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- <div class="section">
- <div class="titlepage"><div><div><h2 class="title" style="clear: both">
- <a name="boost_localfunction.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_localfunction.tutorial.local_functions">Local Functions</a></span></dt>
- <dt><span class="section"><a href="tutorial.html#boost_localfunction.tutorial.Binding">Binding Variables</a></span></dt>
- <dt><span class="section"><a href="tutorial.html#boost_localfunction.tutorial.binding_the_object__this_">Binding
- the Object <code class="computeroutput"><span class="keyword">this</span></code></a></span></dt>
- <dt><span class="section"><a href="tutorial.html#boost_localfunction.tutorial.templates">Templates</a></span></dt>
- </dl></div>
- <p>
- This section illustrates basic usage of this library.
- </p>
- <div class="section">
- <div class="titlepage"><div><div><h3 class="title">
- <a name="boost_localfunction.tutorial.local_functions"></a><a class="link" href="tutorial.html#boost_localfunction.tutorial.local_functions" title="Local Functions">Local Functions</a>
- </h3></div></div></div>
- <p>
- Local functions are defined using macros from the header file <code class="computeroutput">boost/local_function.hpp</code>. The
- macros must be used from within a declarative context (this is a limitation
- with respect to <a href="http://en.wikipedia.org/wiki/C%2B%2B0x#Lambda_functions_and_expressions" target="_top">C++11
- lambda functions</a> which can instead be declared also within expressions):
- </p>
- <pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">local_function</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> <span class="comment">// This library header.</span>
- <span class="special">...</span>
- <span class="special">{</span> <span class="comment">// Some declarative context.</span>
- <span class="special">...</span>
- <span class="emphasis"><em>result-type</em></span> <span class="identifier">BOOST_LOCAL_FUNCTION</span><span class="special">(</span><span class="emphasis"><em>parameters</em></span><span class="special">)</span> <span class="special">{</span>
- <span class="emphasis"><em>body-code</em></span>
- <span class="special">}</span> <span class="identifier">BOOST_LOCAL_FUNCTION_NAME</span><span class="special">(</span><span class="emphasis"><em>name</em></span><span class="special">)</span>
- <span class="special">...</span>
- <span class="special">}</span>
- </pre>
- <p>
- The code expanded by the macros declares a function object (or <a href="http://en.wikipedia.org/wiki/Functor" target="_top">functor</a>)
- with the local function name specified by <code class="computeroutput">BOOST_LOCAL_FUNCTION_NAME</code>.
- <a href="#ftn.boost_localfunction.tutorial.local_functions.f0" class="footnote" name="boost_localfunction.tutorial.local_functions.f0"><sup class="footnote">[5]</sup></a> The usual C++ scope visibility rules apply to local functions
- for which a local function is visible only within the enclosing scope in
- which it is declared.
- </p>
- <p>
- The local function result type is specified just before the <code class="computeroutput">BOOST_LOCAL_FUNCTION</code>
- macro.
- </p>
- <p>
- The local function body is specified using the usual C++ statement syntax
- in a code block <code class="computeroutput"><span class="special">{</span> <span class="special">...</span>
- <span class="special">}</span></code> between the <code class="computeroutput">BOOST_LOCAL_FUNCTION</code>
- and <code class="computeroutput">BOOST_LOCAL_FUNCTION_NAME</code>
- macros. The body is specified outside any of the macros so eventual compiler
- error messages and related line numbers retain their usual meaning and format.
- <a href="#ftn.boost_localfunction.tutorial.local_functions.f1" class="footnote" name="boost_localfunction.tutorial.local_functions.f1"><sup class="footnote">[6]</sup></a>
- </p>
- <p>
- The local function parameters are passed to the <code class="computeroutput">BOOST_LOCAL_FUNCTION</code>
- macro as a comma-separated list of tokens (see the <a class="link" href="no_variadic_macros.html" title="Annex: No Variadic Macros">No
- Variadic Macros</a> section for compilers that do not support variadic
- macros):
- </p>
- <pre class="programlisting"><span class="identifier">BOOST_LOCAL_FUNCTION</span><span class="special">(</span><code class="literal"><span class="emphasis"><em>parameter-type1 parameter-name1</em></span></code><span class="special">,</span> <code class="literal"><span class="emphasis"><em>parameter-type2 parameter-name2, ...</em></span></code><span class="special">)</span>
- </pre>
- <p>
- The maximum number of parameters that can be passed to a local function is
- controlled at compile-time by the configuration macro <code class="computeroutput">BOOST_LOCAL_FUNCTION_CONFIG_ARITY_MAX</code>.
