class="computeroutput"><span class="identifier">result</span></code> to
            <code class="computeroutput"><a class="link" href="../boost/contract/public_f_idm45394998885120.html" title="Function template public_function">boost::contract::public_function</a></code>
            right after <code class="computeroutput"><span class="identifier">v</span></code> for non-void
            functions.
          </p></blockquote></div>
</td></tr>
</table></div>
<p>
        For the rest, considerations made in <a class="link" href="tutorial.html#boost_contract.tutorial.public_functions" title="Public Functions">Public
        Functions</a> apply to virtual public functions as well.
      </p>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../doc/src/images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
          A virtual public function should always call <code class="computeroutput"><a class="link" href="../boost/contract/public_f_idm45394998885120.html" title="Function template public_function">boost::contract::public_function</a></code>
          (even if it has no preconditions, no postconditions, no exception guarantees,
          and its class has no invariants), otherwise this library will not be able
          to correctly use it for subcontracting.
        </p></td></tr>
</table></div>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_contract.tutorial.public_function_overrides__subcontracting_"></a><a class="link" href="tutorial.html#boost_contract.tutorial.public_function_overrides__subcontracting_" title="Public Function Overrides (Subcontracting)">Public
      Function Overrides (Subcontracting)</a>
</h3></div></div></div>
<p>
        Contracts for public functions are programmed using <code class="computeroutput"><a class="link" href="../boost/contract/public_f_idm45394998885120.html" title="Function template public_function">boost::contract::public_function</a></code>.
        In this section, let's consider public functions (virtual or not) that override
        virtual public functions from one or more of their public base classes. For
        example (see <a href="../../../example/features/public.cpp" target="_top"><code class="literal">public.cpp</code></a>):
        <a href="#ftn.boost_contract.tutorial.public_function_overrides__subcontracting_.f0" class="footnote" name="boost_contract.tutorial.public_function_overrides__subcontracting_.f0"><sup class="footnote">[43]</sup></a>
      </p>
<p>
</p>
<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">identifiers</span>
    <span class="preprocessor">#define</span> <span class="identifier">BASES</span> <span class="keyword">public</span> <span class="identifier">unique_identifiers</span>
    <span class="special">:</span> <span class="identifier">BASES</span>
<span class="special">{</span>
<span class="keyword">public</span><span class="special">:</span>
    <span class="keyword">typedef</span> <span class="identifier">BOOST_CONTRACT_BASE_TYPES</span><span class="special">(</span><span class="identifier">BASES</span><span class="special">)</span> <span class="identifier">base_types</span><span class="special">;</span> <span class="comment">// Bases typedef.</span>
    <span class="preprocessor">#undef</span> <span class="identifier">BASES</span>

    <span class="keyword">void</span> <span class="identifier">invariant</span><span class="special">()</span> <span class="keyword">const</span> <span class="special">{</span> <span class="comment">// Check in AND with bases.</span>
        <span class="identifier">BOOST_CONTRACT_ASSERT</span><span class="special">(</span><span class="identifier">empty</span><span class="special">()</span> <span class="special">==</span> <span class="special">(</span><span class="identifier">size</span><span class="special">()</span> <span class="special">==</span> <span class="number">0</span><span class="special">));</span>
    <span class="special">}</span>
</pre>
<p>
      </p>
<p>
</p>
<pre class="programlisting"><span class="keyword">public</span><span class="special">:</span>
    <span class="comment">// Contract for a public function override.</span>
    <span class="keyword">int</span> <span class="identifier">push_back</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">id</span><span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">contract</span><span class="special">::</span><span class="identifier">virtual_</span><span class="special">*</span> <span class="identifier">v</span> <span class="special">=</span> <span class="number">0</span><span class="special">)</span> <span class="comment">/* override */</span> <span class="special">{</span>
        <span class="keyword">int</span> <span class="identifier">result</span><span class="special">;</span>
        <span class="identifier">boost</span><span class="special">::</span><span class="identifier">contract</span><span class="special">::</span><span class="identifier">old_ptr</span><span class="special">&lt;</span><span class="keyword">bool</span><span class="special">&gt;</span> <span class="identifier">old_find</span> <span class="special">=</span>
                <span class="identifier">BOOST_CONTRACT_OLDOF</span><span class="special">(</span><span class="identifier">v</span><span class="special">,</span> <span class="identifier">find</span><span class="special">(</span><span class="identifier">id</span><span class="special">));</span>
        <span class="identifier">boost</span><span class="special">::</span><span class="identifier">contract</span><span class="special">::</span><span class="identifier">old_ptr</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span> <span class="identifier">old_size</span> <span class="special">=</span>
                <span class="identifier">BOOST_CONTRACT_OLDOF</span><span class="special">(</span><span class="identifier">v</span><span class="special">,</span> <span class="identifier">size</span><span class="special">());</span>
        <span class="identifier">boost</span><span class="special">::</span><span class="identifier">contract</span><span class="special">::</span><span class="identifier">check</span> <span class="identifier">c</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">contract</span><span class="special">::</span><span class="identifier">public_function</span><span class="special">&lt;</span>
            <span class="identifier">override_push_back</span> <span class="comment">// Pass override type plus below function pointer...</span>
        <span class="special">&gt;(</span><span class="identifier">v</span><span class="special">,</span> <span class="identifier">result</span><span class="special">,</span> <span class="special">&amp;</span><span class="identifier">identifiers</span><span class="special">::</span><span class="identifier">push_back</span><span class="special">,</span> <span class="keyword">this</span><span class="special">,</span> <span class="identifier">id</span><span class="special">)</span> <span class="comment">// ...and arguments.</span>
            <span class="special">.</span><span class="identifier">precondition</span><span class="special">([&amp;]</span> <span class="special">{</span> <span class="comment">// Check in OR with bases.</span>
                <span class="identifier">BOOST_CONTRACT_ASSERT</span><span class="special">(</span><span class="identifier">id</span> <span class="special">&gt;=</span> <span class="number">0</span><span class="special">);</span>
                <span class="identifier">BOOST_CONTRACT_ASSERT</span><span class="special">(</span><span class="identifier">find</span><span class="special">(</span><span class="identifier">id</span><span class="special">));</span> <span class="comment">// ID can be already present.</span>
            <span class="special">})</span>
            <span class="special">.</span><span class="identifier">postcondition</span><span class="special">([&amp;]</span> <span class="special">(</span><span class="keyword">int</span> <span class="keyword">const</span> <span class="identifier">result</span><span class="special">)</span> <span class="special">{</span> <span class="comment">// Check in AND with bases.</span>
                <span class="keyword">if</span><span class="special">(*</span><span class="identifier">old_find</span><span class="special">)</span> <span class="identifier">BOOST_CONTRACT_ASSERT</span><span class="special">(</span><span class="identifier">size</span><span class="special">()</span> <span class="special">==</span> <span class="special">*</span><span class="identifier">old_size</span><span class="special">);</span>
            <span class="special">})</span>
        <span class="special">;</span>

        <span class="comment">// Function body.</span>
        <span class="keyword">if</span><span class="special">(!</span><span class="identifier">find</span><span class="special">(</span><span class="identifier">id</span><span class="special">))</span> <span class="identifier">unique_identifiers</span><span class="special">::</span><span class="identifier">push_back</span><span class="special">(</span><span class="identifier">id</span><span class="special">);</span> <span class="comment">// Else, do nothing.</span>
        <span class="keyword">return</span> <span class="identifier">result</span> <span class="special">=</span> <span class="identifier">id</span><span class="special">;</span>
    <span class="special">}</span>
    <span class="identifier">BOOST_CONTRACT_OVERRIDE</span><span class="special">(</span><span class="identifier">push_back</span><span class="special">)</span> <span class="comment">// Define `override_push_back`.</span>
</pre>
<p>
      </p>
<p>
</p>
<pre class="programlisting">    <span class="comment">/* ... */</span>
<span class="special">};</span>
</pre>
<p>
      </p>
<p>
        The extra <code class="computeroutput"><span class="keyword">typedef</span></code> declared using
        <code class="computeroutput"><a class="link" href="../BOOST_CONTRACT_BASE_TYPES.html" title="Macro BOOST_CONTRACT_BASE_TYPES">BOOST_CONTRACT_BASE_TYPES</a></code>
        is required by this library for derived classes and it is internally used
        to detect base classes for subcontracting (see <a class="link" href="tutorial.html#boost_contract.tutorial.base_classes__subcontracting_" title="Base Classes (Subcontracting)">Base
        Classes</a>). This library will generate a compile-time error if there
        is no suitable virtual function to override in any of the public base classes
        for subcontracting. <a href="#ftn.boost_contract.tutorial.public_function_overrides__subcontracting_.f1" class="footnote" name="boost_contract.tutorial.public_function_overrides__subcontracting_.f1"><sup class="footnote">[44]</sup></a>
      </p>
<p>
        When called from public function overrides, the <code class="computeroutput"><a class="link" href="../boost/contract/public_f_idm45394998885120.html" title="Function template public_function">boost::contract::public_function</a></code>
        function template takes an explicit template argument <code class="computeroutput"><span class="identifier">override_</span></code><code class="literal"><span class="emphasis"><em>function-name</em></span></code>
        that must be defined using <code class="computeroutput"><a class="link" href="../BOOST_CONTRACT_OVERRIDE.html" title="Macro BOOST_CONTRACT_OVERRIDE">BOOST_CONTRACT_OVERRIDE</a></code>:
      </p>
<pre class="programlisting"><span class="identifier">BOOST_CONTRACT_OVERRIDE</span><span class="special">(</span><code class="literal"><span class="emphasis"><em>function-name</em></span></code><span class="special">)</span>
</pre>
<p>
        This can be declared at any point in the public section of the enclosing
        class (see <a class="link" href="advanced.html#boost_contract.advanced.access_specifiers" title="Access Specifiers">Access
        Specifiers</a> to use <code class="computeroutput"><a class="link" href="../BOOST_CONTRACT_OVERRIDE.html" title="Macro BOOST_CONTRACT_OVERRIDE">BOOST_CONTRACT_OVERRIDE</a></code>
        also in a non-public section of the class). <code class="computeroutput"><a class="link" href="../BOOST_CONTRACT_OVERRIDE.html" title="Macro BOOST_CONTRACT_OVERRIDE">BOOST_CONTRACT_OVERRIDE</a></code>
        is used only once in a class for a given function name and overloaded functions
        can reuse the same <code class="literal">override_<span class="emphasis"><em>function-name</em></span></code>
        definition (see <a class="link" href="advanced.html#boost_contract.advanced.function_overloads" title="Function Overloads">Function
        Overloads</a>). <code class="computeroutput"><a class="link" href="../BOOST_CO_idm45394998912688.html" title="Macro BOOST_CONTRACT_NAMED_OVERRIDE">BOOST_CONTRACT_NAMED_OVERRIDE</a></code>
        can be used to generate a name different than <code class="literal">override_<span class="emphasis"><em>function-name</em></span></code>
        (e.g., to avoid generating C++ reserved names containing double underscores
        "<code class="computeroutput"><span class="identifier">__</span></code>" for function
        names that already start with an underscore "<code class="computeroutput"><span class="identifier">_</span></code>",
        see <a class="link" href="advanced.html#boost_contract.advanced.named_overrides" title="Named Overrides">Named Overrides</a>).
