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- </div>
- <div class="section">
- <div class="titlepage"><div><div><h2 class="title" style="clear: both">
- <a name="boost_localfunction.advanced_topics"></a><a class="link" href="advanced_topics.html" title="Advanced Topics">Advanced Topics</a>
- </h2></div></div></div>
- <div class="toc"><dl class="toc">
- <dt><span class="section"><a href="advanced_topics.html#boost_localfunction.advanced_topics.default_parameters">Default
- Parameters</a></span></dt>
- <dt><span class="section"><a href="advanced_topics.html#boost_localfunction.advanced_topics.commas_and_symbols_in_macros">Commas
- and Symbols in Macros</a></span></dt>
- <dt><span class="section"><a href="advanced_topics.html#boost_localfunction.advanced_topics.assignments_and_returns">Assignments
- and Returns</a></span></dt>
- <dt><span class="section"><a href="advanced_topics.html#boost_localfunction.advanced_topics.nesting">Nesting</a></span></dt>
- <dt><span class="section"><a href="advanced_topics.html#boost_localfunction.advanced_topics.accessing_types__concepts__etc_">Accessing
- Types (concepts, etc)</a></span></dt>
- <dt><span class="section"><a href="advanced_topics.html#boost_localfunction.advanced_topics.specifying_types__no_boost_typeof_">Specifying
- Types (no Boost.Typeof)</a></span></dt>
- <dt><span class="section"><a href="advanced_topics.html#boost_localfunction.advanced_topics.inlining">Inlining</a></span></dt>
- <dt><span class="section"><a href="advanced_topics.html#boost_localfunction.advanced_topics.recursion">Recursion</a></span></dt>
- <dt><span class="section"><a href="advanced_topics.html#boost_localfunction.advanced_topics.overloading">Overloading</a></span></dt>
- <dt><span class="section"><a href="advanced_topics.html#boost_localfunction.advanced_topics.exception_specifications">Exception
- Specifications</a></span></dt>
- <dt><span class="section"><a href="advanced_topics.html#boost_localfunction.advanced_topics.storage_classifiers">Storage
- Classifiers</a></span></dt>
- <dt><span class="section"><a href="advanced_topics.html#boost_localfunction.advanced_topics.same_line_expansions">Same
- Line Expansions</a></span></dt>
- <dt><span class="section"><a href="advanced_topics.html#boost_localfunction.advanced_topics.limitations__operators__etc_">Limitations
- (operators, etc)</a></span></dt>
- </dl></div>
- <p>
- This section illustrates advanced usage of this library. At the bottom there
- is also a list of known limitations of this library.
- </p>
- <div class="section">
- <div class="titlepage"><div><div><h3 class="title">
- <a name="boost_localfunction.advanced_topics.default_parameters"></a><a class="link" href="advanced_topics.html#boost_localfunction.advanced_topics.default_parameters" title="Default Parameters">Default
- Parameters</a>
- </h3></div></div></div>
- <p>
- This library allows to specify default values for the local function parameters.
- However, the usual C++ syntax for default parameters that uses the assignment
- symbol <code class="computeroutput"><span class="special">=</span></code> cannot be used. <a href="#ftn.boost_localfunction.advanced_topics.default_parameters.f0" class="footnote" name="boost_localfunction.advanced_topics.default_parameters.f0"><sup class="footnote">[17]</sup></a> The keyword <code class="computeroutput"><span class="keyword">default</span></code>
- is used instead:
- </p>
- <pre class="programlisting"><code class="literal"><span class="emphasis"><em>parameter-type parameter-name</em></span></code><span class="special">,</span> <span class="keyword">default</span> <code class="literal"><span class="emphasis"><em>parameter-default-value</em></span></code><span class="special">,</span> <span class="special">...</span>
- </pre>
- <p>
- For example, let's program a local function <code class="computeroutput"><span class="identifier">add</span><span class="special">(</span><span class="identifier">x</span><span class="special">,</span>
- <span class="identifier">y</span><span class="special">)</span></code>
- where the second parameter <code class="computeroutput"><span class="identifier">y</span></code>
- is optional and has a default value of <code class="computeroutput"><span class="number">2</span></code>
- (see also <a href="../../../test/add_default.cpp" target="_top"><code class="literal">add_default.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="keyword">default</span> <span class="number">2</span><span class="special">)</span> <span class="special">{</span> <span class="comment">// Default parameter.</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="special">==</span> <span class="number">3</span><span class="special">);</span>
- </pre>
- <p>
- </p>
- <p>
- Programmers can define a <code class="computeroutput"><span class="identifier">WITH_DEFAULT</span></code>
- macro similar to the following if they think it improves readability over
- the above syntax (see also <a href="../../../test/add_with_default.cpp" target="_top"><code class="literal">add_with_default.cpp</code></a>):
- <a href="#ftn.boost_localfunction.advanced_topics.default_parameters.f1" class="footnote" name="boost_localfunction.advanced_topics.default_parameters.f1"><sup class="footnote">[18]</sup></a>
- </p>
- <p>
- </p>
- <pre class="programlisting"><span class="preprocessor">#define</span> <span class="identifier">WITH_DEFAULT</span> <span class="special">,</span> <span class="keyword">default</span>
- </pre>
- <p>
- </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="identifier">WITH_DEFAULT</span> <span class="number">2</span><span class="special">)</span> <span class="special">{</span> <span class="comment">// Default.</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="special">==</span> <span class="number">3</span><span class="special">);</span>
- </pre>
- <p>
- </p>
- </div>
- <div class="section">
- <div class="titlepage"><div><div><h3 class="title">
- <a name="boost_localfunction.advanced_topics.commas_and_symbols_in_macros"></a><a class="link" href="advanced_topics.html#boost_localfunction.advanced_topics.commas_and_symbols_in_macros" title="Commas and Symbols in Macros">Commas
- and Symbols in Macros</a>
- </h3></div></div></div>
- <p>
- The C++ preprocessor does not allow commas <code class="computeroutput"><span class="special">,</span></code>
- within macro parameters unless they are wrapped by round parenthesis <code class="computeroutput"><span class="special">()</span></code> (see the <a href="http://www.boost.org/libs/utility/identity_type" target="_top">Boost.Utility/IdentityType</a>
- documentation for details). Therefore, using commas within local function
- parameters and bindings will generate (cryptic) preprocessor errors unless
- they are wrapped with an extra set of round parenthesis <code class="computeroutput"><span class="special">()</span></code>
- as explained here.
- </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>
- Also macro parameters with commas wrapped by angular parenthesis <code class="computeroutput"><span class="special"><></span></code> (templates, etc) or square parenthesis
- <code class="computeroutput"><span class="special">[]</span></code> (multidimensional array
- access, etc) need to be wrapped by the extra round parenthesis <code class="computeroutput"><span class="special">()</span></code> as explained here (this is because the
- preprocessor only recognizes the round parenthesis and it does not recognize
- angular, square, or any other type of parenthesis). However, macro parameters
- with commas which are already wrapped by round parenthesis <code class="computeroutput"><span class="special">()</span></code> are fine (function calls, some value
- expressions, etc).
- </p></td></tr>
- </table></div>
- <p>
- In addition, local function parameter types cannot start with non-alphanumeric
- symbols (alphanumeric symbols are <code class="computeroutput"><span class="identifier">A</span><span class="special">-</span><span class="identifier">Z</span></code>, <code class="computeroutput"><span class="identifier">a</span><span class="special">-</span><span class="identifier">z</span></code>,
- and <code class="computeroutput"><span class="number">0</span><span class="special">-</span><span class="number">9</span></code>). <a href="#ftn.boost_localfunction.advanced_topics.commas_and_symbols_in_macros.f0" class="footnote" name="boost_localfunction.advanced_topics.commas_and_symbols_in_macros.f0"><sup class="footnote">[19]</sup></a> The library will generate (cryptic) preprocessor errors if a
- parameter type starts with a non-alphanumeric symbol.