- For example, let's program a local function named <code class="computeroutput"><span class="identifier">add</span></code>
- that adds together two integers <code class="computeroutput"><span class="identifier">x</span></code>
- and <code class="computeroutput"><span class="identifier">y</span></code> (see also <a href="../../../test/add_params_only.cpp" target="_top"><code class="literal">add_params_only.cpp</code></a>):
- </p>
- <p>
- </p>
- <pre class="programlisting"><span class="keyword">int</span> <span class="identifier">BOOST_LOCAL_FUNCTION</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">x</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">y</span><span class="special">)</span> <span class="special">{</span> <span class="comment">// Local function.</span>
- <span class="keyword">return</span> <span class="identifier">x</span> <span class="special">+</span> <span class="identifier">y</span><span class="special">;</span>
- <span class="special">}</span> <span class="identifier">BOOST_LOCAL_FUNCTION_NAME</span><span class="special">(</span><span class="identifier">add</span><span class="special">)</span>
- <span class="identifier">BOOST_TEST</span><span class="special">(</span><span class="identifier">add</span><span class="special">(</span><span class="number">1</span><span class="special">,</span> <span class="number">2</span><span class="special">)</span> <span class="special">==</span> <span class="number">3</span><span class="special">);</span> <span class="comment">// Local function call.</span>
- </pre>
- <p>
- </p>
- <p>
- If the local function has no parameter, it is possible to pass <code class="computeroutput"><span class="keyword">void</span></code> to the <code class="computeroutput">BOOST_LOCAL_FUNCTION</code>
- macro (similarly to the C++ syntax that allows to use <code class="literal"><span class="emphasis"><em>result-type
- function-name</em></span></code><code class="computeroutput"><span class="special">(</span><span class="keyword">void</span><span class="special">)</span></code> to declare
- a function with no parameter): <a href="#ftn.boost_localfunction.tutorial.local_functions.f2" class="footnote" name="boost_localfunction.tutorial.local_functions.f2"><sup class="footnote">[7]</sup></a>
- </p>
- <pre class="programlisting"><span class="identifier">BOOST_LOCAL_FUNCTION</span><span class="special">(</span><span class="keyword">void</span><span class="special">)</span> <span class="comment">// No parameter.</span>
- </pre>
- <p>
- For example, let's program a local function that always returns <code class="computeroutput"><span class="number">10</span></code> (see also <a href="../../../test/ten_void.cpp" target="_top"><code class="literal">ten_void.cpp</code></a>):
- </p>
- <p>
- </p>
- <pre class="programlisting"><span class="keyword">int</span> <span class="identifier">BOOST_LOCAL_FUNCTION</span><span class="special">(</span><span class="keyword">void</span><span class="special">)</span> <span class="special">{</span> <span class="comment">// No parameter.</span>
- <span class="keyword">return</span> <span class="number">10</span><span class="special">;</span>
- <span class="special">}</span> <span class="identifier">BOOST_LOCAL_FUNCTION_NAME</span><span class="special">(</span><span class="identifier">ten</span><span class="special">)</span>
- <span class="identifier">BOOST_TEST</span><span class="special">(</span><span class="identifier">ten</span><span class="special">()</span> <span class="special">==</span> <span class="number">10</span><span class="special">);</span>
- </pre>
- <p>
- </p>
- </div>
- <div class="section">
- <div class="titlepage"><div><div><h3 class="title">
- <a name="boost_localfunction.tutorial.Binding"></a><a class="link" href="tutorial.html#boost_localfunction.tutorial.Binding" title="Binding Variables">Binding Variables</a>
- </h3></div></div></div>
- <p>
- Variables in scope (local variables, enclosing function parameters, data
- members, etc) can be bound to a local function declaration. Only bound variables,
- static variables, global variables, functions, and enumerations from the
- enclosing scope are accessible from within the local function body. The types
- of bound variables are deduced automatically by this library using <a href="http://www.boost.org/libs/typeof" target="_top">Boost.Typeof</a>. <a href="#ftn.boost_localfunction.tutorial.Binding.f0" class="footnote" name="boost_localfunction.tutorial.Binding.f0"><sup class="footnote">[8]</sup></a>
- </p>
- <p>
- This library introduces the new "keyword" <code class="computeroutput"><span class="identifier">bind</span></code>
- <a href="#ftn.boost_localfunction.tutorial.Binding.f1" class="footnote" name="boost_localfunction.tutorial.Binding.f1"><sup class="footnote">[9]</sup></a> which is used in place of the parameter type to specify the name
- of a variable in scope to bind (therefore, <code class="computeroutput"><span class="identifier">bind</span></code>
- cannot be used as a local function parameter type). A variable can be bound
- by value:
- </p>
- <pre class="programlisting"><span class="identifier">bind</span> <span class="emphasis"><em>variable-name</em></span> <span class="comment">// Bind by value.</span>
- </pre>
- <p>
- Or by reference prefixing the variable name with <code class="computeroutput"><span class="special">&</span></code>:
- </p>
- <pre class="programlisting"><span class="identifier">bind</span><span class="special">&</span> <span class="emphasis"><em>variable-name</em></span> <span class="comment">// Bind by reference.</span>
- </pre>
- <p>
- Furthermore, the "keyword" <code class="computeroutput"><span class="identifier">bind</span></code>
- can be prefixed by <code class="computeroutput"><span class="keyword">const</span></code> to
- bind the variable by constant value:
- </p>
- <pre class="programlisting"><span class="keyword">const</span> <span class="identifier">bind</span> <span class="emphasis"><em>variable-name</em></span> <span class="comment">// Bind by constant value.</span>
- </pre>
- <p>
- Or by constant reference:
- </p>
- <pre class="programlisting"><span class="keyword">const</span> <span class="identifier">bind</span><span class="special">&</span> <span class="emphasis"><em>variable-name</em></span> <span class="comment">// Bind by constant value.</span>
- </pre>
- <p>
- Note that when <code class="computeroutput"><span class="keyword">const</span></code> is used,
- it must always precede <code class="computeroutput"><span class="identifier">bind</span></code>.