        For convenience <code class="computeroutput"><a class="link" href="../BOOST_CONTRACT_OVERRIDES.html" title="Macro BOOST_CONTRACT_OVERRIDES">BOOST_CONTRACT_OVERRIDES</a></code>
        can be used with multiple function names instead of repeating <code class="computeroutput"><a class="link" href="../BOOST_CONTRACT_OVERRIDE.html" title="Macro BOOST_CONTRACT_OVERRIDE">BOOST_CONTRACT_OVERRIDE</a></code> for each
        function name (on compilers that support variadic macros). For example, for
        three functions named <code class="computeroutput"><span class="identifier">f</span></code>,
        <code class="computeroutput"><span class="identifier">g</span></code>, and <code class="computeroutput"><span class="identifier">h</span></code>
        (but same for any other number of functions), the following:
      </p>
<pre class="programlisting"><span class="identifier">BOOST_CONTRACT_OVERRIDES</span><span class="special">(</span><span class="identifier">f</span><span class="special">,</span> <span class="identifier">g</span><span class="special">,</span> <span class="identifier">h</span><span class="special">)</span>
</pre>
<p>
        Is equivalent to: <a href="#ftn.boost_contract.tutorial.public_function_overrides__subcontracting_.f2" class="footnote" name="boost_contract.tutorial.public_function_overrides__subcontracting_.f2"><sup class="footnote">[45]</sup></a>
      </p>
<pre class="programlisting"><span class="identifier">BOOST_CONTRACT_OVERRIDE</span><span class="special">(</span><span class="identifier">f</span><span class="special">)</span>
<span class="identifier">BOOST_CONTRACT_OVERRIDE</span><span class="special">(</span><span class="identifier">g</span><span class="special">)</span>
<span class="identifier">BOOST_CONTRACT_OVERRIDE</span><span class="special">(</span><span class="identifier">h</span><span class="special">)</span>
</pre>
<p>
        Public function overrides must always list the extra trailing parameter of
        type <code class="computeroutput"><a class="link" href="../boost/contract/virtual_.html" title="Class virtual_">boost::contract::virtual_</a></code><code class="computeroutput"><span class="special">*</span></code> with default value <code class="computeroutput"><span class="number">0</span></code>
        (i.e., <code class="computeroutput"><span class="keyword">nullptr</span></code>), even when they
        are not declared <code class="computeroutput"><span class="keyword">virtual</span></code>, if
        this parameter is present in the signature of the virtual function being
        overridden from base classes. Programmers must pass the extra virtual parameter
        as the very first argument to all <code class="computeroutput"><a class="link" href="../BOOST_CONTRACT_OLDOF.html" title="Macro BOOST_CONTRACT_OLDOF">BOOST_CONTRACT_OLDOF</a></code>
        and <code class="computeroutput"><a class="link" href="../boost/contract/public_f_idm45394998885120.html" title="Function template public_function">boost::contract::public_function</a></code>
        calls in the public function override definition (see <a class="link" href="tutorial.html#boost_contract.tutorial.virtual_public_functions" title="Virtual Public Functions">Virtual
        Public Functions</a>).
      </p>
<p>
        When called from public function overrides, the <code class="computeroutput"><a class="link" href="../boost/contract/public_f_idm45394998885120.html" title="Function template public_function">boost::contract::public_function</a></code>
        function takes a pointer to the enclosing function, the object <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> (because
        public function overrides check class invariants, see <a class="link" href="tutorial.html#boost_contract.tutorial.class_invariants" title="Class Invariants">Class
        Invariants</a>), and references to each function argument in the order
        they appear in the function declaration. <a href="#ftn.boost_contract.tutorial.public_function_overrides__subcontracting_.f3" class="footnote" name="boost_contract.tutorial.public_function_overrides__subcontracting_.f3"><sup class="footnote">[46]</sup></a> For public function overrides returning <code class="computeroutput"><span class="keyword">void</span></code>:
      </p>
<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">u</span> <span class="special">{</span>
<span class="keyword">public</span><span class="special">:</span>
    <span class="comment">// A void public function override.</span>
    <span class="keyword">void</span> <span class="identifier">f</span><span class="special">(</span><span class="identifier">t_1</span> <span class="identifier">a_1</span><span class="special">,</span> <span class="special">...,</span> <span class="identifier">t_n</span> <span class="identifier">a_n</span><span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">contract</span><span class="special">::</span><span class="identifier">virtual_</span><span class="special">*</span> <span class="identifier">v</span> <span class="special">=</span> <span class="number">0</span><span class="special">)</span> <span class="comment">/* override */</span> <span class="special">{</span>
        <span class="identifier">boost</span><span class="special">::</span><span class="identifier">contract</span><span class="special">::</span><span class="identifier">check</span> <span class="identifier">c</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">contract</span><span class="special">::</span><span class="identifier">public_function</span><span class="special">&lt;</span><span class="identifier">override_f</span><span class="special">&gt;(</span>
                <span class="identifier">v</span><span class="special">,</span> <span class="special">&amp;</span><span class="identifier">u</span><span class="special">::</span><span class="identifier">f</span><span class="special">,</span> <span class="keyword">this</span><span class="special">,</span> <span class="identifier">a_1</span><span class="special">,</span> <span class="special">...,</span> <span class="identifier">a_n</span><span class="special">)</span>              <span class="comment">// No result parameter...</span>
            <span class="special">.</span><span class="identifier">precondition</span><span class="special">([&amp;]</span> <span class="special">{</span> <span class="special">...</span> <span class="special">})</span>
            <span class="special">.</span><span class="identifier">postcondition</span><span class="special">([&amp;]</span> <span class="special">{</span> <span class="special">...</span> <span class="special">})</span>                     <span class="comment">// ...so nullary functor.</span>
            <span class="special">.</span><span class="identifier">except</span><span class="special">([&amp;]</span> <span class="special">{</span> <span class="special">...</span> <span class="special">})</span>
        <span class="special">;</span>

        <span class="special">...</span>
    <span class="special">}</span>
    <span class="identifier">BOOST_CONTRACT_OVERRIDE</span><span class="special">(</span><span class="identifier">f</span><span class="special">)</span>

    <span class="special">...</span>
<span class="special">}</span>
</pre>
<p>
        For public function overrides not returning <code class="computeroutput"><span class="keyword">void</span></code>,
        programmers must also pass a reference to the function return value as the
        second argument to <code class="computeroutput"><a class="link" href="../boost/contract/public_f_idm45394998885120.html" title="Function template public_function">boost::contract::public_function</a></code>
        (this library will generate a compile-time error otherwise). <a href="#ftn.boost_contract.tutorial.public_function_overrides__subcontracting_.f4" class="footnote" name="boost_contract.tutorial.public_function_overrides__subcontracting_.f4"><sup class="footnote">[47]</sup></a> In this case, the library will pass this return value reference
        to the postcondition functor that must therefore take one single argument
        matching the return type, otherwise this library will generate a compile-time
        error (the functor parameter can be a constant reference <code class="computeroutput"><span class="keyword">const</span><span class="special">&amp;</span></code> to avoid extra copies of the return
        value, similarly to non-overriding non-void <a class="link" href="tutorial.html#boost_contract.tutorial.virtual_public_functions" title="Virtual Public Functions">Virtual
        Public Functions</a>):
      </p>
<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">u</span> <span class="special">{</span>
<span class="keyword">public</span><span class="special">:</span>
    <span class="comment">// A non-void public function override.</span>
    <span class="identifier">t</span> <span class="identifier">f</span><span class="special">(</span><span class="identifier">t_1</span> <span class="identifier">a_1</span><span class="special">,</span> <span class="special">...,</span> <span class="identifier">t_n</span> <span class="identifier">a_n</span><span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">contract</span><span class="special">::</span><span class="identifier">virtual_</span><span class="special">*</span> <span class="identifier">v</span> <span class="special">=</span> <span class="number">0</span><span class="special">)</span> <span class="comment">/* override */</span> <span class="special">{</span>
        <span class="identifier">t</span> <span class="identifier">result</span><span class="special">;</span>
        <span class="identifier">boost</span><span class="special">::</span><span class="identifier">contract</span><span class="special">::</span><span class="identifier">check</span> <span class="identifier">c</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">contract</span><span class="special">::</span><span class="identifier">public_function</span><span class="special">&lt;</span><span class="identifier">override_f</span><span class="special">&gt;(</span>
                <span class="identifier">v</span><span class="special">,</span> <span class="identifier">result</span><span class="special">,</span> <span class="special">&amp;</span><span class="identifier">u</span><span class="special">::</span><span class="identifier">f</span><span class="special">,</span> <span class="keyword">this</span><span class="special">,</span> <span class="identifier">a_1</span><span class="special">,</span> <span class="special">...,</span> <span class="identifier">a_n</span><span class="special">)</span>      <span class="comment">// Result parameter...</span>
            <span class="special">.</span><span class="identifier">precondition</span><span class="special">([&amp;]</span> <span class="special">{</span> <span class="special">...</span> <span class="special">})</span>