- </p>
- <p>
- Let's consider the following example:
- </p>
- <pre class="programlisting"><span class="keyword">void</span> <span class="identifier">BOOST_LOCAL_FUNCTION</span><span class="special">(</span>
- <span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">map</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">,</span> <span class="identifier">size_t</span><span class="special">>&</span> <span class="identifier">m</span><span class="special">,</span> <span class="comment">// (1) Error.</span>
- <span class="special">::</span><span class="identifier">sign_t</span> <span class="identifier">sign</span><span class="special">,</span> <span class="comment">// (2) Error.</span>
- <span class="keyword">const</span> <span class="identifier">size_t</span><span class="special">&</span> <span class="identifier">factor</span><span class="special">,</span>
- <span class="keyword">default</span> <span class="identifier">key_sizeof</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">,</span> <span class="identifier">size_t</span><span class="special">>::</span><span class="identifier">value</span><span class="special">,</span> <span class="comment">// (3) Error.</span>
- <span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">&</span> <span class="identifier">separator</span><span class="special">,</span> <span class="keyword">default</span> <span class="identifier">cat</span><span class="special">(</span><span class="string">":"</span><span class="special">,</span> <span class="string">" "</span><span class="special">)</span> <span class="comment">// (4) OK.</span>
- <span class="special">)</span> <span class="special">{</span>
- <span class="special">...</span>
- <span class="special">}</span> <span class="identifier">BOOST_LOCAL_FUNCTION_NAME</span><span class="special">(</span><span class="identifier">f</span><span class="special">)</span>
- </pre>
- <p>
- <span class="bold"><strong>(1)</strong></span> The parameter type <code class="computeroutput"><span class="keyword">const</span>
- <span class="identifier">std</span><span class="special">::</span><span class="identifier">map</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">,</span> <span class="identifier">size_t</span><span class="special">>&</span></code> contains a comma <code class="computeroutput"><span class="special">,</span></code>
- after the first template parameter <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span></code>.
- This comma is not wrapped by any round parenthesis <code class="computeroutput"><span class="special">()</span></code>
- thus it will cause a preprocessor error. <a href="#ftn.boost_localfunction.advanced_topics.commas_and_symbols_in_macros.f1" class="footnote" name="boost_localfunction.advanced_topics.commas_and_symbols_in_macros.f1"><sup class="footnote">[20]</sup></a> The <a href="http://www.boost.org/libs/utility/identity_type" target="_top">Boost.Utility/IdentityType</a>
- macro <code class="computeroutput"><span class="identifier">BOOST_IDENTITY_TYPE</span><span class="special">((</span></code><code class="literal"><span class="emphasis"><em>type-with-commas</em></span></code><code class="computeroutput"><span class="special">))</span></code> defined in the <code class="literal">boost/utility/identity_type.hpp</code>
- header can be used to wrap a type within extra parenthesis <code class="computeroutput"><span class="special">()</span></code> so to overcome this problem:
- </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">utility</span><span class="special">/</span><span class="identifier">identity_type</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
- <span class="keyword">void</span> <span class="identifier">BOOST_LOCAL_FUNCTION</span><span class="special">(</span>
- <span class="identifier">BOOST_IDENTITY_TYPE</span><span class="special">((</span><span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">map</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">,</span> <span class="identifier">size_t</span><span class="special">>&))</span> <span class="identifier">m</span><span class="special">,</span> <span class="comment">// OK.</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_LOCAL_FUNCTION_NAME</span><span class="special">(</span><span class="identifier">f</span><span class="special">)</span>
- </pre>
- <p>
- This macro expands to an expression that evaluates (at compile-time) exactly
- to the specified type (furthermore, this macro does not use variadic macros
- so it works on any <a href="http://www.open-std.org/JTC1/SC22/WG21/docs/standards" target="_top">C++03</a>
- compiler). Note that a total of two set of parenthesis <code class="computeroutput"><span class="special">()</span></code>
- are needed: The parenthesis to invoke the <code class="computeroutput"><span class="identifier">BOOST_IDENTITY_TYPE</span><span class="special">(...)</span></code> macro plus the parenthesis to wrap the
- type expression (and therefore any comma <code class="computeroutput"><span class="special">,</span></code>
- that it contains) passed as parameter to the <code class="computeroutput"><span class="identifier">BOOST_IDENTITY_TYPE</span><span class="special">((...))</span></code> macro. Finally, the <code class="computeroutput"><span class="identifier">BOOST_IDENTITY_TYPE</span></code> macro must be prefixed
- by the <code class="computeroutput"><span class="keyword">typename</span></code> keyword <code class="computeroutput"><span class="keyword">typename</span> <span class="identifier">BOOST_IDENTITY_TYPE</span><span class="special">(</span></code><code class="literal"><span class="emphasis"><em>parenthesized-type</em></span></code><code class="computeroutput"><span class="special">)</span></code> when used together with the <code class="computeroutput">BOOST_LOCAL_FUNCTION_TPL</code>
- macro within templates.
- </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>
- Often, there might be better ways to overcome this limitation that lead
- to code which is more readable than the one using the <code class="computeroutput"><span class="identifier">BOOST_IDENTITY_TYPE</span></code>
- macro.
- </p></td></tr>
- </table></div>
- <p>
- For example, in this case a <code class="computeroutput"><span class="keyword">typedef</span></code>
- from the enclosing scope could have been used to obtain the following valid
- and perhaps more readable code:
- </p>
- <pre class="programlisting"><span class="keyword">typedef</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">map</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">,</span> <span class="identifier">size_t</span><span class="special">></span> <span class="identifier">map_type</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">map_type</span><span class="special">&</span> <span class="identifier">m</span><span class="special">,</span> <span class="comment">// OK (and more readable).</span>
- <span class="special">...</span>
- <span class="special">)</span> <span class="identifier">BOOST_LOCAL_FUNCTION_NAME</span><span class="special">(</span><span class="identifier">f</span><span class="special">)</span>
- </pre>
- <p>
- <span class="bold"><strong>(2)</strong></span> The parameter type <code class="computeroutput"><span class="special">::</span><span class="identifier">sign_t</span></code> starts with the non-alphanumeric
- symbols <code class="computeroutput"><span class="special">::</span></code> thus it will generate
- preprocessor errors if used as a local function parameter type. The <code class="computeroutput"><span class="identifier">BOOST_IDENTITY_TYPE</span></code> macro can also be used
- to overcome this issue:
- </p>
- <pre class="programlisting"><span class="keyword">void</span> <span class="identifier">BOOST_LOCAL_FUNCTION</span><span class="special">(</span>
- <span class="special">...</span>
- <span class="identifier">BOOST_IDENTITY_TYPE</span><span class="special">((::</span><span class="identifier">sign_t</span><span class="special">))</span> <span class="identifier">sign</span><span class="special">,</span> <span class="comment">// OK.</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_LOCAL_FUNCTION_NAME</span><span class="special">(</span><span class="identifier">f</span><span class="special">)</span>
- </pre>
- <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>
- Often, there might be better ways to overcome this limitation that lead
- to code which is more readable than the one using the <code class="computeroutput"><span class="identifier">BOOST_IDENTITY_TYPE</span></code>
- macro.
- </p></td></tr>
- </table></div>
- <p>
- For example, in this case the symbols <code class="computeroutput"><span class="special">::</span></code>
- could have been simply dropped to obtain the following valid and perhaps
- more readable code:
- </p>
- <pre class="programlisting"><span class="keyword">void</span> <span class="identifier">BOOST_LOCAL_FUNCTION</span><span class="special">(</span>
- <span class="special">...</span>
- <span class="identifier">sign_t</span> <span class="identifier">sign</span><span class="special">,</span> <span class="comment">// OK (and more readable).</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_LOCAL_FUNCTION_NAME</span><span class="special">(</span><span class="identifier">f</span><span class="special">)</span>
- </pre>
- <p>
- <span class="bold"><strong>(3)</strong></span> The default parameter value <code class="computeroutput"><span class="identifier">key_sizeof</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">,</span> <span class="identifier">size_t</span><span class="special">>::</span><span class="identifier">value</span></code>
- contains a comma <code class="computeroutput"><span class="special">,</span></code> after the
- first template parameter <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span></code>.