- <a href="#ftn.boost_localfunction.tutorial.Binding.f2" class="footnote" name="boost_localfunction.tutorial.Binding.f2"><sup class="footnote">[10]</sup></a>
- </p>
- <p>
- If a variable is bound by value, then a copy of the variable value is taken
- at the point of the local function declaration. If a variable is bound by
- reference instead, the variable will refer to the value it has at the point
- of the local function call. Furthermore, it is the programmers' responsibility
- to ensure that variables bound by reference survive the existence scope of
- the local function otherwise the bound references will be invalid when the
- local function is called resulting in undefined behaviour (in other words,
- the usual care in using C++ references must be taken for variables bound
- by reference).
- </p>
- <p>
- The type of a bound variable is automatically deduced using <a href="http://www.boost.org/libs/typeof" target="_top">Boost.Typeof</a>
- and it is the exact same type used to declare such a variable in the enclosing
- scope with the following notes:
- </p>
- <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
- <li class="listitem">
- If a bound variable was declared constant in the enclosing scope, it
- will always be bound by constant value or constant reference even if
- <code class="computeroutput"><span class="identifier">bind</span><span class="special">...</span></code>
- is used instead of <code class="computeroutput"><span class="keyword">const</span> <span class="identifier">bind</span><span class="special">...</span></code>
- . However, if a bound variable was not declared constant in the enclosing
- scope then it will not be bound as constant unless constant binding is
- forced using <code class="computeroutput"><span class="keyword">const</span> <span class="identifier">bind</span><span class="special">...</span></code>. (Note that binding by constant reference
- is not supported by <a href="http://en.wikipedia.org/wiki/C%2B%2B0x#Lambda_functions_and_expressions" target="_top">C++11
- lambda functions</a> but it is supported by this library.) <a href="#ftn.boost_localfunction.tutorial.Binding.f3" class="footnote" name="boost_localfunction.tutorial.Binding.f3"><sup class="footnote">[11]</sup></a>
- </li>
- <li class="listitem">
- If a bound variable was declared as a reference in the enclosing scope,
- it will still be bound by value unless it is explicitly bound by reference
- using <code class="computeroutput"><span class="identifier">bind</span><span class="special">&</span></code>
- or <code class="computeroutput"><span class="keyword">const</span> <span class="identifier">bind</span><span class="special">&</span></code>. <a href="#ftn.boost_localfunction.tutorial.Binding.f4" class="footnote" name="boost_localfunction.tutorial.Binding.f4"><sup class="footnote">[12]</sup></a>
- </li>
- </ul></div>
- <p>
- When a variable is bound by value (constant or not), its type must be <a href="http://www.boost.org/doc/libs/release/doc/html/CopyConstructible.html" target="_top"><code class="computeroutput"><span class="identifier">CopyConstructible</span></code></a> (i.e., its must
- provide a copy constructor). As with passing parameters to usual C++ functions,
- programmers might want to bind variables of complex types by (possibly constant)
- reference instead of by value to avoid expensive copy operations when these
- variables are bound to a local function.
- </p>
- <p>
- For example, let's program the local function <code class="computeroutput"><span class="identifier">add</span></code>
- from the example in the <a class="link" href="../index.html#boost_localfunction.introduction" title="Introduction">Introduction</a>
- section. We bind the local variable <code class="computeroutput"><span class="identifier">factor</span></code>
- by constant value (because its value should not be modified by the local
- function), the local variable <code class="computeroutput"><span class="identifier">sum</span></code>
- by non-constant reference (because its value needs to be updated with the
- summation result), and program the body to perform the summation (see also
- <a href="../../../test/add.cpp" target="_top"><code class="literal">add.cpp</code></a>):
- </p>
- <p>
- </p>
- <pre class="programlisting"><span class="keyword">int</span> <span class="identifier">main</span><span class="special">(</span><span class="keyword">void</span><span class="special">)</span> <span class="special">{</span> <span class="comment">// Some local scope.</span>
- <span class="keyword">int</span> <span class="identifier">sum</span> <span class="special">=</span> <span class="number">0</span><span class="special">,</span> <span class="identifier">factor</span> <span class="special">=</span> <span class="number">10</span><span class="special">;</span> <span class="comment">// Variables in scope to bind.</span>
- <span class="keyword">void</span> <span class="identifier">BOOST_LOCAL_FUNCTION</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">bind</span> <span class="identifier">factor</span><span class="special">,</span> <span class="identifier">bind</span><span class="special">&</span> <span class="identifier">sum</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">num</span><span class="special">)</span> <span class="special">{</span>
- <span class="identifier">sum</span> <span class="special">+=</span> <span class="identifier">factor</span> <span class="special">*</span> <span class="identifier">num</span><span class="special">;</span>