            <span class="special">.</span><span class="identifier">postcondition</span><span class="special">([&amp;]</span> <span class="special">(</span><span class="identifier">t</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">result</span><span class="special">)</span> <span class="special">{</span> <span class="special">...</span> <span class="special">})</span>   <span class="comment">// ...so unary functor.</span>
            <span class="special">.</span><span class="identifier">except</span><span class="special">([&amp;]</span> <span class="special">{</span> <span class="special">...</span> <span class="special">})</span>
        <span class="special">;</span>

        <span class="special">...</span>                                                 <span class="comment">// Assign `result` at each return.</span>
    <span class="special">}</span>
    <span class="identifier">BOOST_CONTRACT_OVERRIDE</span><span class="special">(</span><span class="identifier">f</span><span class="special">)</span>

    <span class="special">...</span>
<span class="special">}</span>
</pre>
<p>
        This library will throw <code class="computeroutput"><a class="link" href="../boost/contract/bad_virtual_result_cast.html" title="Class bad_virtual_result_cast">boost::contract::bad_virtual_result_cast</a></code>
        if programmers specify return values for public function overrides in derived
        classes that are not consistent with the return types of the virtual public
        functions being overridden in the base classes. <a href="#ftn.boost_contract.tutorial.public_function_overrides__subcontracting_.f5" class="footnote" name="boost_contract.tutorial.public_function_overrides__subcontracting_.f5"><sup class="footnote">[48]</sup></a>
      </p>
<div class="important"><table border="0" summary="Important">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Important]" src="../../../../../doc/src/images/important.png"></td>
<th align="left">Important</th>
</tr>
<tr><td align="left" valign="top">
<p>
          It is the responsibility of the programmers to pass the extra virtual parameter
          <code class="computeroutput"><span class="identifier">v</span></code> to all <code class="computeroutput"><a class="link" href="../BOOST_CONTRACT_OLDOF.html" title="Macro BOOST_CONTRACT_OLDOF">BOOST_CONTRACT_OLDOF</a></code>
          and <code class="computeroutput"><a class="link" href="../boost/contract/public_f_idm45394998885120.html" title="Function template public_function">boost::contract::public_function</a></code>
          calls within public function overrides, and also to pass the return value
          reference after <code class="computeroutput"><span class="identifier">v</span></code> to <code class="computeroutput"><a class="link" href="../boost/contract/public_f_idm45394998885120.html" title="Function template public_function">boost::contract::public_function</a></code>
          for non-void public function overrides. This library cannot always generate
          compile-time errors if programmers fail to do so (but in general this will
          prevent the library from correctly checking contracts at run-time).
        </p>
<p>
          <span class="bold"><strong>Mnemonics:</strong></span>
        </p>
<div class="blockquote"><blockquote class="blockquote"><p>
            When <code class="computeroutput"><span class="identifier">override_</span><span class="special">...</span></code>
            is present, always pass it as template parameter to <code class="computeroutput"><a class="link" href="../boost/contract/public_f_idm45394998885120.html" title="Function template public_function">boost::contract::public_function</a></code>.
          </p></blockquote></div>
<div class="blockquote"><blockquote class="blockquote"><p>
            When <code class="computeroutput"><span class="identifier">v</span></code> is present, always
            pass it as the first argument to <code class="computeroutput"><a class="link" href="../boost/contract/public_f_idm45394998885120.html" title="Function template public_function">boost::contract::public_function</a></code>
            and <code class="computeroutput"><a class="link" href="../BOOST_CONTRACT_OLDOF.html" title="Macro BOOST_CONTRACT_OLDOF">BOOST_CONTRACT_OLDOF</a></code>.
          </p></blockquote></div>
<div class="blockquote"><blockquote class="blockquote"><p>
            Always pass <code class="computeroutput"><span class="identifier">result</span></code> to
            <code class="computeroutput"><a class="link" href="../boost/contract/public_f_idm45394998885120.html" title="Function template public_function">boost::contract::public_function</a></code>
            right after <code class="computeroutput"><span class="identifier">v</span></code> for non-void
            functions.
          </p></blockquote></div>
</td></tr>
</table></div>
<p>
        At construction, the <code class="computeroutput"><a class="link" href="../boost/contract/check.html" title="Class check">boost::contract::check</a></code>
        RAII object for public function overrides does the following (enclosing public
        function override entry):
      </p>
<div class="orderedlist"><ol class="orderedlist" type="1">
<li class="listitem">
            Check static and non-static class invariants for all overridden bases
            and for the derived class in <a class="link" href="contract_programming_overview.html#and_anchor"><code class="literal"><span class="emphasis"><em>AND</em></span></code></a>
            with each other, by calling <code class="literal"><span class="emphasis"><em>type-of</em></span>(<span class="emphasis"><em>overridden-base_1</em></span>)</code><code class="computeroutput"><span class="special">::</span><span class="identifier">static_invariant</span><span class="special">()</span></code> <a class="link" href="contract_programming_overview.html#and_anchor"><code class="literal"><span class="emphasis"><em>AND</em></span></code></a>
            <code class="literal"><span class="emphasis"><em>overridden-base_1</em></span></code><code class="computeroutput"><span class="special">.</span><span class="identifier">invariant</span><span class="special">()</span></code> <a class="link" href="contract_programming_overview.html#and_anchor"><code class="literal"><span class="emphasis"><em>AND</em></span></code></a>...
            <code class="literal"><span class="emphasis"><em>type-of</em></span>(<span class="emphasis"><em>overridden-base_n</em></span>)</code><code class="computeroutput"><span class="special">::</span><span class="identifier">static_invariant</span><span class="special">()</span></code> <a class="link" href="contract_programming_overview.html#and_anchor"><code class="literal"><span class="emphasis"><em>AND</em></span></code></a>
            <code class="literal"><span class="emphasis"><em>overridden-base_n</em></span></code><code class="computeroutput"><span class="special">.</span><span class="identifier">invariant</span><span class="special">()</span></code> <a class="link" href="contract_programming_overview.html#and_anchor"><code class="literal"><span class="emphasis"><em>AND</em></span></code></a>
            <code class="literal"><span class="emphasis"><em>type-of</em></span></code><code class="computeroutput"><span class="special">(*</span><span class="keyword">this</span><span class="special">)::</span><span class="identifier">static_invariant</span><span class="special">()</span></code>
            <a class="link" href="contract_programming_overview.html#and_anchor"><code class="literal"><span class="emphasis"><em>AND</em></span></code></a>
            <code class="computeroutput"><span class="keyword">this</span><span class="special">-&gt;</span><span class="identifier">invariant</span><span class="special">()</span></code>.
          </li>
<li class="listitem">
            Check preconditions for all overridden base functions and for the overriding
            derived function in <a class="link" href="contract_programming_overview.html#or_anchor"><code class="literal"><span class="emphasis"><em>OR</em></span></code></a>
            with each other, by calling the nullary functors <code class="literal"><span class="emphasis"><em>r_1</em></span></code><code class="computeroutput"><span class="special">()</span></code> <a class="link" href="contract_programming_overview.html#or_anchor"><code class="literal"><span class="emphasis"><em>OR</em></span></code></a>...
            <code class="literal"><span class="emphasis"><em>r_n</em></span></code><code class="computeroutput"><span class="special">()</span></code>
            <a class="link" href="contract_programming_overview.html#or_anchor"><code class="literal"><span class="emphasis"><em>OR</em></span></code></a>
            <code class="literal"><span class="emphasis"><em>r</em></span></code><code class="computeroutput"><span class="special">()</span></code>
            passed to <code class="computeroutput"><span class="special">.</span><span class="identifier">precondition</span><span class="special">(</span></code><code class="literal"><span class="emphasis"><em>r_1</em></span></code><code class="computeroutput"><span class="special">)</span></code>, ... <code class="computeroutput"><span class="special">.</span><span class="identifier">precondition</span><span class="special">(</span></code><code class="literal"><span class="emphasis"><em>r_n</em></span></code><code class="computeroutput"><span class="special">)</span></code>, <code class="computeroutput"><span class="special">.</span><span class="identifier">precondition</span><span class="special">(</span></code><code class="literal"><span class="emphasis"><em>r</em></span></code><code class="computeroutput"><span class="special">)</span></code> for all of the overridden and overriding
            functions respectively.
          </li>
</ol></div>
<p>
        At destruction instead (enclosing public function override exit):
      </p>
<div class="orderedlist"><ol class="orderedlist" type="1">
<li class="listitem">
            Check static and non-static class invariants for all overridden bases
            and for the derived class in <a class="link" href="contract_programming_overview.html#and_anchor"><code class="literal"><span class="emphasis"><em>AND</em></span></code></a>
            with each other, by calling <code class="literal"><span class="emphasis"><em>type-of</em></span>(<span class="emphasis"><em>overridden-base_1</em></span>)</code><code class="computeroutput"><span class="special">::</span><span class="identifier">static_invariant</span><span class="special">()</span></code> <a class="link" href="contract_programming_overview.html#and_anchor"><code class="literal"><span class="emphasis"><em>AND</em></span></code></a>
            <code class="literal"><span class="emphasis"><em>overridden-base_1</em></span></code><code class="computeroutput"><span class="special">.</span><span class="identifier">invariant</span><span class="special">()</span></code> <a class="link" href="contract_programming_overview.html#and_anchor"><code class="literal"><span class="emphasis"><em>AND</em></span></code></a>...