- Again, this comma is not wrapped by any parenthesis <code class="computeroutput"><span class="special">()</span></code>
- so it will cause a preprocessor error. Because this is a value expression
- (and not a type expression), it can simply be wrapped within an extra set
- of round parenthesis <code class="computeroutput"><span class="special">()</span></code>:
- </p>
- <pre class="programlisting"><span class="keyword">void</span> <span class="identifier">BOOST_LOCAL_FUNCTION</span><span class="special">(</span>
- <span class="special">...</span>
- <span class="keyword">const</span> <span class="identifier">size_t</span><span class="special">&</span> <span class="identifier">factor</span><span class="special">,</span>
- <span class="keyword">default</span> <span class="special">(</span><span class="identifier">key_sizeof</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">,</span> <span class="identifier">size_t</span><span class="special">>::</span><span class="identifier">value</span><span class="special">),</span> <span class="comment">// OK.</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_LOCAL_FUNCTION_NAME</span><span class="special">(</span><span class="identifier">f</span><span class="special">)</span>
- </pre>
- <p>
- <span class="bold"><strong>(4)</strong></span> The default parameter value <code class="computeroutput"><span class="identifier">cat</span><span class="special">(</span><span class="string">":"</span><span class="special">,</span> <span class="string">" "</span><span class="special">)</span></code> is instead fine because it contains a comma
- <code class="computeroutput"><span class="special">,</span></code> which is already wrapped by
- the parenthesis <code class="computeroutput"><span class="special">()</span></code> of the function
- call <code class="computeroutput"><span class="identifier">cat</span><span class="special">(...)</span></code>.
- </p>
- <p>
- Consider the following complete example (see also <a href="../../../test/macro_commas.cpp" target="_top"><code class="literal">macro_commas.cpp</code></a>):
- </p>
- <p>
- </p>
- <pre class="programlisting"><span class="keyword">void</span> <span class="identifier">BOOST_LOCAL_FUNCTION</span><span class="special">(</span>
- <span class="identifier">BOOST_IDENTITY_TYPE</span><span class="special">((</span><span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">map</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">,</span> <span class="identifier">size_t</span><span class="special">>&))</span> <span class="identifier">m</span><span class="special">,</span>
- <span class="identifier">BOOST_IDENTITY_TYPE</span><span class="special">((::</span><span class="identifier">sign_t</span><span class="special">))</span> <span class="identifier">sign</span><span class="special">,</span>
- <span class="keyword">const</span> <span class="identifier">size_t</span><span class="special">&</span> <span class="identifier">factor</span><span class="special">,</span>
- <span class="keyword">default</span> <span class="special">(</span><span class="identifier">key_sizeof</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">,</span> <span class="identifier">size_t</span><span class="special">>::</span><span class="identifier">value</span><span class="special">),</span>
- <span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">&</span> <span class="identifier">separator</span><span class="special">,</span> <span class="keyword">default</span> <span class="identifier">cat</span><span class="special">(</span><span class="string">":"</span><span class="special">,</span> <span class="string">" "</span><span class="special">)</span>
- <span class="special">)</span> <span class="special">{</span>
- <span class="comment">// Do something...</span>
- <span class="special">}</span> <span class="identifier">BOOST_LOCAL_FUNCTION_NAME</span><span class="special">(</span><span class="identifier">f</span><span class="special">)</span>
- </pre>
- <p>
- </p>
- </div>
- <div class="section">
- <div class="titlepage"><div><div><h3 class="title">
- <a name="boost_localfunction.advanced_topics.assignments_and_returns"></a><a class="link" href="advanced_topics.html#boost_localfunction.advanced_topics.assignments_and_returns" title="Assignments and Returns">Assignments
- and Returns</a>
- </h3></div></div></div>
- <p>
- Local functions are function objects so it is possible to assign them to
- other functors like <a href="http://www.boost.org/libs/function" target="_top">Boost.Function</a>'s
- <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">function</span></code> in order to store the local function
- into a variable, pass it as a parameter to another function, or return it
- from the enclosing function.
- </p>
- <p>
- For example (see also <a href="../../../test/return_assign.cpp" target="_top"><code class="literal">return_assign.cpp</code></a>):
- </p>
- <p>
- </p>
- <pre class="programlisting"><span class="keyword">void</span> <span class="identifier">call1</span><span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">function</span><span class="special"><</span><span class="keyword">int</span> <span class="special">(</span><span class="keyword">int</span><span class="special">)</span> <span class="special">></span> <span class="identifier">f</span><span class="special">)</span> <span class="special">{</span> <span class="identifier">BOOST_TEST</span><span class="special">(</span><span class="identifier">f</span><span class="special">(</span><span class="number">1</span><span class="special">)</span> <span class="special">==</span> <span class="number">5</span><span class="special">);</span> <span class="special">}</span>
- <span class="keyword">void</span> <span class="identifier">call0</span><span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">function</span><span class="special"><</span><span class="keyword">int</span> <span class="special">(</span><span class="keyword">void</span><span class="special">)></span> <span class="identifier">f</span><span class="special">)</span> <span class="special">{</span> <span class="identifier">BOOST_TEST</span><span class="special">(</span><span class="identifier">f</span><span class="special">()</span> <span class="special">==</span> <span class="number">5</span><span class="special">);</span> <span class="special">}</span>
- <span class="identifier">boost</span><span class="special">::</span><span class="identifier">function</span><span class="special"><</span><span class="keyword">int</span> <span class="special">(</span><span class="keyword">int</span><span class="special">,</span> <span class="keyword">int</span><span class="special">)></span> <span class="identifier">linear</span><span class="special">(</span><span class="keyword">const</span> <span class="keyword">int</span><span class="special">&</span> <span class="identifier">slope</span><span class="special">)</span> <span class="special">{</span>
- <span class="keyword">int</span> <span class="identifier">BOOST_LOCAL_FUNCTION</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">bind</span><span class="special">&</span> <span class="identifier">slope</span><span class="special">,</span>
- <span class="keyword">int</span> <span class="identifier">x</span><span class="special">,</span> <span class="keyword">default</span> <span class="number">1</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">y</span><span class="special">,</span> <span class="keyword">default</span> <span class="number">2</span><span class="special">)</span> <span class="special">{</span>
- <span class="keyword">return</span> <span class="identifier">x</span> <span class="special">+</span> <span class="identifier">slope</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">lin</span><span class="special">)</span>
- <span class="identifier">boost</span><span class="special">::</span><span class="identifier">function</span><span class="special"><</span><span class="keyword">int</span> <span class="special">(</span><span class="keyword">int</span><span class="special">,</span> <span class="keyword">int</span><span class="special">)></span> <span class="identifier">f</span> <span class="special">=</span> <span class="identifier">lin</span><span class="special">;</span> <span class="comment">// Assign to local variable.</span>
- <span class="identifier">BOOST_TEST</span><span class="special">(</span><span class="identifier">f</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">5</span><span class="special">);</span>
- <span class="identifier">call1</span><span class="special">(</span><span class="identifier">lin</span><span class="special">);</span> <span class="comment">// Pass to other functions.</span>
- <span class="identifier">call0</span><span class="special">(</span><span class="identifier">lin</span><span class="special">);</span>
- <span class="keyword">return</span> <span class="identifier">lin</span><span class="special">;</span> <span class="comment">// Return.</span>
- <span class="special">}</span>
- <span class="keyword">void</span> <span class="identifier">call</span><span class="special">(</span><span class="keyword">void</span><span class="special">)</span> <span class="special">{</span>
- <span class="identifier">boost</span><span class="special">::</span><span class="identifier">function</span><span class="special"><</span><span class="keyword">int</span> <span class="special">(</span><span class="keyword">int</span><span class="special">,</span> <span class="keyword">int</span><span class="special">)></span> <span class="identifier">f</span> <span class="special">=</span> <span class="identifier">linear</span><span class="special">(</span><span class="number">2</span><span class="special">);</span>
- <span class="identifier">BOOST_TEST</span><span class="special">(</span><span class="identifier">f</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">5</span><span class="special">);</span>
- <span class="special">}</span>
- </pre>
- <p>
- </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>
- As with <a href="http://en.wikipedia.org/wiki/C%2B%2B0x#Lambda_functions_and_expressions" target="_top">C++11
- lambda functions</a>, programmers are responsible to ensure that bound
- variables are valid in any scope where the local function object is called.