- <span class="special">}</span> <span class="identifier">BOOST_LOCAL_FUNCTION_NAME</span><span class="special">(</span><span class="identifier">add</span><span class="special">)</span>
- <span class="identifier">add</span><span class="special">(</span><span class="number">1</span><span class="special">);</span> <span class="comment">// Call the local function.</span>
- <span class="keyword">int</span> <span class="identifier">nums</span><span class="special">[]</span> <span class="special">=</span> <span class="special">{</span><span class="number">2</span><span class="special">,</span> <span class="number">3</span><span class="special">};</span>
- <span class="identifier">std</span><span class="special">::</span><span class="identifier">for_each</span><span class="special">(</span><span class="identifier">nums</span><span class="special">,</span> <span class="identifier">nums</span> <span class="special">+</span> <span class="number">2</span><span class="special">,</span> <span class="identifier">add</span><span class="special">);</span> <span class="comment">// Pass it to an algorithm.</span>
- <span class="identifier">BOOST_TEST</span><span class="special">(</span><span class="identifier">sum</span> <span class="special">==</span> <span class="number">60</span><span class="special">);</span> <span class="comment">// Assert final summation value.</span>
- <span class="keyword">return</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">report_errors</span><span class="special">();</span>
- <span class="special">}</span>
- </pre>
- <p>
- </p>
- </div>
- <div class="section">
- <div class="titlepage"><div><div><h3 class="title">
- <a name="boost_localfunction.tutorial.binding_the_object__this_"></a><a class="link" href="tutorial.html#boost_localfunction.tutorial.binding_the_object__this_" title="Binding the Object this">Binding
- the Object <code class="computeroutput"><span class="keyword">this</span></code></a>
- </h3></div></div></div>
- <p>
- It is also possible to bind the object <code class="computeroutput"><span class="keyword">this</span></code>
- when it is in scope (e.g., from an enclosing non-static member function).
- This is done by using the special symbol <code class="computeroutput"><span class="identifier">this_</span></code>
- (instead of <code class="computeroutput"><span class="keyword">this</span></code>) as the name
- of the variable to bind in the local function declaration and also to access
- the object within the local function body. <a href="#ftn.boost_localfunction.tutorial.binding_the_object__this_.f0" class="footnote" name="boost_localfunction.tutorial.binding_the_object__this_.f0"><sup class="footnote">[13]</sup></a>
- </p>
- <div class="warning"><table border="0" summary="Warning">
- <tr>
- <td rowspan="2" align="center" valign="top" width="25"><img alt="[Warning]" src="../../../../../doc/src/images/warning.png"></td>
- <th align="left">Warning</th>
- </tr>
- <tr><td align="left" valign="top"><p>
- The library will generate a compile-time error if <code class="computeroutput"><span class="keyword">this</span></code>
- is mistakenly used instead of <code class="computeroutput"><span class="identifier">this_</span></code>
- to bind the object in the local function declaration. However, mistakenly
- using <code class="computeroutput"><span class="keyword">this</span></code> instead of <code class="computeroutput"><span class="identifier">this_</span></code> to access the object within the
- local function body will leads to undefined behaviour and it will not necessarily
- generate a compile-time error. <a href="#ftn.boost_localfunction.tutorial.binding_the_object__this_.f1" class="footnote" name="boost_localfunction.tutorial.binding_the_object__this_.f1"><sup class="footnote">[14]</sup></a> Programmers are ultimately responsible to make sure that <code class="computeroutput"><span class="keyword">this</span></code> is never used within a local function.
- </p></td></tr>
- </table></div>
- <p>
- The object <code class="computeroutput"><span class="keyword">this</span></code> can be bound
- by value:
- </p>
- <pre class="programlisting"><span class="identifier">bind</span> <span class="identifier">this_</span> <span class="comment">// Bind the object `this` by value.</span>
- </pre>
- <p>
- In this case the local function will be able to modify the object when the
- enclosing scope is not a constant member and it will not be able to modify
- the object when the enclosing scope is a constant member. Otherwise, the
- object <code class="computeroutput"><span class="keyword">this</span></code> can be bound by
- constant value:
- </p>
- <pre class="programlisting"><span class="keyword">const</span> <span class="identifier">bind</span> <span class="identifier">this_</span> <span class="comment">// Bind the object `this` by constant value.</span>
- </pre>
- <p>
- In this case the local function will never be able to modify the object (regardless
- of whether the enclosing scope is a constant member or not).
- </p>
- <p>
- Note that the object <code class="computeroutput"><span class="keyword">this</span></code> can
- never be bound by reference because C++ does not allow to obtain a reference
- to <code class="computeroutput"><span class="keyword">this</span></code> (the library will generate
- a compile-time error if programmers try to use <code class="computeroutput"><span class="identifier">bind</span><span class="special">&</span> <span class="identifier">this_</span></code>
- or <code class="computeroutput"><span class="keyword">const</span> <span class="identifier">bind</span><span class="special">&</span> <span class="identifier">this_</span></code>).