            <code class="literal"><span class="emphasis"><em>type-of</em></span>(<span class="emphasis"><em>overridden-base_n</em></span>)</code><code class="computeroutput"><span class="special">::</span><span class="identifier">static_invariant</span><span class="special">()</span></code> <a class="link" href="contract_programming_overview.html#and_anchor"><code class="literal"><span class="emphasis"><em>AND</em></span></code></a>
            <code class="literal"><span class="emphasis"><em>overridden-base_n</em></span></code><code class="computeroutput"><span class="special">.</span><span class="identifier">invariant</span><span class="special">()</span></code> <a class="link" href="contract_programming_overview.html#and_anchor"><code class="literal"><span class="emphasis"><em>AND</em></span></code></a>
            <code class="literal"><span class="emphasis"><em>type-of</em></span></code><code class="computeroutput"><span class="special">(*</span><span class="keyword">this</span><span class="special">)::</span><span class="identifier">static_invariant</span><span class="special">()</span></code>
            <a class="link" href="contract_programming_overview.html#and_anchor"><code class="literal"><span class="emphasis"><em>AND</em></span></code></a>
            <code class="computeroutput"><span class="keyword">this</span><span class="special">-&gt;</span><span class="identifier">invariant</span><span class="special">()</span></code>
            (even if the function body threw an exception).
          </li>
<li class="listitem">
            If the function body did not throw an exception:
            <div class="orderedlist"><ol class="orderedlist" type="a"><li class="listitem">
                  Check postconditions for all overridden base functions and for
                  the overriding derived function in <a class="link" href="contract_programming_overview.html#and_anchor"><code class="literal"><span class="emphasis"><em>AND</em></span></code></a>
                  with each other, by calling the nullary functors <code class="literal"><span class="emphasis"><em>s_1</em></span></code><code class="computeroutput"><span class="special">()</span></code> <a class="link" href="contract_programming_overview.html#and_anchor"><code class="literal"><span class="emphasis"><em>AND</em></span></code></a>...
                  <code class="literal"><span class="emphasis"><em>s_n</em></span></code><code class="computeroutput"><span class="special">()</span></code>
                  <a class="link" href="contract_programming_overview.html#and_anchor"><code class="literal"><span class="emphasis"><em>AND</em></span></code></a>
                  <code class="literal"><span class="emphasis"><em>s</em></span></code><code class="computeroutput"><span class="special">()</span></code>
                  passed to <code class="computeroutput"><span class="special">.</span><span class="identifier">postcondition</span><span class="special">(</span></code><code class="literal"><span class="emphasis"><em>s_1</em></span></code><code class="computeroutput"><span class="special">)</span></code>, ... <code class="computeroutput"><span class="special">.</span><span class="identifier">postcondition</span><span class="special">(</span></code><code class="literal"><span class="emphasis"><em>s_n</em></span></code><code class="computeroutput"><span class="special">)</span></code>, <code class="computeroutput"><span class="special">.</span><span class="identifier">postcondition</span><span class="special">(</span></code><code class="literal"><span class="emphasis"><em>s</em></span></code><code class="computeroutput"><span class="special">)</span></code> for all of the overridden and
                  overriding functions respectively (or the unary functors <code class="literal"><span class="emphasis"><em>s_1</em></span></code><code class="computeroutput"><span class="special">(</span></code><code class="literal"><span class="emphasis"><em>result</em></span></code><code class="computeroutput"><span class="special">)</span></code> <a class="link" href="contract_programming_overview.html#and_anchor"><code class="literal"><span class="emphasis"><em>AND</em></span></code></a>...
                  <code class="literal"><span class="emphasis"><em>s_n</em></span></code><code class="computeroutput"><span class="special">(</span></code><code class="literal"><span class="emphasis"><em>result</em></span></code><code class="computeroutput"><span class="special">)</span></code> <a class="link" href="contract_programming_overview.html#and_anchor"><code class="literal"><span class="emphasis"><em>AND</em></span></code></a>
                  <code class="literal"><span class="emphasis"><em>s</em></span></code><code class="computeroutput"><span class="special">(</span></code><code class="literal"><span class="emphasis"><em>result</em></span></code><code class="computeroutput"><span class="special">)</span></code> for non-void public function overrides).
                </li></ol></div>
          </li>
<li class="listitem">
            Else:
            <div class="orderedlist"><ol class="orderedlist" type="a"><li class="listitem">
                  Check exception guarantees for all overridden base functions and
                  for the overriding derived function in <a class="link" href="contract_programming_overview.html#and_anchor"><code class="literal"><span class="emphasis"><em>AND</em></span></code></a>
                  with each other, by calling the nullary functors <code class="literal"><span class="emphasis"><em>e_1</em></span></code><code class="computeroutput"><span class="special">()</span></code> <a class="link" href="contract_programming_overview.html#and_anchor"><code class="literal"><span class="emphasis"><em>AND</em></span></code></a>...
                  <code class="literal"><span class="emphasis"><em>e_n</em></span></code><code class="computeroutput"><span class="special">()</span></code>
                  <a class="link" href="contract_programming_overview.html#and_anchor"><code class="literal"><span class="emphasis"><em>AND</em></span></code></a>
                  <code class="literal"><span class="emphasis"><em>e</em></span></code><code class="computeroutput"><span class="special">()</span></code>
                  passed to <code class="computeroutput"><span class="special">.</span><span class="identifier">except</span><span class="special">(</span></code><code class="literal"><span class="emphasis"><em>e_1</em></span></code><code class="computeroutput"><span class="special">)</span></code>, ... <code class="computeroutput"><span class="special">.</span><span class="identifier">except</span><span class="special">(</span></code><code class="literal"><span class="emphasis"><em>e_n</em></span></code><code class="computeroutput"><span class="special">)</span></code>, <code class="computeroutput"><span class="special">.</span><span class="identifier">except</span><span class="special">(</span></code><code class="literal"><span class="emphasis"><em>e</em></span></code><code class="computeroutput"><span class="special">)</span></code> for all of the overridden and
                  overriding functions respectively.
                </li></ol></div>
          </li>
</ol></div>
<p>
        This ensures that contracts and subcontracts of public function overrides
        are correctly checked at run-time in accordance with the <a href="http://en.wikipedia.org/wiki/Liskov_substitution_principle" target="_top">substitution
        principle</a> (see <a class="link" href="contract_programming_overview.html#boost_contract.contract_programming_overview.public_function_calls" title="Public Function Calls">Public
        Function Calls</a>).
      </p>
<p>
        For the rest, considerations made in <a class="link" href="tutorial.html#boost_contract.tutorial.virtual_public_functions" title="Virtual Public Functions">Virtual
        Public Functions</a> apply to public function overrides as well.
      </p>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../doc/src/images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
          A public function override should always call <code class="computeroutput"><a class="link" href="../boost/contract/public_f_idm45394998885120.html" title="Function template public_function">boost::contract::public_function</a></code>
          (even if it has no preconditions, no postconditions, no exception guarantees,
          and its class has no invariants), otherwise this library will not be able
          to correctly use it for subcontracting.
        </p></td></tr>
</table></div>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_contract.tutorial.base_classes__subcontracting_"></a><a class="link" href="tutorial.html#boost_contract.tutorial.base_classes__subcontracting_" title="Base Classes (Subcontracting)">Base
      Classes (Subcontracting)</a>
</h3></div></div></div>
<p>
        In order for this library to support subcontracting, programmers must specify
        the bases of a derived class declaring a public member type named <code class="computeroutput"><span class="identifier">base_types</span></code> via a <code class="computeroutput"><span class="keyword">typedef</span></code>
        using <code class="computeroutput"><a class="link" href="../BOOST_CONTRACT_BASE_TYPES.html" title="Macro BOOST_CONTRACT_BASE_TYPES">BOOST_CONTRACT_BASE_TYPES</a></code>.