- Returning and calling a local function outside its declaration scope will
- lead to undefined behaviour if any of the bound variable is no longer valid
- in the scope where the local function is called (see the <a class="link" href="examples.html" title="Examples">Examples</a>
- section for more examples on the extra care needed when returning a local
- function as a closure). It is always safe instead to call a local function
- within its enclosing scope.
- </p></td></tr>
- </table></div>
- <p>
- In addition, a local function can bind and call other local functions. Local
- functions should always be bound by constant reference <code class="computeroutput"><span class="keyword">const</span>
- <span class="identifier">bind</span><span class="special">&</span></code>
- to avoid unnecessary copies. For example, the following local function <code class="computeroutput"><span class="identifier">inc_sum</span></code> binds the local function <code class="computeroutput"><span class="identifier">inc</span></code> so <code class="computeroutput"><span class="identifier">inc_sum</span></code>
- can call <code class="computeroutput"><span class="identifier">inc</span></code> (see aslo <a href="../../../test/transform.cpp" target="_top"><code class="literal">transform.cpp</code></a>):
- </p>
- <p>
- </p>
- <pre class="programlisting"><span class="keyword">int</span> <span class="identifier">offset</span> <span class="special">=</span> <span class="number">5</span><span class="special">;</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">v</span><span class="special">;</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">w</span><span class="special">;</span>
- <span class="keyword">for</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">1</span><span class="special">;</span> <span class="identifier">i</span> <span class="special"><=</span> <span class="number">2</span><span class="special">;</span> <span class="special">++</span><span class="identifier">i</span><span class="special">)</span> <span class="identifier">v</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="identifier">i</span> <span class="special">*</span> <span class="number">10</span><span class="special">);</span>
- <span class="identifier">BOOST_TEST</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="number">10</span><span class="special">);</span> <span class="identifier">BOOST_TEST</span><span class="special">(</span><span class="identifier">v</span><span class="special">[</span><span class="number">1</span><span class="special">]</span> <span class="special">==</span> <span class="number">20</span><span class="special">);</span>
- <span class="identifier">w</span><span class="special">.</span><span class="identifier">resize</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="keyword">int</span> <span class="identifier">BOOST_LOCAL_FUNCTION</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">bind</span><span class="special">&</span> <span class="identifier">offset</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">i</span><span class="special">)</span> <span class="special">{</span>
- <span class="keyword">return</span> <span class="special">++</span><span class="identifier">i</span> <span class="special">+</span> <span class="identifier">offset</span><span class="special">;</span>
- <span class="special">}</span> <span class="identifier">BOOST_LOCAL_FUNCTION_NAME</span><span class="special">(</span><span class="identifier">inc</span><span class="special">)</span>
- <span class="identifier">std</span><span class="special">::</span><span class="identifier">transform</span><span class="special">(</span><span class="identifier">v</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">v</span><span class="special">.</span><span class="identifier">end</span><span class="special">(),</span> <span class="identifier">w</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">inc</span><span class="special">);</span>
- <span class="identifier">BOOST_TEST</span><span class="special">(</span><span class="identifier">w</span><span class="special">[</span><span class="number">0</span><span class="special">]</span> <span class="special">==</span> <span class="number">16</span><span class="special">);</span> <span class="identifier">BOOST_TEST</span><span class="special">(</span><span class="identifier">w</span><span class="special">[</span><span class="number">1</span><span class="special">]</span> <span class="special">==</span> <span class="number">26</span><span class="special">);</span>
- <span class="keyword">int</span> <span class="identifier">BOOST_LOCAL_FUNCTION</span><span class="special">(</span><span class="identifier">bind</span><span class="special">&</span> <span class="identifier">inc</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">i</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">j</span><span class="special">)</span> <span class="special">{</span>
- <span class="keyword">return</span> <span class="identifier">inc</span><span class="special">(</span><span class="identifier">i</span> <span class="special">+</span> <span class="identifier">j</span><span class="special">);</span> <span class="comment">// Call the other bound local function.</span>
- <span class="special">}</span> <span class="identifier">BOOST_LOCAL_FUNCTION_NAME</span><span class="special">(</span><span class="identifier">inc_sum</span><span class="special">)</span>
- <span class="identifier">offset</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span>
- <span class="identifier">std</span><span class="special">::</span><span class="identifier">transform</span><span class="special">(</span><span class="identifier">v</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">v</span><span class="special">.</span><span class="identifier">end</span><span class="special">(),</span> <span class="identifier">w</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">v</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">inc_sum</span><span class="special">);</span>
- <span class="identifier">BOOST_TEST</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="number">27</span><span class="special">);</span> <span class="identifier">BOOST_TEST</span><span class="special">(</span><span class="identifier">v</span><span class="special">[</span><span class="number">1</span><span class="special">]</span> <span class="special">==</span> <span class="number">47</span><span class="special">);</span>
- </pre>
- <p>
- </p>
- </div>
- <div class="section">
- <div class="titlepage"><div><div><h3 class="title">
- <a name="boost_localfunction.advanced_topics.nesting"></a><a class="link" href="advanced_topics.html#boost_localfunction.advanced_topics.nesting" title="Nesting">Nesting</a>
- </h3></div></div></div>
- <p>
- It is possible to nest local functions into one another. For example (see
- also <a href="../../../test/nesting.cpp" target="_top"><code class="literal">nesting.cpp</code></a>):
- </p>
- <p>
- </p>
- <pre class="programlisting"><span class="keyword">int</span> <span class="identifier">x</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span>
- <span class="keyword">void</span> <span class="identifier">BOOST_LOCAL_FUNCTION</span><span class="special">(</span><span class="identifier">bind</span><span class="special">&</span> <span class="identifier">x</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="identifier">bind</span><span class="special">&</span> <span class="identifier">x</span><span class="special">)</span> <span class="special">{</span> <span class="comment">// Nested.</span>
- <span class="identifier">x</span><span class="special">++;</span>
- <span class="special">}</span> <span class="identifier">BOOST_LOCAL_FUNCTION_NAME</span><span class="special">(</span><span class="identifier">g</span><span class="special">)</span>
- <span class="identifier">x</span><span class="special">--;</span>
- <span class="identifier">g</span><span class="special">();</span> <span class="comment">// Nested local function call.</span>
- <span class="special">}</span> <span class="identifier">BOOST_LOCAL_FUNCTION_NAME</span><span class="special">(</span><span class="identifier">f</span><span class="special">)</span>
- <span class="identifier">f</span><span class="special">();</span>
- </pre>
- <p>
- </p>
- </div>
- <div class="section">
- <div class="titlepage"><div><div><h3 class="title">
- <a name="boost_localfunction.advanced_topics.accessing_types__concepts__etc_"></a><a class="link" href="advanced_topics.html#boost_localfunction.advanced_topics.accessing_types__concepts__etc_" title="Accessing Types (concepts, etc)">Accessing
- Types (concepts, etc)</a>
- </h3></div></div></div>
- <p>
- This library never requires to explicitly specify the type of bound variables
- (e.g., this reduces maintenance because the local function declaration and
- definition do not have to change even if the bound variable types change
- as long as the semantics of the local function remain valid). From within
- local functions, programmers can access the type of a bound variable using
- the following macro:
- </p>
- <pre class="programlisting"><span class="identifier">BOOST_LOCAL_FUNCTION_TYPEOF</span><span class="special">(</span><span class="emphasis"><em>bound-variable-name</em></span><span class="special">)</span>
- </pre>
- <p>
- The <code class="computeroutput">BOOST_LOCAL_FUNCTION_TYPEOF</code>
- macro expands to a type expression that evaluates (at compile-time) to the
- fully qualified type of the bound variable with the specified name. This
- type expression is fully qualified in the sense that it will be constant
- if the variable is bound by constant <code class="computeroutput"><span class="keyword">const</span>
- <span class="identifier">bind</span><span class="special">[&]</span></code>
- and it will also be a reference if the variable is bound by 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>
- (if needed, programmers can remove the <code class="computeroutput"><span class="keyword">const</span></code>
- and <code class="computeroutput"><span class="special">&</span></code> qualifiers using
- <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">remove_const</span></code> and <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">remove_reference</span></code>,
- see <a href="http://www.boost.org/libs/type_traits" target="_top">Boost.TypeTraits</a>).