- Note that <code class="computeroutput"><span class="keyword">this</span></code> is a pointer
- so the pointed object is never copied even if <code class="computeroutput"><span class="keyword">this</span></code>
- is bound by value (also it is not possible to directly bind <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> because
- <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
- is an expression and not a variable name).
- </p>
- <p>
- For example, let's program a local function <code class="computeroutput"><span class="identifier">add</span></code>
- similar to the one in the example from the <a class="link" href="../index.html#boost_localfunction.introduction" title="Introduction">Introduction</a>
- section but using a member function to illustrate how to bind the object
- <code class="computeroutput"><span class="keyword">this</span></code> (see also <a href="../../../test/add_this.cpp" target="_top"><code class="literal">add_this.cpp</code></a>):
- </p>
- <p>
- </p>
- <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">adder</span> <span class="special">{</span>
- <span class="identifier">adder</span><span class="special">()</span> <span class="special">:</span> <span class="identifier">sum_</span><span class="special">(</span><span class="number">0</span><span class="special">)</span> <span class="special">{}</span>
- <span class="keyword">int</span> <span class="identifier">sum</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="keyword">int</span><span class="special">>&</span> <span class="identifier">nums</span><span class="special">,</span> <span class="keyword">const</span> <span class="keyword">int</span> <span class="identifier">factor</span> <span class="special">=</span> <span class="number">10</span><span class="special">)</span> <span class="special">{</span>
- <span class="keyword">void</span> <span class="identifier">BOOST_LOCAL_FUNCTION</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">bind</span> <span class="identifier">factor</span><span class="special">,</span> <span class="identifier">bind</span> <span class="identifier">this_</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">num</span><span class="special">)</span> <span class="special">{</span>
- <span class="identifier">this_</span><span class="special">-></span><span class="identifier">sum_</span> <span class="special">+=</span> <span class="identifier">factor</span> <span class="special">*</span> <span class="identifier">num</span><span class="special">;</span> <span class="comment">// Use `this_` instead of `this`.</span>
- <span class="special">}</span> <span class="identifier">BOOST_LOCAL_FUNCTION_NAME</span><span class="special">(</span><span class="identifier">add</span><span class="special">)</span>
- <span class="identifier">std</span><span class="special">::</span><span class="identifier">for_each</span><span class="special">(</span><span class="identifier">nums</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">nums</span><span class="special">.</span><span class="identifier">end</span><span class="special">(),</span> <span class="identifier">add</span><span class="special">);</span>
- <span class="keyword">return</span> <span class="identifier">sum_</span><span class="special">;</span>
- <span class="special">}</span>
- <span class="keyword">private</span><span class="special">:</span>
- <span class="keyword">int</span> <span class="identifier">sum_</span><span class="special">;</span>
- <span class="special">};</span>
- </pre>
- <p>
- </p>
- <p>
- Note that the local function has access to all class members via the bound
- object <code class="computeroutput"><span class="identifier">this_</span></code> regardless of
- their 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>). <a href="#ftn.boost_localfunction.tutorial.binding_the_object__this_.f2" class="footnote" name="boost_localfunction.tutorial.binding_the_object__this_.f2"><sup class="footnote">[15]</sup></a> Specifically, in the example above the local function updates
- the private data member <code class="computeroutput"><span class="identifier">sum_</span></code>.
- </p>
- </div>
- <div class="section">
- <div class="titlepage"><div><div><h3 class="title">
- <a name="boost_localfunction.tutorial.templates"></a><a class="link" href="tutorial.html#boost_localfunction.tutorial.templates" title="Templates">Templates</a>
- </h3></div></div></div>
- <p>
- When local functions are programmed within templates, they need to be declared
- using the special macros <code class="computeroutput">BOOST_LOCAL_FUNCTION_TPL</code>
- and <code class="computeroutput">BOOST_LOCAL_FUNCTION_NAME_TPL</code>:
- <a href="#ftn.boost_localfunction.tutorial.templates.f0" class="footnote" name="boost_localfunction.tutorial.templates.f0"><sup class="footnote">[16]</sup></a>
- </p>
- <pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">local_function</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> <span class="comment">// This library header.</span>
- <span class="special">...</span>
- <span class="special">{</span> <span class="comment">// Some declarative context within a template.</span>
- <span class="special">...</span>
- <span class="emphasis"><em>result-type</em></span> <span class="identifier">BOOST_LOCAL_FUNCTION_TPL</span><span class="special">(</span><span class="emphasis"><em>parameters</em></span><span class="special">)</span> <span class="special">{</span>
- <span class="emphasis"><em>body-code</em></span>
- <span class="special">}</span> <span class="identifier">BOOST_LOCAL_FUNCTION_NAME_TPL</span><span class="special">(</span><span class="emphasis"><em>name</em></span><span class="special">)</span>
- <span class="special">...</span>
- <span class="special">}</span>
- </pre>
- <p>
- The <code class="computeroutput">BOOST_LOCAL_FUNCTION_TPL</code>
- and <code class="computeroutput">BOOST_LOCAL_FUNCTION_NAME_TPL</code>
- macros have the exact same syntax of the <code class="computeroutput">BOOST_LOCAL_FUNCTION</code>
- and <code class="computeroutput">BOOST_LOCAL_FUNCTION_NAME</code>
- macros that we have seen so far.