        For example (see <a href="../../../example/features/base_types.cpp" target="_top"><code class="literal">base_types.cpp</code></a>):
      </p>
<p>
</p>
<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">chars</span>
    <span class="preprocessor">#define</span> <span class="identifier">BASES</span> <span class="comment">/* local macro (for convenience) */</span> <span class="special">\</span>
        <span class="keyword">private</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">contract</span><span class="special">::</span><span class="identifier">constructor_precondition</span><span class="special">&lt;</span><span class="identifier">chars</span><span class="special">&gt;,</span> <span class="special">\</span>
        <span class="keyword">public</span> <span class="identifier">unique_chars</span><span class="special">,</span> <span class="special">\</span>
        <span class="keyword">public</span> <span class="keyword">virtual</span> <span class="identifier">pushable</span><span class="special">&lt;</span><span class="keyword">char</span><span class="special">&gt;,</span> <span class="special">\</span>
        <span class="keyword">virtual</span> <span class="keyword">protected</span> <span class="identifier">has_size</span><span class="special">,</span> <span class="special">\</span>
        <span class="keyword">private</span> <span class="identifier">has_empty</span>
    <span class="special">:</span> <span class="identifier">BASES</span> <span class="comment">// Bases of this class.</span>
<span class="special">{</span>
<span class="keyword">public</span><span class="special">:</span>
    <span class="keyword">typedef</span> <span class="identifier">BOOST_CONTRACT_BASE_TYPES</span><span class="special">(</span><span class="identifier">BASES</span><span class="special">)</span> <span class="identifier">base_types</span><span class="special">;</span> <span class="comment">// Bases typedef.</span>
    <span class="preprocessor">#undef</span> <span class="identifier">BASES</span> <span class="comment">// Undefine local macro.</span>

    <span class="comment">/* ... */</span>
</pre>
<p>
      </p>
<p>
        For convenience, a <span class="emphasis"><em>local macro</em></span> named <code class="computeroutput"><span class="identifier">BASES</span></code>
        can be used to avoid repeating the base list twice (first in the derived
        class declaration <code class="computeroutput"><span class="keyword">class</span> </code><code class="literal"><span class="emphasis"><em>class-name</em></span></code><code class="computeroutput">
        <span class="special">:</span> </code><code class="literal"><span class="emphasis"><em>base-list</em></span></code>
        and then again when invoking <code class="computeroutput"><span class="identifier">BOOST_CONTRACT_BASE_TYPES</span><span class="special">(</span></code><code class="literal"><span class="emphasis"><em>base-list</em></span></code><code class="computeroutput"><span class="special">)</span></code>). Being a local macro, <code class="computeroutput"><span class="identifier">BASES</span></code>
        must be undefined using <code class="computeroutput"><span class="preprocessor">#undef</span>
        <span class="identifier">BASES</span></code> after it is used to declare
        the <code class="computeroutput"><span class="identifier">base_types</span></code> <code class="computeroutput"><span class="keyword">typedef</span></code> (to avoid name clashes and macro redefinition
        errors). <a href="#ftn.boost_contract.tutorial.base_classes__subcontracting_.f0" class="footnote" name="boost_contract.tutorial.base_classes__subcontracting_.f0"><sup class="footnote">[49]</sup></a>
      </p>
<p>
        <code class="computeroutput"><a class="link" href="../BOOST_CONTRACT_BASE_TYPES.html" title="Macro BOOST_CONTRACT_BASE_TYPES">BOOST_CONTRACT_BASE_TYPES</a></code>
        is a variadic macro and accepts a list of bases separated by commas (see
        <a class="link" href="extras.html#boost_contract.extras.no_macros__and_no_variadic_macros_" title="No Macros (and No Variadic Macros)">No
        Macros</a> to program <code class="computeroutput"><span class="identifier">base_types</span></code>
        without using macros). As already noted in <a class="link" href="tutorial.html#boost_contract.tutorial.constructors" title="Constructors">Constructors</a>,
        when the extra base <code class="computeroutput"><a class="link" href="../boost/contract/constructor_precondition.html" title="Class template constructor_precondition">boost::contract::constructor_precondition</a></code>
        is used to program constructor preconditions, its inheritance access level
        must always be <code class="computeroutput"><span class="keyword">private</span></code> and it
        must be specified as the very first base.
      </p>
<div class="important"><table border="0" summary="Important">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Important]" src="../../../../../doc/src/images/important.png"></td>
<th align="left">Important</th>
</tr>
<tr><td align="left" valign="top">
<p>
          Each base passed to <code class="computeroutput"><a class="link" href="../BOOST_CONTRACT_BASE_TYPES.html" title="Macro BOOST_CONTRACT_BASE_TYPES">BOOST_CONTRACT_BASE_TYPES</a></code>
          must <span class="emphasis"><em>explicitly</em></span> specify its inheritance access level
          <code class="computeroutput"><span class="keyword">public</span></code>, <code class="computeroutput"><span class="keyword">protected</span></code>,
          or <code class="computeroutput"><span class="keyword">private</span></code> (but <code class="computeroutput"><span class="keyword">virtual</span></code> is optional and can be specified
          either before or after the access level as usual in C++). This library
          will generate a compile-time error if the first base is missing its inheritance
          access level, but this library will not be able to always generate an error
          if the access level is missing for bases after the first one. <a href="#ftn.boost_contract.tutorial.base_classes__subcontracting_.f1" class="footnote" name="boost_contract.tutorial.base_classes__subcontracting_.f1"><sup class="footnote">[50]</sup></a> It is the responsibility of the programmers to make sure that
          all bases passed to <code class="computeroutput"><a class="link" href="../BOOST_CONTRACT_BASE_TYPES.html" title="Macro BOOST_CONTRACT_BASE_TYPES">BOOST_CONTRACT_BASE_TYPES</a></code>
          explicitly specify their inheritance access level (inheritance access levels
          are instead optional in C++ because <code class="computeroutput"><span class="keyword">private</span></code>
          is implicitly assumed for <code class="computeroutput"><span class="keyword">class</span></code>
          types and <code class="computeroutput"><span class="keyword">public</span></code> for <code class="computeroutput"><span class="keyword">struct</span></code> types).
        </p>
<p>
          <span class="bold"><strong>Mnemonics:</strong></span>
        </p>
<div class="blockquote"><blockquote class="blockquote"><p>
            Always explicitly specify the inheritance access level <code class="computeroutput"><span class="keyword">public</span></code>, <code class="computeroutput"><span class="keyword">protected</span></code>,
            or <code class="computeroutput"><span class="keyword">private</span></code> for base classes
            passed to <code class="computeroutput"><a class="link" href="../BOOST_CONTRACT_BASE_TYPES.html" title="Macro BOOST_CONTRACT_BASE_TYPES">BOOST_CONTRACT_BASE_TYPES</a></code>.
          </p></blockquote></div>
</td></tr>
</table></div>
<p>
        See <a class="link" href="advanced.html#boost_contract.advanced.access_specifiers" title="Access Specifiers">Access Specifiers</a>
        to avoid making the <code class="computeroutput"><span class="identifier">base_types</span></code>
        member type <code class="computeroutput"><span class="keyword">public</span></code>. <a href="#ftn.boost_contract.tutorial.base_classes__subcontracting_.f2" class="footnote" name="boost_contract.tutorial.base_classes__subcontracting_.f2"><sup class="footnote">[51]</sup></a> See <code class="computeroutput"><a class="link" href="../BOOST_CO_idm45394999732400.html" title="Macro BOOST_CONTRACT_BASES_TYPEDEF">BOOST_CONTRACT_BASES_TYPEDEF</a></code>
        to use a name different from <code class="computeroutput"><span class="identifier">base_types</span></code>
        (e.g., because <code class="computeroutput"><span class="identifier">base_types</span></code>
        clashes with other names in user-defined classes).
      </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_contract.tutorial.static_public_functions"></a><a class="link" href="tutorial.html#boost_contract.tutorial.static_public_functions" title="Static Public Functions">Static
      Public Functions</a>
</h3></div></div></div>
<p>
        Contracts for public functions are programmed using <code class="computeroutput"><a class="link" href="../boost/contract/public_f_idm45394998885120.html" title="Function template public_function">boost::contract::public_function</a></code>.
        In this section, let's consider static public functions. For example (see
        <a href="../../../example/features/static_public.cpp" target="_top"><code class="literal">static_public.cpp</code></a>):
      </p>
<p>
</p>
<pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">C</span><span class="special">&gt;</span>
<span class="keyword">class</span> <span class="identifier">make</span> <span class="special">{</span>
<span class="keyword">public</span><span class="special">:</span>
    <span class="keyword">static</span> <span class="keyword">void</span> <span class="identifier">static_invariant</span><span class="special">()</span> <span class="special">{</span> <span class="comment">// Static class invariants.</span>
        <span class="identifier">BOOST_CONTRACT_ASSERT</span><span class="special">(</span><span class="identifier">instances</span><span class="special">()</span> <span class="special">&gt;=</span> <span class="number">0</span><span class="special">);</span>
    <span class="special">}</span>

    <span class="comment">// Contract for a static public function.</span>
    <span class="keyword">static</span> <span class="keyword">int</span> <span class="identifier">instances</span><span class="special">()</span> <span class="special">{</span>
        <span class="comment">// Explicit template parameter `make` (check static invariants).</span>
        <span class="identifier">boost</span><span class="special">::</span><span class="identifier">contract</span><span class="special">::</span><span class="identifier">check</span> <span class="identifier">c</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">contract</span><span class="special">::</span><span class="identifier">public_function</span><span class="special">&lt;</span><span class="identifier">make</span><span class="special">&gt;();</span>

        <span class="keyword">return</span> <span class="identifier">instances_</span><span class="special">;</span> <span class="comment">// Function body.</span>
    <span class="special">}</span>

    <span class="comment">/* ... */</span>
</pre>
<p>
      </p>
<p>
        It is possible to specify preconditions, postconditions, and exception guarantees
        for static public functions (see <a class="link" href="tutorial.html#boost_contract.tutorial.preconditions" title="Preconditions">Preconditions</a>,
        <a class="link" href="tutorial.html#boost_contract.tutorial.postconditions" title="Postconditions">Postconditions</a>,
        and <a class="link" href="tutorial.html#boost_contract.tutorial.exception_guarantees" title="Exception Guarantees">Exception
        Guarantees</a>). When called from static public functions, <code class="computeroutput"><a class="link" href="../boost/contract/public_f_idm45394998885120.html" title="Function template public_function">boost::contract::public_function</a></code>
        cannot take the object <code class="computeroutput"><span class="keyword">this</span></code>
        as a parameter (because there is no object <code class="computeroutput"><span class="keyword">this</span></code>
        in static member functions) so the enclosing class type is specified via
        an explicit template parameter as in <code class="computeroutput"><a class="link" href="../boost/contract/public_f_idm45394998885120.html" title="Function template public_function">boost::contract::public_function</a></code><code class="literal">&lt;<span class="emphasis"><em>class-type</em></span>&gt;</code>
        (the class type is required to check static class invariants, see <a class="link" href="tutorial.html#boost_contract.tutorial.class_invariants" title="Class Invariants">Class
        Invariants</a>):
      </p>
<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">u</span> <span class="special">{</span>
<span class="keyword">public</span><span class="special">:</span>
    <span class="comment">// A static public function.</span>
    <span class="keyword">static</span> <span class="keyword">void</span> <span class="identifier">f</span><span class="special">()</span> <span class="special">{</span>
        <span class="identifier">boost</span><span class="special">::</span><span class="identifier">contract</span><span class="special">::</span><span class="identifier">check</span> <span class="identifier">c</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">contract</span><span class="special">::</span><span class="identifier">public_function</span><span class="special">&lt;</span><span class="identifier">u</span><span class="special">&gt;()</span> <span class="comment">// Class type `u` as explicit template parameter.</span>
            <span class="special">.</span><span class="identifier">precondition</span><span class="special">([&amp;]</span> <span class="special">{</span> <span class="special">...</span> <span class="special">})</span>
            <span class="special">.</span><span class="identifier">postcondition</span><span class="special">([&amp;]</span> <span class="special">{</span> <span class="special">...</span> <span class="special">})</span>
            <span class="special">.</span><span class="identifier">except</span><span class="special">([&amp;]</span> <span class="special">{</span> <span class="special">...</span> <span class="special">})</span>
        <span class="special">;</span>

        <span class="special">...</span>
    <span class="special">}</span>

    <span class="special">...</span>
<span class="special">};</span>
</pre>
<p>
        The <code class="computeroutput"><a class="link" href="../boost/contract/public_f_idm45394998885120.html" title="Function template public_function">boost::contract::public_function</a></code>
        function returns an RAII object that must be assigned to a local variable
        of type <code class="computeroutput"><a class="link" href="../boost/contract/check.html" title="Class check">boost::contract::check</a></code>
        (otherwise this library will generate a run-time error, see <code class="computeroutput"><a class="link" href="../BOOST_CO_idm45394999694064.html" title="Macro BOOST_CONTRACT_ON_MISSING_CHECK_DECL">BOOST_CONTRACT_ON_MISSING_CHECK_DECL</a></code>).