- </p>
- <p>
- The deduced bound type can be used within the body to check concepts, declare
- local variables, etc. For example (see also <a href="../../../test/typeof.cpp" target="_top"><code class="literal">typeof.cpp</code></a>
- and <a href="../../../test/addable.hpp" target="_top"><code class="literal">addable.hpp</code></a>):
- </p>
- <p>
- </p>
- <pre class="programlisting"><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="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="comment">// Type-of for concept checking.</span>
- <span class="identifier">BOOST_CONCEPT_ASSERT</span><span class="special">((</span><span class="identifier">Addable</span><span class="special"><</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">remove_reference</span><span class="special"><</span>
- <span class="identifier">BOOST_LOCAL_FUNCTION_TYPEOF</span><span class="special">(</span><span class="identifier">sum</span><span class="special">)>::</span><span class="identifier">type</span><span class="special">>));</span>
- <span class="comment">// Type-of for declarations.</span>
- <span class="identifier">boost</span><span class="special">::</span><span class="identifier">remove_reference</span><span class="special"><</span><span class="identifier">BOOST_LOCAL_FUNCTION_TYPEOF</span><span class="special">(</span>
- <span class="identifier">factor</span><span class="special">)>::</span><span class="identifier">type</span> <span class="identifier">mult</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="identifier">sum</span> <span class="special">+=</span> <span class="identifier">mult</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">6</span><span class="special">);</span>
- </pre>
- <p>
- </p>
- <p>
- Within templates, <code class="computeroutput">BOOST_LOCAL_FUNCTION_TYPEOF</code>
- should not be prefixed by the <code class="computeroutput"><span class="keyword">typename</span></code>
- keyword but eventual type manipulations need the <code class="computeroutput"><span class="keyword">typename</span></code>
- prefix as usual (see also <a href="../../../test/typeof_template.cpp" target="_top"><code class="literal">typeof_template.cpp</code></a>
- and <a href="../../../test/addable.hpp" target="_top"><code class="literal">addable.hpp</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">calculate</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">T</span><span class="special">&</span> <span class="identifier">factor</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="number">0</span><span class="special">;</span>
- <span class="keyword">void</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="comment">// Local function `TYPEOF` does not need `typename`.</span>
- <span class="identifier">BOOST_CONCEPT_ASSERT</span><span class="special">((</span><span class="identifier">Addable</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">remove_reference</span><span class="special"><</span>
- <span class="identifier">BOOST_LOCAL_FUNCTION_TYPEOF</span><span class="special">(</span><span class="identifier">sum</span><span class="special">)>::</span><span class="identifier">type</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_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="number">6</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>
- <p>
- In this context, it is best to use the <code class="computeroutput">BOOST_LOCAL_FUNCTION_TYPEOF</code>
- macro instead of using <a href="http://www.boost.org/libs/typeof" target="_top">Boost.Typeof</a>
- to reduce the number of times that <a href="http://www.boost.org/libs/typeof" target="_top">Boost.Typeof</a>
- is invoked (either the library already internally used <a href="http://www.boost.org/libs/typeof" target="_top">Boost.Typeof</a>
- once, in which case using this macro will not use <a href="http://www.boost.org/libs/typeof" target="_top">Boost.Typeof</a>
- again, or the bound variable type is explicitly specified by programmers
- as shown be below, in which case using this macro will not use <a href="http://www.boost.org/libs/typeof" target="_top">Boost.Typeof</a>
- at all).
- </p>
- <p>
- Furthermore, within the local function body it possible to access the result
- type using <code class="computeroutput"><span class="identifier">result_type</span></code>, the
- type of the first parameter using <code class="computeroutput"><span class="identifier">arg1_type</span></code>,
- the type of the second parameter using <code class="computeroutput"><span class="identifier">arg2_type</span></code>,
- etc. <a href="#ftn.boost_localfunction.advanced_topics.accessing_types__concepts__etc_.f0" class="footnote" name="boost_localfunction.advanced_topics.accessing_types__concepts__etc_.f0"><sup class="footnote">[21]</sup></a>
- </p>
- </div>
- <div class="section">
- <div class="titlepage"><div><div><h3 class="title">
- <a name="boost_localfunction.advanced_topics.specifying_types__no_boost_typeof_"></a><a class="link" href="advanced_topics.html#boost_localfunction.advanced_topics.specifying_types__no_boost_typeof_" title="Specifying Types (no Boost.Typeof)">Specifying
- Types (no Boost.Typeof)</a>
- </h3></div></div></div>
- <p>
- While not required, it is possible to explicitly specify the type of bound
- variables so the library will not internally use <a href="http://www.boost.org/libs/typeof" target="_top">Boost.Typeof</a>
- to automatically deduce the types. When specified, the bound variable type
- must follow the <code class="computeroutput"><span class="identifier">bind</span></code> "keyword"
- and it must be wrapped within round parenthesis <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-type</em></span><span class="special">)</span> <span class="emphasis"><em>variable-name</em></span> <span class="comment">// Bind by value with explicit type.</span>
- <span class="identifier">bind</span><span class="special">(</span><span class="emphasis"><em>variable-type</em></span><span class="special">)&</span> <span class="emphasis"><em>variable-name</em></span> <span class="comment">// Bind by reference with explicit type.</span>
- <span class="keyword">const</span> <span class="identifier">bind</span><span class="special">(</span><span class="emphasis"><em>variable-type</em></span><span class="special">)</span> <span class="emphasis"><em>variable-name</em></span> <span class="comment">// Bind by constant value with explicit type.</span>
- <span class="keyword">const</span> <span class="identifier">bind</span><span class="special">(</span><span class="emphasis"><em>variable-type</em></span><span class="special">)&</span> <span class="emphasis"><em>variable-name</em></span> <span class="comment">// Bind by constant reference with explicit type.</span>
- <span class="identifier">bind</span><span class="special">(</span><span class="emphasis"><em>class-type</em></span><span class="special">*)</span> <span class="identifier">this_</span> <span class="comment">// Bind object `this` with explicit type.</span>
- <span class="keyword">const</span> <span class="identifier">bind</span><span class="special">(</span><span class="emphasis"><em>class-type</em></span><span class="special">*)</span> <span class="identifier">this_</span> <span class="comment">// Bind object `this` by constant with explicit type.</span>
- </pre>
- <p>
- Note that within the local function body it is always possible to abstract
- the access to the type of a bound variable using <code class="computeroutput">BOOST_LOCAL_FUNCTION_TYPEOF</code>
- (even when the bound variable type is explicitly specified in the local function
- declaration).