- </p>
- <p>
- For example, let's program a local function similar to the one from the
- <a class="link" href="../index.html#boost_localfunction.introduction" title="Introduction">Introduction</a> section
- but within a template (see also <a href="../../../test/add_template.cpp" target="_top"><code class="literal">add_template.cpp</code></a>):
- </p>
- <p>
- </p>
- <pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">></span>
- <span class="identifier">T</span> <span class="identifier">total</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">T</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="keyword">const</span> <span class="identifier">T</span><span class="special">&</span> <span class="identifier">y</span><span class="special">,</span> <span class="keyword">const</span> <span class="identifier">T</span><span class="special">&</span> <span class="identifier">z</span><span class="special">)</span> <span class="special">{</span>
- <span class="identifier">T</span> <span class="identifier">sum</span> <span class="special">=</span> <span class="identifier">T</span><span class="special">(),</span> <span class="identifier">factor</span> <span class="special">=</span> <span class="number">10</span><span class="special">;</span>
- <span class="comment">// Must use the `..._TPL` macros within templates.</span>
- <span class="identifier">T</span> <span class="identifier">BOOST_LOCAL_FUNCTION_TPL</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">bind</span> <span class="identifier">factor</span><span class="special">,</span> <span class="identifier">bind</span><span class="special">&</span> <span class="identifier">sum</span><span class="special">,</span> <span class="identifier">T</span> <span class="identifier">num</span><span class="special">)</span> <span class="special">{</span>
- <span class="keyword">return</span> <span class="identifier">sum</span> <span class="special">+=</span> <span class="identifier">factor</span> <span class="special">*</span> <span class="identifier">num</span><span class="special">;</span>
- <span class="special">}</span> <span class="identifier">BOOST_LOCAL_FUNCTION_NAME_TPL</span><span class="special">(</span><span class="identifier">add</span><span class="special">)</span>
- <span class="identifier">add</span><span class="special">(</span><span class="identifier">x</span><span class="special">);</span>
- <span class="identifier">T</span> <span class="identifier">nums</span><span class="special">[</span><span class="number">2</span><span class="special">];</span> <span class="identifier">nums</span><span class="special">[</span><span class="number">0</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">y</span><span class="special">;</span> <span class="identifier">nums</span><span class="special">[</span><span class="number">1</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">z</span><span class="special">;</span>
- <span class="identifier">std</span><span class="special">::</span><span class="identifier">for_each</span><span class="special">(</span><span class="identifier">nums</span><span class="special">,</span> <span class="identifier">nums</span> <span class="special">+</span> <span class="number">2</span><span class="special">,</span> <span class="identifier">add</span><span class="special">);</span>
- <span class="keyword">return</span> <span class="identifier">sum</span><span class="special">;</span>
- <span class="special">}</span>
- </pre>
- <p>
- </p>
- </div>
- <div class="footnotes">
- <br><hr style="width:100; text-align:left;margin-left: 0">
- <div id="ftn.boost_localfunction.tutorial.local_functions.f0" class="footnote"><p><a href="#boost_localfunction.tutorial.local_functions.f0" class="para"><sup class="para">[5] </sup></a>
- <span class="bold"><strong>Rationale.</strong></span> The local function name must
- be passed to the macro <code class="computeroutput">BOOST_LOCAL_FUNCTION_NAME</code>
- ending the function definition so this macro can declare a local variable
- with the local function name to hold the local function object. Therefore
- the local function name cannot be specified within the <code class="computeroutput">BOOST_LOCAL_FUNCTION</code>
- and it must appear instead after the local function body (even if that
- differs from the usual C++ function declaration syntax).
- </p></div>
- <div id="ftn.boost_localfunction.tutorial.local_functions.f1" class="footnote"><p><a href="#boost_localfunction.tutorial.local_functions.f1" class="para"><sup class="para">[6] </sup></a>
- <span class="bold"><strong>Rationale.</strong></span> If the local function body
- were instead passed as a macro parameter, it would be expanded on a single
- line of code (because macros always expand as a single line of code). Therefore,
- eventual compiler error line numbers would all report the same value and
- would no longer be useful to pinpoint errors.
- </p></div>
- <div id="ftn.boost_localfunction.tutorial.local_functions.f2" class="footnote"><p><a href="#boost_localfunction.tutorial.local_functions.f2" class="para"><sup class="para">[7] </sup></a>
- <span class="bold"><strong>Rationale.</strong></span> The <a href="http://www.open-std.org/JTC1/SC22/WG21/docs/standards" target="_top">C++03</a>
- standard does not allow to pass empty parameters to a macro so the macro
- cannot be invoked as <code class="computeroutput"><span class="identifier">BOOST_LOCAL_FUNCTION</span><span class="special">()</span></code>. On <a href="http://www.open-std.org/jtc1/sc22/wg14/www/projects#9899" target="_top">C99</a>
- compilers with properly implemented empty macro parameter support, it would
- be possible to allow <code class="computeroutput"><span class="identifier">BOOST_LOCAL_FUNCTION</span><span class="special">()</span></code> but this is already not the case for
- MSVC so this syntax is never allowed to ensure better portability.