        Furthermore, C++11 <code class="computeroutput"><span class="keyword">auto</span></code> declarations
        cannot be used here and the <code class="computeroutput"><a class="link" href="../boost/contract/check.html" title="Class check">boost::contract::check</a></code>
        type must be explicitly specified (otherwise this library will generate a
        compile-time error prior C++17 and a run-time error post C++17). The static
        public functions body is programmed right after the declaration of this RAII
        object.
      </p>
<p>
        At construction, the <code class="computeroutput"><a class="link" href="../boost/contract/check.html" title="Class check">boost::contract::check</a></code>
        RAII object for static public functions does the following (enclosing static
        public function entry):
      </p>
<div class="orderedlist"><ol class="orderedlist" type="1">
<li class="listitem">
            Check static class invariants, by calling <code class="literal"><span class="emphasis"><em>class-type</em></span></code><code class="computeroutput"><span class="special">::</span><span class="identifier">static_invariant</span><span class="special">()</span></code> (but never non-static class invariants).
          </li>
<li class="listitem">
            Check preconditions, by calling the nullary functor <code class="literal"><span class="emphasis"><em>r</em></span></code><code class="computeroutput"><span class="special">()</span></code> passed to <code class="computeroutput"><span class="special">.</span><span class="identifier">precondition</span><span class="special">(</span></code><code class="literal"><span class="emphasis"><em>r</em></span></code><code class="computeroutput"><span class="special">)</span></code>.
          </li>
</ol></div>
<p>
        At destruction instead (enclosing static public function exit):
      </p>
<div class="orderedlist"><ol class="orderedlist" type="1">
<li class="listitem">
            Check static class invariants, by calling <code class="literal"><span class="emphasis"><em>class-type</em></span></code><code class="computeroutput"><span class="special">::</span><span class="identifier">static_invariant</span><span class="special">()</span></code> (even if the function body threw an
            exception, but never non-static class invariants).
          </li>
<li class="listitem">
            If the function body did not throw an exception:
            <div class="orderedlist"><ol class="orderedlist" type="a"><li class="listitem">
                  Check postconditions, by calling the nullary functor <code class="literal"><span class="emphasis"><em>s</em></span></code><code class="computeroutput"><span class="special">()</span></code> passed to <code class="computeroutput"><span class="special">.</span><span class="identifier">postcondition</span><span class="special">(</span></code><code class="literal"><span class="emphasis"><em>s</em></span></code><code class="computeroutput"><span class="special">)</span></code>.
                </li></ol></div>
          </li>
<li class="listitem">
            Else:
            <div class="orderedlist"><ol class="orderedlist" type="a"><li class="listitem">
                  Check exception guarantees, by calling the nullary functor <code class="literal"><span class="emphasis"><em>e</em></span></code><code class="computeroutput"><span class="special">()</span></code> passed to <code class="computeroutput"><span class="special">.</span><span class="identifier">except</span><span class="special">(</span></code><code class="literal"><span class="emphasis"><em>e</em></span></code><code class="computeroutput"><span class="special">)</span></code>.
                </li></ol></div>
          </li>
</ol></div>
<p>
        This ensures that static public function contracts are correctly checked
        at run-time (static public functions do not subcontract because they have
        no object <code class="computeroutput"><span class="keyword">this</span></code> and therefore
        there is no inheritance, see <a class="link" href="contract_programming_overview.html#boost_contract.contract_programming_overview.public_function_calls" title="Public Function Calls">Public
        Function Calls</a>).
      </p>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../doc/src/images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
          A static public function can avoid calling <code class="computeroutput"><a class="link" href="../boost/contract/public_f_idm45394998885120.html" title="Function template public_function">boost::contract::public_function</a></code>
          for efficiency but only when it has no preconditions, no postconditions,
          no exception guarantees, and its class has no static invariants (the class
          can still have non-static invariants or base classes instead).
        </p></td></tr>
</table></div>
</div>
<div class="footnotes">
<br><hr style="width:100; text-align:left;margin-left: 0">
<div id="ftn.boost_contract.tutorial.non_member_functions.f0" class="footnote"><p><a href="#boost_contract.tutorial.non_member_functions.f0" class="para"><sup class="para">[19] </sup></a>
          The name of this local variable is arbitrary, but <code class="computeroutput"><span class="identifier">c</span></code>
          is often used in this documentation for <span class="quote">&#8220;<span class="quote">c</span>&#8221;</span>heck or <span class="quote">&#8220;<span class="quote">c</span>&#8221;</span>aminiti
          <code class="literal">;-)</code> .
        </p></div>
<div id="ftn.boost_contract.tutorial.non_member_functions.f1" class="footnote"><p><a href="#boost_contract.tutorial.non_member_functions.f1" class="para"><sup class="para">[20] </sup></a>
          <span class="bold"><strong>Rationale:</strong></span> C++17 guaranteed copy elision
          on function return value voids the trick this library uses to force a compile-time
          error when <code class="computeroutput"><span class="keyword">auto</span></code> is incorrectly
          used instead of <code class="computeroutput"><a class="link" href="../boost/contract/check.html" title="Class check">boost::contract::check</a></code>.
          The library still generates a run-time error in this case (also on C++17).
          In any case, after reading this documentation it should be evident to programmers
          that <code class="computeroutput"><span class="keyword">auto</span></code> should not be used
          in <code class="computeroutput"><a class="link" href="../boost/contract/check.html" title="Class check">boost::contract::check</a></code>
          declarations so this misuse of <code class="computeroutput"><span class="keyword">auto</span></code>
          should not be an issue in practice.
        </p></div>
<div id="ftn.boost_contract.tutorial.preconditions.f0" class="footnote"><p><a href="#boost_contract.tutorial.preconditions.f0" class="para"><sup class="para">[21] </sup></a>
          Lambda functions with no parameters can be programmed in C++11 as <code class="computeroutput"><span class="special">[...]</span> <span class="special">()</span> <span class="special">{</span> <span class="special">...</span> <span class="special">}</span></code>
          but also equivalently as <code class="computeroutput"><span class="special">[...]</span> <span class="special">{</span> <span class="special">...</span> <span class="special">}</span></code>.
          This second from is often used in this documentation omitting the empty
          parameter list <code class="computeroutput"><span class="special">()</span></code> for brevity.
        </p></div>
<div id="ftn.boost_contract.tutorial.preconditions.f1" class="footnote"><p><a href="#boost_contract.tutorial.preconditions.f1" class="para"><sup class="para">[22] </sup></a>
          In this documentation preconditions often capture variables by reference
          to avoid extra copies.
        </p></div>
<div id="ftn.boost_contract.tutorial.return_values.f0" class="footnote"><p><a href="#boost_contract.tutorial.return_values.f0" class="para"><sup class="para">[23] </sup></a>
          The name of the local variable that holds the return value is arbitrary,
          but <code class="computeroutput"><span class="identifier">result</span></code> is often used
          in this documentation.
        </p></div>
<div id="ftn.boost_contract.tutorial.old_values.f0" class="footnote"><p><a href="#boost_contract.tutorial.old_values.f0" class="para"><sup class="para">[24] </sup></a>
          The name of a local variable that holds an old value is arbitrary, but
          <code class="literal">old_<span class="emphasis"><em>variable-name</em></span></code> is often used
          in this documentation.
        </p></div>
<div id="ftn.boost_contract.tutorial.old_values.f1" class="footnote"><p><a href="#boost_contract.tutorial.old_values.f1" class="para"><sup class="para">[25] </sup></a>
          <span class="bold"><strong>Rationale:</strong></span> Old values have to be optional
          values because they need to be left uninitialized when they are not used
          because both postconditions and exception guarantees are disabled (defining
          <code class="computeroutput"><a class="link" href="../BOOST_CO_idm45394999656032.html" title="Macro BOOST_CONTRACT_NO_POSTCONDITIONS">BOOST_CONTRACT_NO_POSTCONDITIONS</a></code>
          and <code class="computeroutput"><a class="link" href="../BOOST_CONTRACT_NO_EXCEPTS.html" title="Macro BOOST_CONTRACT_NO_EXCEPTS">BOOST_CONTRACT_NO_EXCEPTS</a></code>).