- </p>
- <p>
- The library also uses <a href="http://www.boost.org/libs/typeof" target="_top">Boost.Typeof</a>
- to determine the local function result type (because this type is specified
- outside the <code class="computeroutput">BOOST_LOCAL_FUNCTION</code>
- macro). Thus it is also possible to specify the local function result type
- as one of the <code class="computeroutput">BOOST_LOCAL_FUNCTION</code>
- macro parameters prefixing it by <code class="computeroutput"><span class="keyword">return</span></code>
- so the library will not use <a href="http://www.boost.org/libs/typeof" target="_top">Boost.Typeof</a>
- to deduce the result type:
- </p>
- <pre class="programlisting"><span class="identifier">BOOST_LOCAL_FUNCTION_TYPE</span><span class="special">(</span><span class="keyword">return</span> <code class="literal"><span class="emphasis"><em>result-type</em></span></code><span class="special">,</span> <span class="special">...)</span>
- </pre>
- <p>
- Note that the result type must be specified only once either before the macro
- (without the <code class="computeroutput"><span class="keyword">return</span></code> prefix)
- or as one of the macro parameters (with the <code class="computeroutput"><span class="keyword">return</span></code>
- prefix). As always, the result type can be <code class="computeroutput"><span class="keyword">void</span></code>
- to declare a function that returns nothing (so <code class="computeroutput"><span class="keyword">return</span>
- <span class="keyword">void</span></code> is allowed when the result type
- is specified as one of the macro parameters).
- </p>
- <p>
- The following example specifies all bound variables and result types (see
- also <a href="../../../test/add_typed.cpp" target="_top"><code class="literal">add_typed.cpp</code></a>):
- <a href="#ftn.boost_localfunction.advanced_topics.specifying_types__no_boost_typeof_.f0" class="footnote" name="boost_localfunction.advanced_topics.specifying_types__no_boost_typeof_.f0"><sup class="footnote">[22]</sup></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="keyword">void</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="special">&</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="comment">// Explicitly specify bound variable and return types (no type-of).</span>
- <span class="identifier">BOOST_LOCAL_FUNCTION</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">bind</span><span class="special">(</span><span class="keyword">const</span> <span class="keyword">int</span><span class="special">&)</span> <span class="identifier">factor</span><span class="special">,</span>
- <span class="identifier">bind</span><span class="special">(</span><span class="identifier">adder</span><span class="special">*)</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="keyword">return</span> <span class="keyword">int</span><span class="special">)</span> <span class="special">{</span>
- <span class="keyword">return</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="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>
- Unless necessary, it is recommended to not specify the bound variable and
- result types. Let the library deduce these types so the local function syntax
- will be more concise and the local function declaration will not have to
- change if a bound variable type changes (reducing maintenance).
- </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>
- When all bound variable and result types are explicitly specified, the
- library implementation will not use <a href="http://www.boost.org/libs/typeof" target="_top">Boost.Typeof</a>.
- </p></td></tr>
- </table></div>
- </div>
- <div class="section">
- <div class="titlepage"><div><div><h3 class="title">
- <a name="boost_localfunction.advanced_topics.inlining"></a><a class="link" href="advanced_topics.html#boost_localfunction.advanced_topics.inlining" title="Inlining">Inlining</a>
- </h3></div></div></div>
- <p>
- Local functions can be declared <a href="http://en.wikipedia.org/wiki/Inline_function" target="_top">inline</a>
- to increase the chances that the compiler will be able to reduce the run-time
- of the local function call by inlining the generated assembly code. A local
- function is declared inline by prefixing its name with the keyword <code class="computeroutput"><span class="keyword">inline</span></code>:
- </p>
- <pre class="programlisting"><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="special">...</span> <span class="comment">// Body.</span>
- <span class="special">}</span> <span class="identifier">BOOST_LOCAL_FUNCTION_NAME</span><span class="special">(</span><span class="keyword">inline</span> <span class="emphasis"><em>name</em></span><span class="special">)</span> <span class="comment">// Inlining.</span>
- </pre>
- <p>
- When inlining a local function, note the following:
- </p>
- <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
- <li class="listitem">
- On <a href="http://www.open-std.org/JTC1/SC22/WG21/docs/standards" target="_top">C++03</a>
- compliant compilers, inline local functions always have a run-time comparable
- to their equivalent implementation that uses local functors (see the
- <a class="link" href="alternatives.html" title="Annex: Alternatives">Alternatives</a>
- section). However, inline local functions have the important limitation
- that they cannot be assigned to other functors (like <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">function</span></code>)
- and they cannot be passed as template parameters.
- </li>
- <li class="listitem">
- On <a href="http://www.open-std.org/JTC1/SC22/WG21/" target="_top">C++11</a>
- compilers, <code class="computeroutput"><span class="keyword">inline</span></code> has no
- effect because this library will automatically generate code that uses
- <a href="http://www.open-std.org/JTC1/SC22/WG21/" target="_top">C++11</a> specific
- features to inline the local function calls whenever possible even if
- the local function is not declared inline. Furthermore, non <a href="http://www.open-std.org/JTC1/SC22/WG21/" target="_top">C++11</a>
- local functions can always be passes as template parameters even when
- they are declared inline. <a href="#ftn.boost_localfunction.advanced_topics.inlining.f0" class="footnote" name="boost_localfunction.advanced_topics.inlining.f0"><sup class="footnote">[23]</sup></a>
- </li>
- </ul></div>
- <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 recommended to not declare a local function inline unless it is strictly
- necessary for optimizing pure <a href="http://www.open-std.org/JTC1/SC22/WG21/docs/standards" target="_top">C++03</a>
- compliant code (because in all other cases this library will automatically
- take advantage of <a href="http://www.open-std.org/JTC1/SC22/WG21/" target="_top">C++11</a>
- features to optimize the local function calls while always allowing to
- pass the local function as a template parameter).
- </p></td></tr>
- </table></div>
- <p>
- For example, the following local function is declared inline (thus a for-loop
- needs to be used for portability instead of passing the local function as
- a template parameter to the <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">for_each</span></code>
- algorithm, see also <a href="../../../test/add_inline.cpp" target="_top"><code class="literal">add_inline.cpp</code></a>):
- </p>
- <p>
- </p>
- <pre class="programlisting"><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="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="keyword">inline</span> <span class="identifier">add</span><span class="special">)</span> <span class="comment">// Inlining.</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">v</span><span class="special">(</span><span class="number">100</span><span class="special">);</span>
- <span class="identifier">std</span><span class="special">::</span><span class="identifier">fill</span><span class="special">(</span><span class="identifier">v</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">v</span><span class="special">.</span><span class="identifier">end</span><span class="special">(),</span> <span class="number">1</span><span class="special">);</span>
- <span class="keyword">for</span><span class="special">(</span><span class="identifier">size_t</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> <span class="identifier">i</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="special">++</span><span class="identifier">i</span><span class="special">)</span> <span class="identifier">add</span><span class="special">(</span><span class="identifier">v</span><span class="special">[</span><span class="identifier">i</span><span class="special">]);</span> <span class="comment">// Cannot use for_each.</span>
- </pre>
- <p>
- </p>
- </div>
- <div class="section">
- <div class="titlepage"><div><div><h3 class="title">
- <a name="boost_localfunction.advanced_topics.recursion"></a><a class="link" href="advanced_topics.html#boost_localfunction.advanced_topics.recursion" title="Recursion">Recursion</a>
- </h3></div></div></div>
- <p>
- Local functions can be declared <a href="http://en.wikipedia.org/wiki/Recursion_(computer_science)#Recursive_procedures" target="_top">recursive</a>
- so a local function can recursively call itself from its body (as usual with
- C++ functions). A local function is declared recursive by prefixing its name
- with the <code class="computeroutput"><span class="identifier">recursive</span></code> "keyword"
- (thus <code class="computeroutput"><span class="identifier">recursive</span></code> cannot be
- used as a local function name):
- </p>
- <pre class="programlisting"><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="special">...</span> <span class="comment">// Body.</span>
- <span class="special">}</span> <span class="identifier">BOOST_LOCAL_FUNCTION_NAME</span><span class="special">(</span><span class="identifier">recursive</span> <span class="emphasis"><em>name</em></span><span class="special">)</span> <span class="comment">// Recursive.</span>
- </pre>
- <p>
- For example, the following local function is used to recursively calculate
- the factorials of all the numbers in the specified vector (see also <a href="../../../test/factorial.cpp" target="_top"><code class="literal">factorial.cpp</code></a>):
- </p>
- <p>
- </p>
- <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">calculator</span> <span class="special">{</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">results</span><span class="special">;</span>
- <span class="keyword">void</span> <span class="identifier">factorials</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="special">{</span>
- <span class="keyword">int</span> <span class="identifier">BOOST_LOCAL_FUNCTION</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="keyword">bool</span> <span class="identifier">recursion</span><span class="special">,</span> <span class="keyword">default</span> <span class="keyword">false</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="number">0</span><span class="special">;</span>
- <span class="keyword">if</span><span class="special">(</span><span class="identifier">num</span> <span class="special"><=</span> <span class="number">0</span><span class="special">)</span> <span class="identifier">result</span> <span class="special">=</span> <span class="number">1</span><span class="special">;</span>
- <span class="keyword">else</span> <span class="identifier">result</span> <span class="special">=</span> <span class="identifier">num</span> <span class="special">*</span> <span class="identifier">factorial</span><span class="special">(</span><span class="identifier">num</span> <span class="special">-</span> <span class="number">1</span><span class="special">,</span> <span class="keyword">true</span><span class="special">);</span> <span class="comment">// Recursive call.</span>
- <span class="keyword">if</span><span class="special">(!</span><span class="identifier">recursion</span><span class="special">)</span> <span class="identifier">this_</span><span class="special">-></span><span class="identifier">results</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="identifier">result</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="identifier">BOOST_LOCAL_FUNCTION_NAME</span><span class="special">(</span><span class="identifier">recursive</span> <span class="identifier">factorial</span><span class="special">)</span> <span class="comment">// Recursive.</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">factorial</span><span class="special">);</span>
- <span class="special">}</span>
- <span class="special">};</span>
- </pre>
- <p>
- </p>
- <p>
- Compilers have not been observed to be able to inline recursive local function
- calls not even when the recursive local function is also declared inline:
- </p>
- <pre class="programlisting"><span class="special">...</span> <span class="identifier">BOOST_LOCAL_FUNCTION_NAME</span><span class="special">(</span><span class="keyword">inline</span> <span class="identifier">recursive</span> <span class="identifier">factorial</span><span class="special">)</span>
- </pre>
- <p>
- Recursive local functions should never be called outside their declaration
- scope. <a href="#ftn.boost_localfunction.advanced_topics.recursion.f0" class="footnote" name="boost_localfunction.advanced_topics.recursion.f0"><sup class="footnote">[24]</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>
- If a local function is returned from the enclosing function and called
- in a different scope, the behaviour is undefined (and it will likely result
- in a run-time error).