- </p></div>
- <div id="ftn.boost_localfunction.tutorial.Binding.f0" class="footnote"><p><a href="#boost_localfunction.tutorial.Binding.f0" class="para"><sup class="para">[8] </sup></a>
- <span class="bold"><strong>Rationale.</strong></span> By binding a variable in scope,
- the local function declaration is specifying that such a variable should
- be accessible within the local function body regardless of its type. Semantically,
- this binding should be seen as an "extension" of the scope of
- the bound variable from the enclosing scope to the scope of the local function
- body. Therefore, contrary to the semantic of passing a function parameter,
- the semantic of binding a variable does not depend on the variable type
- but just on the variable name: "The variable in scope named <span class="emphasis"><em>x</em></span>
- should be accessible within the local function named <span class="emphasis"><em>f</em></span>".
- For example, this reduces maintenance because if a bound variable type
- is changed, the local function declaration does not have to change.
- </p></div>
- <div id="ftn.boost_localfunction.tutorial.Binding.f1" class="footnote"><p><a href="#boost_localfunction.tutorial.Binding.f1" class="para"><sup class="para">[9] </sup></a>
- Obviously, the token <code class="computeroutput"><span class="identifier">bind</span></code>
- is not a keyword of the C++ language. This library parses the token <code class="computeroutput"><span class="identifier">bind</span></code> during macro expansion using preprocessor
- meta-programming (see the <a class="link" href="implementation.html" title="Annex: Implementation">Implementation</a>
- section). Therefore, <code class="computeroutput"><span class="identifier">bind</span></code>
- can be considered a new "keyword" only at the preprocessor meta-programming
- level within the syntax defined by the macros of this library (thus it
- is referred to as a "keyword" only within quotes).
- </p></div>
- <div id="ftn.boost_localfunction.tutorial.Binding.f2" class="footnote"><p><a href="#boost_localfunction.tutorial.Binding.f2" class="para"><sup class="para">[10] </sup></a>
- <span class="bold"><strong>Rationale.</strong></span> The library macros could have
- been implemented to accept both syntaxes <code class="computeroutput"><span class="keyword">const</span>
- <span class="identifier">bind</span> <span class="special">...</span></code>
- and <code class="computeroutput"><span class="identifier">bind</span> <span class="keyword">const</span>
- <span class="special">...</span></code> equivalently. However, handling
- both syntaxes would have complicated the macro implementation without adding
- any feature so only one syntax <code class="computeroutput"><span class="keyword">const</span>
- <span class="identifier">bind</span> <span class="special">...</span></code>
- is supported.
- </p></div>
- <div id="ftn.boost_localfunction.tutorial.Binding.f3" class="footnote"><p><a href="#boost_localfunction.tutorial.Binding.f3" class="para"><sup class="para">[11] </sup></a>
- An historical note: Constant binding of variables in scope was the
- main use case that originally motivated the authors in developing this
- library. The authors needed to locally create a chuck of code to assert
- some correctness conditions while these assertions were not supposed
- to modify any of the variables they were using (see the <a href="http://sourceforge.net/projects/contractpp" target="_top">Contract++</a>
- library). This was achieved by binding by constant reference <code class="computeroutput"><span class="keyword">const</span> <span class="identifier">bind</span><span class="special">&</span></code> the variables needed by the assertions
- and then by programming the local function body to check the assertions.
- This way if any of the assertions mistakenly changes a bound variable
- (for example confusing the operator <code class="computeroutput"><span class="special">==</span></code>
- with <code class="computeroutput"><span class="special">=</span></code>), the compiler
- correctly generates an error because the bound variable is of <code class="computeroutput"><span class="keyword">const</span></code> type within the local function
- body (see also <span class="emphasis"><em>constant blocks</em></span> in the <a class="link" href="examples.html" title="Examples">Examples</a>
- section).
- </p></div>
- <div id="ftn.boost_localfunction.tutorial.Binding.f4" class="footnote"><p><a href="#boost_localfunction.tutorial.Binding.f4" class="para"><sup class="para">[12] </sup></a>
- <span class="bold"><strong>Rationale.</strong></span> Variables originally declared
- as references are bound by value unless <code class="computeroutput"><span class="special">[</span><span class="keyword">const</span><span class="special">]</span> <span class="identifier">bind</span><span class="special">&</span></code>
- is used so that references can be bound by both value <code class="computeroutput"><span class="special">[</span><span class="keyword">const</span><span class="special">]</span> <span class="identifier">bind</span></code>
- and reference <code class="computeroutput"><span class="special">[</span><span class="keyword">const</span><span class="special">]</span> <span class="identifier">bind</span><span class="special">&</span></code> (this is the same binding semantic
- adopted by <a href="http://www.boost.org/libs/scope_exit" target="_top">Boost.ScopeExit</a>).