          That is to avoid old value copies when old values are not used, either
          a pointer or (better) a <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span></code>
          could have been used to achieve that. In addition, old values need to be
          pointers internally allocated by this library so that they are never copied
          twice even when calling an overridden function multiple times to check
          preconditions, postconditions, etc. to implement subcontracting, so a smart
          pointer class template was used.
        </p></div>
<div id="ftn.boost_contract.tutorial.old_values.f2" class="footnote"><p><a href="#boost_contract.tutorial.old_values.f2" class="para"><sup class="para">[26] </sup></a>
          For example, old value pointers might be null in preconditions when postconditions
          and exception guarantees are disabled defining <code class="computeroutput"><a class="link" href="../BOOST_CO_idm45394999656032.html" title="Macro BOOST_CONTRACT_NO_POSTCONDITIONS">BOOST_CONTRACT_NO_POSTCONDITIONS</a></code>
          and <code class="computeroutput"><a class="link" href="../BOOST_CONTRACT_NO_EXCEPTS.html" title="Macro BOOST_CONTRACT_NO_EXCEPTS">BOOST_CONTRACT_NO_EXCEPTS</a></code>
          (but also when checking an overridden virtual public function contract
          via subcontracting, etc.).
        </p></div>
<div id="ftn.boost_contract.tutorial.class_invariants.f0" class="footnote"><p><a href="#boost_contract.tutorial.class_invariants.f0" class="para"><sup class="para">[27] </sup></a>
          This library uses template meta-programming (SFINAE-based introspection
          techniques) to check invariants only for classes that declare a member
          function named by <code class="computeroutput"><a class="link" href="../BOOST_CO_idm45394999724496.html" title="Macro BOOST_CONTRACT_INVARIANT_FUNC">BOOST_CONTRACT_INVARIANT_FUNC</a></code>.
        </p></div>
<div id="ftn.boost_contract.tutorial.class_invariants.f1" class="footnote"><p><a href="#boost_contract.tutorial.class_invariants.f1" class="para"><sup class="para">[28] </sup></a>
          In this documentation the <code class="computeroutput"><span class="identifier">invariant</span></code>
          member function is often declared <code class="computeroutput"><span class="keyword">public</span></code>
          for simplicity. However, in production code it might not be acceptable
          to augment the public members of a class adding the <code class="computeroutput"><span class="identifier">invariant</span></code>
          function (and that can be avoided using <code class="computeroutput"><a class="link" href="../boost/contract/access.html" title="Class access">boost::contract::access</a></code>
          as explained in <a class="link" href="advanced.html#boost_contract.advanced.access_specifiers" title="Access Specifiers">Access
          Specifiers</a>).
        </p></div>
<div id="ftn.boost_contract.tutorial.class_invariants.f2" class="footnote"><p><a href="#boost_contract.tutorial.class_invariants.f2" class="para"><sup class="para">[29] </sup></a>
          This library uses template meta-programming (SFINAE-based introspection
          techniques) to check static invariants only for classes that declare a
          member function named by <code class="computeroutput"><a class="link" href="../BOOST_CO_idm45394999714768.html" title="Macro BOOST_CONTRACT_STATIC_INVARIANT_FUNC">BOOST_CONTRACT_STATIC_INVARIANT_FUNC</a></code>.
        </p></div>
<div id="ftn.boost_contract.tutorial.class_invariants.f3" class="footnote"><p><a href="#boost_contract.tutorial.class_invariants.f3" class="para"><sup class="para">[30] </sup></a>
          In this documentation the <code class="computeroutput"><span class="identifier">static_invariant</span></code>
          member function is often declared <code class="computeroutput"><span class="keyword">public</span></code>
          for simplicity. However, in production code it might not be acceptable
          to augment the public members of a class adding the <code class="computeroutput"><span class="identifier">static_invariant</span></code>
          function (and that can be avoided using <code class="computeroutput"><a class="link" href="../boost/contract/access.html" title="Class access">boost::contract::access</a></code>
          as explained in <a class="link" href="advanced.html#boost_contract.advanced.access_specifiers" title="Access Specifiers">Access
          Specifiers</a>).
        </p></div>
<div id="ftn.boost_contract.tutorial.class_invariants.f4" class="footnote"><p><a href="#boost_contract.tutorial.class_invariants.f4" class="para"><sup class="para">[31] </sup></a>
          <span class="bold"><strong>Rationale:</strong></span> In C++, it is not possible
          to overload a member function based on the <code class="computeroutput"><span class="keyword">static</span></code>
          classifier. Therefore, this library has to use different names for the
          member functions checking non-static and static class invariants (namely
          for <code class="computeroutput"><a class="link" href="../BOOST_CO_idm45394999724496.html" title="Macro BOOST_CONTRACT_INVARIANT_FUNC">BOOST_CONTRACT_INVARIANT_FUNC</a></code>
          and for <code class="computeroutput"><a class="link" href="../BOOST_CO_idm45394999714768.html" title="Macro BOOST_CONTRACT_STATIC_INVARIANT_FUNC">BOOST_CONTRACT_STATIC_INVARIANT_FUNC</a></code>).
        </p></div>
<div id="ftn.boost_contract.tutorial.constructors.f0" class="footnote"><p><a href="#boost_contract.tutorial.constructors.f0" class="para"><sup class="para">[32] </sup></a>
          See <a class="link" href="extras.html#boost_contract.extras.no_lambda_functions__no_c__11_" title="No Lambda Functions (No C++11)">No
          Lambda Functions</a> to enforce this constraint at compile-time (but
          not recommended because of extra boiler-plate code).
        </p></div>
<div id="ftn.boost_contract.tutorial.constructors.f1" class="footnote"><p><a href="#boost_contract.tutorial.constructors.f1" class="para"><sup class="para">[33] </sup></a>
          There is a MSVC bug that was fixed in MSVC 2013 for which lambdas cannot
          be used in constructor member initialization lists for templates. On MSVC
          compilers with that bug, an extra (static) member function can be used
          (together with <code class="computeroutput"><span class="identifier">bind</span></code> and
          <code class="computeroutput"><span class="identifier">cref</span></code> as needed) to program
          constructor preconditions instead of using lambdas (see <a class="link" href="extras.html#boost_contract.extras.no_lambda_functions__no_c__11_" title="No Lambda Functions (No C++11)">No
          Lambda Functions</a>).
        </p></div>
<div id="ftn.boost_contract.tutorial.constructors.f2" class="footnote"><p><a href="#boost_contract.tutorial.constructors.f2" class="para"><sup class="para">[34] </sup></a>
              <span class="bold"><strong>Rationale:</strong></span> The <code class="computeroutput"><a class="link" href="../boost/contract/constructor_precondition.html" title="Class template constructor_precondition">boost::contract::constructor_precondition</a></code>
              takes the derived class as its template parameter (using the Curiously
              Recursive Template Pattern, CRTP) so the instantiated template type
              is unique for each derived class. This always avoids base class ambiguity
              resolution errors even when multiple inheritance is used. Note that,
              as already mentioned, virtual inheritance could not be used instead
              of the template parameter here to resolve ambiguities (because virtual
              bases are initialized only once by the outer-most derived class, and
              that would not allow to properly check preconditions of all base classes).
            </p></div>
<div id="ftn.boost_contract.tutorial.constructors.f3" class="footnote"><p><a href="#boost_contract.tutorial.constructors.f3" class="para"><sup class="para">[35] </sup></a>
          See <a class="link" href="extras.html#boost_contract.extras.no_lambda_functions__no_c__11_" title="No Lambda Functions (No C++11)">No
          Lambda Functions</a> to enforce this constraint at compile-time (but
          not recommended because of extra boiler-plate code).
        </p></div>
<div id="ftn.boost_contract.tutorial.constructors.f4" class="footnote"><p><a href="#boost_contract.tutorial.constructors.f4" class="para"><sup class="para">[36] </sup></a>
          See <a class="link" href="extras.html#boost_contract.extras.no_lambda_functions__no_c__11_" title="No Lambda Functions (No C++11)">No
          Lambda Functions</a> to enforce these constraints at compile-time (but
          not recommended because of extra boiler-plate code).
        </p></div>
<div id="ftn.boost_contract.tutorial.destructors.f0" class="footnote"><p><a href="#boost_contract.tutorial.destructors.f0" class="para"><sup class="para">[37] </sup></a>
          See <a class="link" href="extras.html#boost_contract.extras.no_lambda_functions__no_c__11_" title="No Lambda Functions (No C++11)">No
          Lambda Functions</a> to enforce this constraint at compile-time (but
          not recommended because of extra boiler-plate code).
        </p></div>
<div id="ftn.boost_contract.tutorial.destructors.f1" class="footnote"><p><a href="#boost_contract.tutorial.destructors.f1" class="para"><sup class="para">[38] </sup></a>
          Exceptions guarantees in destructors can access both the object <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> and
          its old value because the object existed before executing the destructor
          body and it still exists given the destructor body failed throwing an exception
          so technically the object should still be properly constructed and satisfy
          its class invariants.
        </p></div>
<div id="ftn.boost_contract.tutorial.virtual_public_functions.f0" class="footnote"><p><a href="#boost_contract.tutorial.virtual_public_functions.f0" class="para"><sup class="para">[39] </sup></a>
          The name of this extra parameter is arbitrary, but <code class="computeroutput"><span class="identifier">v</span></code>
          is often used in this documentation.
        </p></div>
<div id="ftn.boost_contract.tutorial.virtual_public_functions.f1" class="footnote"><p><a href="#boost_contract.tutorial.virtual_public_functions.f1" class="para"><sup class="para">[40] </sup></a>
          <span class="bold"><strong>Rationale:</strong></span> The <code class="computeroutput"><a class="link" href="../boost/contract/virtual_.html" title="Class virtual_">boost::contract::virtual_</a></code><code class="computeroutput"><span class="special">*</span></code> parameter is used by this library to determine
          that a function is virtual (in C++ it is not possible to introspect if
          a function is declared <code class="computeroutput"><span class="keyword">virtual</span></code>).