- </p></td></tr>
- </table></div>
- <p>
- This is not 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> because lambdas can never call themselves recursively
- (in other words, there is no recursive lambda function that can successfully
- be called outside its declaration scope because there is no recursive lambda
- function at all).
- </p>
- </div>
- <div class="section">
- <div class="titlepage"><div><div><h3 class="title">
- <a name="boost_localfunction.advanced_topics.overloading"></a><a class="link" href="advanced_topics.html#boost_localfunction.advanced_topics.overloading" title="Overloading">Overloading</a>
- </h3></div></div></div>
- <p>
- Because local functions are functors, it is possible to overload them using
- the <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">overloaded_function</span></code> functor of <a href="http://www.boost.org/libs/functional/overloaded_function" target="_top">Boost.Functional/OverloadedFunction</a>
- from the <code class="literal">boost/functional/overloaded_function.hpp</code> header
- (see the <a href="http://www.boost.org/libs/functional/overloaded_function" target="_top">Boost.Functional/OverloadedFunction</a>
- documentation for details).
- </p>
- <p>
- In the following example, the overloaded function object <code class="computeroutput"><span class="identifier">add</span></code>
- can be called with signatures from either the local function <code class="computeroutput"><span class="identifier">add_s</span></code>, or the local function <code class="computeroutput"><span class="identifier">add_d</span></code>, or the local function <code class="computeroutput"><span class="identifier">add_d</span></code> with its extra default parameter,
- or the function pointer <code class="computeroutput"><span class="identifier">add_i</span></code>
- (see also <a href="../../../test/overload.cpp" target="_top"><code class="literal">overload.cpp</code></a>):
- </p>
- <p>
- </p>
- <pre class="programlisting"><span class="keyword">int</span> <span class="identifier">add_i</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="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>
- </pre>
- <p>
- </p>
- <p>
- </p>
- <pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">s</span> <span class="special">=</span> <span class="string">"abc"</span><span class="special">;</span>
- <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">BOOST_LOCAL_FUNCTION</span><span class="special">(</span>
- <span class="keyword">const</span> <span class="identifier">bind</span><span class="special">&</span> <span class="identifier">s</span><span class="special">,</span> <span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">&</span> <span class="identifier">x</span><span class="special">)</span> <span class="special">{</span>
- <span class="keyword">return</span> <span class="identifier">s</span> <span class="special">+</span> <span class="identifier">x</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_s</span><span class="special">)</span>
- <span class="keyword">double</span> <span class="identifier">d</span> <span class="special">=</span> <span class="number">1.23</span><span class="special">;</span>
- <span class="keyword">double</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">d</span><span class="special">,</span> <span class="keyword">double</span> <span class="identifier">x</span><span class="special">,</span> <span class="keyword">double</span> <span class="identifier">y</span><span class="special">,</span> <span class="keyword">default</span> <span class="number">0</span><span class="special">)</span> <span class="special">{</span>
- <span class="keyword">return</span> <span class="identifier">d</span> <span class="special">+</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_d</span><span class="special">)</span>
- <span class="identifier">boost</span><span class="special">::</span><span class="identifier">overloaded_function</span><span class="special"><</span>
- <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="special">(</span><span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">&)</span>
- <span class="special">,</span> <span class="keyword">double</span> <span class="special">(</span><span class="keyword">double</span><span class="special">)</span>
- <span class="special">,</span> <span class="keyword">double</span> <span class="special">(</span><span class="keyword">double</span><span class="special">,</span> <span class="keyword">double</span><span class="special">)</span> <span class="comment">// Overload giving default param.</span>
- <span class="special">,</span> <span class="keyword">int</span> <span class="special">(</span><span class="keyword">int</span><span class="special">,</span> <span class="keyword">int</span><span class="special">)</span>
- <span class="special">></span> <span class="identifier">add</span><span class="special">(</span><span class="identifier">add_s</span><span class="special">,</span> <span class="identifier">add_d</span><span class="special">,</span> <span class="identifier">add_d</span><span class="special">,</span> <span class="identifier">add_i</span><span class="special">);</span> <span class="comment">// Overloaded function object.</span>
- <span class="identifier">BOOST_TEST</span><span class="special">(</span><span class="identifier">add</span><span class="special">(</span><span class="string">"xyz"</span><span class="special">)</span> <span class="special">==</span> <span class="string">"abcxyz"</span><span class="special">);</span> <span class="comment">// Call `add_s`.</span>
- <span class="identifier">BOOST_TEST</span><span class="special">((</span><span class="number">4.44</span> <span class="special">-</span> <span class="identifier">add</span><span class="special">(</span><span class="number">3.21</span><span class="special">))</span> <span class="special"><=</span> <span class="number">0.001</span><span class="special">);</span> <span class="comment">// Call `add_d` (no default).</span>
- <span class="identifier">BOOST_TEST</span><span class="special">((</span><span class="number">44.44</span> <span class="special">-</span> <span class="identifier">add</span><span class="special">(</span><span class="number">3.21</span><span class="special">,</span> <span class="number">40.0</span><span class="special">))</span> <span class="special"><=</span> <span class="number">0.001</span><span class="special">);</span> <span class="comment">// Call `add_d`.</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">// Call `add_i`.</span>
- </pre>
- <p>
- </p>
- </div>
- <div class="section">
- <div class="titlepage"><div><div><h3 class="title">
- <a name="boost_localfunction.advanced_topics.exception_specifications"></a><a class="link" href="advanced_topics.html#boost_localfunction.advanced_topics.exception_specifications" title="Exception Specifications">Exception
- Specifications</a>
- </h3></div></div></div>
- <p>
- It is possible to program exception specifications for local functions by
- specifying them after the <code class="computeroutput">BOOST_LOCAL_FUNCTION</code>
- macro and before the body code block <code class="computeroutput"><span class="special">{</span>
- <span class="special">...</span> <span class="special">}</span></code>.