- However, variables originally declared as constants should never loose
- their <code class="computeroutput"><span class="keyword">const</span></code> qualifier
- (to prevent their modification not just in the enclosing scope but
- also in the local scope) thus they are always bound by constant even
- if <code class="computeroutput"><span class="identifier">bind</span><span class="special">[&]</span></code>
- is used instead of <code class="computeroutput"><span class="keyword">const</span> <span class="identifier">bind</span><span class="special">[&]</span></code>.
- </p></div>
- <div id="ftn.boost_localfunction.tutorial.binding_the_object__this_.f0" class="footnote"><p><a href="#boost_localfunction.tutorial.binding_the_object__this_.f0" class="para"><sup class="para">[13] </sup></a>
- <span class="bold"><strong>Rationale.</strong></span> The special name <code class="computeroutput"><span class="identifier">this_</span></code> was chosen following <a href="http://lists.boost.org/Archives/boost/2011/04/179729.php" target="_top">Boost
- practise</a> to postfix with an underscore identifiers that are named
- after keywords (the C++ keyword <code class="computeroutput"><span class="keyword">this</span></code>
- in this case). The special symbol <code class="computeroutput"><span class="identifier">this_</span></code>
- is needed because <code class="computeroutput"><span class="keyword">this</span></code> is
- a reserved C++ keyword so it cannot be used as the name of the internal
- parameter that passes the bound object to the local function body. It would
- have been possible to use <code class="computeroutput"><span class="keyword">this</span></code>
- (instead of <code class="computeroutput"><span class="identifier">this_</span></code>) within
- the local function body either at the expenses of copying the bound object
- (which would introduce run-time overhead and also the stringent requirement
- that the bound object must have a deep copy constructor) or by relying
- on an <a href="http://groups.google.com/group/comp.lang.c++.moderated/browse_thread/thread/d3a86f27277f713b" target="_top">undefined
- behaviour of <code class="computeroutput"><span class="keyword">static_cast</span></code></a>
- (which might not work on all platforms at the cost of portability).
- </p></div>
- <div id="ftn.boost_localfunction.tutorial.binding_the_object__this_.f1" class="footnote"><p><a href="#boost_localfunction.tutorial.binding_the_object__this_.f1" class="para"><sup class="para">[14] </sup></a>
- <span class="bold"><strong>Rationale.</strong></span> The local function body cannot
- be a static member function of the local functor object in order to support
- recursion (because the local function name is specified by the <code class="computeroutput">BOOST_LOCAL_FUNCTION_NAME</code>
- macro only after the body so it must be made available via a functor
- data member named after the local function and local classes cannot have
- static data members in C++) and nesting (because the argument binding
- variable must be declared as a data member so it is visible in a local
- function nested within the body member function) -- see the <a class="link" href="implementation.html" title="Annex: Implementation">Implementation</a>
- section. Therefore, from within the local function body the variable
- <code class="computeroutput"><span class="keyword">this</span></code> is visible but it refers
- to the local functor and not to the bound object.
- </p></div>
- <div id="ftn.boost_localfunction.tutorial.binding_the_object__this_.f2" class="footnote"><p><a href="#boost_localfunction.tutorial.binding_the_object__this_.f2" class="para"><sup class="para">[15] </sup></a>
- <span class="bold"><strong>Rationale.</strong></span> This is possible because of
- the fix to C++ <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#45" target="_top">defect
- 45</a> that made inner and local types able to access all outer class
- members regardless of their access level.
- </p></div>
- <div id="ftn.boost_localfunction.tutorial.templates.f0" class="footnote"><p><a href="#boost_localfunction.tutorial.templates.f0" class="para"><sup class="para">[16] </sup></a>
- <span class="bold"><strong>Rationale.</strong></span> Within templates, this library
- needs to use <code class="computeroutput"><span class="keyword">typename</span></code> to explicitly
- indicate that some expressions evaluate to a type. Because <a href="http://www.open-std.org/JTC1/SC22/WG21/docs/standards" target="_top">C++03</a>
- does not allow to use <code class="computeroutput"><span class="keyword">typename</span></code>
- outside templates, the special <code class="computeroutput"><span class="special">...</span><span class="identifier">_TPL</span></code> macros are used to indicate that
- the enclosing scope is a template so this library can safely use <code class="computeroutput"><span class="keyword">typename</span></code> to resolve expression type ambiguities.
- <a href="http://www.open-std.org/JTC1/SC22/WG21/" target="_top">C++11</a> and
- other compilers might compile local functions within templates even when
- the <code class="computeroutput"><span class="special">...</span><span class="identifier">_TPL</span></code>
- macros are not used. However, it is recommended to always use the <code class="computeroutput"><span class="special">...</span><span class="identifier">_TPL</span></code>
- macros within templates to maximize portability.
- </p></div>
- </div>
- </div>
- <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
- <td align="left"></td>
- <td align="right"><div class="copyright-footer">Copyright © 2009-2012 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>
- </div></td>
- </tr></table>
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