          Furthermore, this parameter is internally used by this library to implement
          subcontracting (specifically to pass result and old values that are evaluated
          by the overriding function to the contracts of overridden virtual functions
          in base classes, and also to check preconditions, postconditions, and exception
          guarantees of overridden virtual functions in <a class="link" href="contract_programming_overview.html#or_anchor"><code class="literal"><span class="emphasis"><em>OR</em></span></code></a>
          and <a class="link" href="contract_programming_overview.html#and_anchor"><code class="literal"><span class="emphasis"><em>AND</em></span></code></a>
          with contracts of the overriding virtual function).
        </p></div>
<div id="ftn.boost_contract.tutorial.virtual_public_functions.f2" class="footnote"><p><a href="#boost_contract.tutorial.virtual_public_functions.f2" class="para"><sup class="para">[41] </sup></a>
          <span class="bold"><strong>Rationale:</strong></span> The extra function result parameter
          taken by the functor passed to <code class="computeroutput"><span class="special">.</span><span class="identifier">postcondition</span><span class="special">(...)</span></code>
          is used by this library to pass the return value evaluated by the overriding
          function to all its overridden virtual functions to support subcontracting.
        </p></div>
<div id="ftn.boost_contract.tutorial.virtual_public_functions.f3" class="footnote"><p><a href="#boost_contract.tutorial.virtual_public_functions.f3" class="para"><sup class="para">[42] </sup></a>
            <span class="bold"><strong>Rationale:</strong></span> This library does not require
            programmers to specify the function type when using <code class="computeroutput"><a class="link" href="../boost/contract/public_f_idm45394998885120.html" title="Function template public_function">boost::contract::public_function</a></code>
            for non-overriding virtual public functions. Therefore, this library
            does not know if the enclosing function has a non-void return type so
            it cannot check if the return value reference is passed as required for
            non-overriding virtual public functions. Instead the function type is
            passed to this library for virtual public function overrides and that
            also allows this library to give a compile-time error if the return value
            reference is missing in those cases.
          </p></div>
<div id="ftn.boost_contract.tutorial.public_function_overrides__subcontracting_.f0" class="footnote"><p><a href="#boost_contract.tutorial.public_function_overrides__subcontracting_.f0" class="para"><sup class="para">[43] </sup></a>
          In this documentation, function overrides are often marked with the code
          comment <code class="computeroutput"><span class="comment">/* override */</span></code>. On
          compilers that support C++11 virtual specifiers, the <code class="computeroutput"><span class="identifier">override</span></code>
          identifier can be used instead (<code class="computeroutput"><span class="identifier">override</span></code>
          is not used in the documentation simply because virtual specifiers are
          not widely supported yet, even by compilers that support C++11 lambda functions).
        </p></div>
<div id="ftn.boost_contract.tutorial.public_function_overrides__subcontracting_.f1" class="footnote"><p><a href="#boost_contract.tutorial.public_function_overrides__subcontracting_.f1" class="para"><sup class="para">[44] </sup></a>
          The compile-time error generated by the library in this case is similar
          in principle to the error generated by the C++11 <code class="computeroutput"><span class="identifier">override</span></code>
          specifier, but it is limited to functions with the extra <code class="computeroutput"><a class="link" href="../boost/contract/virtual_.html" title="Class virtual_">boost::contract::virtual_</a></code><code class="computeroutput"><span class="special">*</span></code> parameter and searched recursively only
          in <code class="computeroutput"><span class="keyword">public</span></code> base classes passed
          to <code class="computeroutput"><a class="link" href="../BOOST_CONTRACT_BASE_TYPES.html" title="Macro BOOST_CONTRACT_BASE_TYPES">BOOST_CONTRACT_BASE_TYPES</a></code>
          because only those are considered for subcontracting.
        </p></div>
<div id="ftn.boost_contract.tutorial.public_function_overrides__subcontracting_.f2" class="footnote"><p><a href="#boost_contract.tutorial.public_function_overrides__subcontracting_.f2" class="para"><sup class="para">[45] </sup></a>
          There is no equivalent of <code class="computeroutput"><a class="link" href="../BOOST_CO_idm45394998912688.html" title="Macro BOOST_CONTRACT_NAMED_OVERRIDE">BOOST_CONTRACT_NAMED_OVERRIDE</a></code>
          that operates on multiple function names at once (<code class="computeroutput"><a class="link" href="../BOOST_CO_idm45394998912688.html" title="Macro BOOST_CONTRACT_NAMED_OVERRIDE">BOOST_CONTRACT_NAMED_OVERRIDE</a></code>
          is not expected to be used often so it can simply be repeated multiple
          times when needed).
        </p></div>
<div id="ftn.boost_contract.tutorial.public_function_overrides__subcontracting_.f3" class="footnote"><p><a href="#boost_contract.tutorial.public_function_overrides__subcontracting_.f3" class="para"><sup class="para">[46] </sup></a>
          <span class="bold"><strong>Rationale:</strong></span> The object <code class="computeroutput"><span class="keyword">this</span></code>
          is passed after the function pointer to follow <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">bind</span></code>'s
          syntax. The function pointer and references to all function arguments are
          needed for public function overrides because this library has to internally
          call overridden virtual public functions to check their contracts for subcontracting
          (even if this library will not actually execute the bodies of the overridden
          functions).
        </p></div>
<div id="ftn.boost_contract.tutorial.public_function_overrides__subcontracting_.f4" class="footnote"><p><a href="#boost_contract.tutorial.public_function_overrides__subcontracting_.f4" class="para"><sup class="para">[47] </sup></a>
          <span class="bold"><strong>Rationale:</strong></span> As for non-overriding virtual
          public functions, also public function overrides use the extra return value
          parameter to pass it to the overridden functions when subcontracting. In
          the case of public function overrides, this library has the function pointer
          so it will generate a compile-time error if the function is non-void and
          programmers forget to specify the extra return value parameter (this extra
          error checking is not possible instead for non-overriding virtual public
          functions because their contracts do not take the function pointer as a
          parameter, see <a class="link" href="tutorial.html#boost_contract.tutorial.virtual_public_functions" title="Virtual Public Functions">Virtual
          Public Functions</a>).
        </p></div>
<div id="ftn.boost_contract.tutorial.public_function_overrides__subcontracting_.f5" class="footnote"><p><a href="#boost_contract.tutorial.public_function_overrides__subcontracting_.f5" class="para"><sup class="para">[48] </sup></a>
          <span class="bold"><strong>Rationale:</strong></span> The <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">bad_any_cast</span></code>
          exception was not used here because it does not print the from- and to-
          type names (so it is not descriptive enough).
        </p></div>
<div id="ftn.boost_contract.tutorial.base_classes__subcontracting_.f0" class="footnote"><p><a href="#boost_contract.tutorial.base_classes__subcontracting_.f0" class="para"><sup class="para">[49] </sup></a>
          The name of this local macro is arbitrary, but <code class="computeroutput"><span class="identifier">BASES</span></code>
          is often used in this documentation.
        </p></div>
<div id="ftn.boost_contract.tutorial.base_classes__subcontracting_.f1" class="footnote"><p><a href="#boost_contract.tutorial.base_classes__subcontracting_.f1" class="para"><sup class="para">[50] </sup></a>
            <span class="bold"><strong>Rationale:</strong></span> This library explicitly requires
            the inheritance access level because derived classes must subcontract
            only from public bases, but not from protected or private bases (see
            <a class="link" href="contract_programming_overview.html#boost_contract.contract_programming_overview.public_function_calls" title="Public Function Calls">Public
            Function Calls</a>). <code class="computeroutput"><a class="link" href="../BOOST_CONTRACT_BASE_TYPES.html" title="Macro BOOST_CONTRACT_BASE_TYPES">BOOST_CONTRACT_BASE_TYPES</a></code>
            inspects each inheritance access level using preprocessor meta-programming
            and removes non-public bases from the list of bases internally used for
            subcontracting. However, this library cannot always detect when programmers
            forget to specify the inheritance access level because, when commas are
            used to separate template parameters passed to base classes, the preprocessor
            will not be able to correctly use commas to identify the next base class
            token in the inheritance list (the preprocessor cannot distinguish between
            commas that are not protected by round parenthesis, like the ones used
            in templates). Therefore, this library uses the inheritance access level
            keyword <code class="computeroutput"><span class="keyword">public</span></code>, <code class="computeroutput"><span class="keyword">protected</span></code>, or <code class="computeroutput"><span class="keyword">private</span></code>
            instead of commas <code class="computeroutput"><span class="special">,</span></code> for
            the preprocessor to correctly find the next base class token in the inheritance
            list (thus inheritance access levels must always be explicit specified
            by programmers for each base).
          </p></div>
<div id="ftn.boost_contract.tutorial.base_classes__subcontracting_.f2" class="footnote"><p><a href="#boost_contract.tutorial.base_classes__subcontracting_.f2" class="para"><sup class="para">[51] </sup></a>
          In this documentation the <code class="computeroutput"><span class="identifier">base_type</span></code>
          member type is often declared <code class="computeroutput"><span class="keyword">public</span></code>
          for simplicity. However, in production code it might not be acceptable
          to augment the public members of a class adding the <code class="computeroutput"><span class="identifier">base_types</span></code>
          type (and that can be avoided using <code class="computeroutput"><a class="link" href="../boost/contract/access.html" title="Class access">boost::contract::access</a></code>
          as explained in <a class="link" href="advanced.html#boost_contract.advanced.access_specifiers" title="Access Specifiers">Access
          Specifiers</a>).
        </p></div>
</div>
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<td align="right"><div class="copyright-footer">Copyright &#169; 2008-2019 Lorenzo Caminiti<p>
        Distributed under the Boost Software License, Version 1.0 (see accompanying
        file LICENSE_1_0.txt or a copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
      </p>
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