- </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>
- Note that the exception specifications only apply to the body code specified
- by programmers and they do not apply to the rest of the code automatically
- generated by the macro expansions to implement local functions. For example,
- even if the body code is specified to throw no exception using <code class="computeroutput"><span class="keyword">throw</span> <span class="special">()</span> <span class="special">{</span> <span class="special">...</span> <span class="special">}</span></code>,
- the execution of the library code automatically generated by the macros
- could still throw (if there is no memory, etc).
- </p></td></tr>
- </table></div>
- <p>
- For example (see also <a href="../../../test/add_except.cpp" target="_top"><code class="literal">add_except.cpp</code></a>):
- </p>
- <p>
- </p>
- <pre class="programlisting"><span class="keyword">double</span> <span class="identifier">sum</span> <span class="special">=</span> <span class="number">0.0</span><span class="special">;</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="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">double</span> <span class="identifier">num</span><span class="special">)</span> <span class="keyword">throw</span><span class="special">()</span> <span class="special">{</span> <span class="comment">// Throw nothing.</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">100</span><span class="special">);</span>
- </pre>
- <p>
- </p>
- </div>
- <div class="section">
- <div class="titlepage"><div><div><h3 class="title">
- <a name="boost_localfunction.advanced_topics.storage_classifiers"></a><a class="link" href="advanced_topics.html#boost_localfunction.advanced_topics.storage_classifiers" title="Storage Classifiers">Storage
- Classifiers</a>
- </h3></div></div></div>
- <p>
- Local function parameters support the storage classifiers as usual in <a href="http://www.open-std.org/JTC1/SC22/WG21/docs/standards" target="_top">C++03</a>.
- The <code class="computeroutput"><span class="keyword">auto</span></code> storage classifier
- is specified as: <a href="#ftn.boost_localfunction.advanced_topics.storage_classifiers.f0" class="footnote" name="boost_localfunction.advanced_topics.storage_classifiers.f0"><sup class="footnote">[25]</sup></a>
- </p>
- <pre class="programlisting"><span class="keyword">auto</span> <span class="emphasis"><em>parameter-type parameter-name</em></span>
- </pre>
- <p>
- The <code class="computeroutput"><span class="keyword">register</span></code> storage classifier
- is specified as:
- </p>
- <pre class="programlisting"><span class="keyword">register</span> <span class="emphasis"><em>parameter-type parameter-name</em></span>
- </pre>
- <p>
- For example (see also <a href="../../../test/add_classifiers.cpp" target="_top"><code class="literal">add_classifiers.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">auto</span> <span class="keyword">int</span> <span class="identifier">x</span><span class="special">,</span> <span class="keyword">register</span> <span class="keyword">int</span> <span class="identifier">y</span><span class="special">)</span> <span class="special">{</span> <span class="comment">// Classifiers.</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>
- </pre>
- <p>
- </p>
- </div>
- <div class="section">
- <div class="titlepage"><div><div><h3 class="title">
- <a name="boost_localfunction.advanced_topics.same_line_expansions"></a><a class="link" href="advanced_topics.html#boost_localfunction.advanced_topics.same_line_expansions" title="Same Line Expansions">Same
- Line Expansions</a>
- </h3></div></div></div>
- <p>
- In general, it is not possible to expand the <code class="computeroutput">BOOST_LOCAL_FUNCTION</code>,
- <code class="computeroutput">BOOST_LOCAL_FUNCTION_TPL</code>
- macros multiple times on the same line. <a href="#ftn.boost_localfunction.advanced_topics.same_line_expansions.f0" class="footnote" name="boost_localfunction.advanced_topics.same_line_expansions.f0"><sup class="footnote">[26]</sup></a>
- </p>
- <p>
- Therefore, this library provides additional macros <code class="computeroutput">BOOST_LOCAL_FUNCTION_ID</code>
- and <code class="computeroutput">BOOST_LOCAL_FUNCTION_ID_TPL</code>
- which can be expanded multiple times on the same line as long as programmers
- specify unique identifiers as the macros' first parameters. The unique identifier
- can be any token (not just numeric) that can be successfully concatenated
- by the preprocessor (e.g., <code class="computeroutput"><span class="identifier">local_function_number_1_at_line_123</span></code>).
- <a href="#ftn.boost_localfunction.advanced_topics.same_line_expansions.f1" class="footnote" name="boost_localfunction.advanced_topics.same_line_expansions.f1"><sup class="footnote">[27]</sup></a>
- </p>
- <p>
- The <code class="computeroutput">BOOST_LOCAL_FUNCTION_ID</code>
- and <code class="computeroutput">BOOST_LOCAL_FUNCTION_ID_TPL</code>
- macros accept local function parameter declaration lists using the exact
- same syntax as <code class="computeroutput">BOOST_LOCAL_FUNCTION</code>.
- For example (see also <a href="../../../test/same_line.cpp" target="_top"><code class="literal">same_line.cpp</code></a>):
- </p>
- <p>
- </p>
- <pre class="programlisting"><span class="preprocessor">#define</span> <span class="identifier">LOCAL_INC_DEC</span><span class="special">(</span><span class="identifier">offset</span><span class="special">)</span> <span class="special">\</span>
- <span class="keyword">int</span> <span class="identifier">BOOST_LOCAL_FUNCTION_ID</span><span class="special">(</span><span class="identifier">BOOST_PP_CAT</span><span class="special">(</span><span class="identifier">inc</span><span class="special">,</span> <span class="identifier">__LINE__</span><span class="special">),</span> <span class="comment">/* unique ID */</span> <span class="special">\</span>
- <span class="keyword">const</span> <span class="identifier">bind</span> <span class="identifier">offset</span><span class="special">,</span> <span class="keyword">const</span> <span class="keyword">int</span> <span class="identifier">x</span><span class="special">)</span> <span class="special">{</span> <span class="special">\</span>
- <span class="keyword">return</span> <span class="identifier">x</span> <span class="special">+</span> <span class="identifier">offset</span><span class="special">;</span> <span class="special">\</span>
- <span class="special">}</span> <span class="identifier">BOOST_LOCAL_FUNCTION_NAME</span><span class="special">(</span><span class="identifier">inc</span><span class="special">)</span> <span class="special">\</span>
- <span class="special">\</span>
- <span class="keyword">int</span> <span class="identifier">BOOST_LOCAL_FUNCTION_ID</span><span class="special">(</span><span class="identifier">BOOST_PP_CAT</span><span class="special">(</span><span class="identifier">dec</span><span class="special">,</span> <span class="identifier">__LINE__</span><span class="special">),</span> <span class="special">\</span>
- <span class="keyword">const</span> <span class="identifier">bind</span> <span class="identifier">offset</span><span class="special">,</span> <span class="keyword">const</span> <span class="keyword">int</span> <span class="identifier">x</span><span class="special">)</span> <span class="special">{</span> <span class="special">\</span>
- <span class="keyword">return</span> <span class="identifier">x</span> <span class="special">-</span> <span class="identifier">offset</span><span class="special">;</span> <span class="special">\</span>
- <span class="special">}</span> <span class="identifier">BOOST_LOCAL_FUNCTION_NAME</span><span class="special">(</span><span class="identifier">dec</span><span class="special">)</span>
- <span class="preprocessor">#define</span> <span class="identifier">LOCAL_INC_DEC_TPL</span><span class="special">(</span><span class="identifier">offset</span><span class="special">)</span> <span class="special">\</span>
- <span class="identifier">T</span> <span class="identifier">BOOST_LOCAL_FUNCTION_ID_TPL</span><span class="special">(</span><span class="identifier">BOOST_PP_CAT</span><span class="special">(</span><span class="identifier">inc</span><span class="special">,</span> <span class="identifier">__LINE__</span><span class="special">),</span> <span class="special">\</span>
- <span class="keyword">const</span> <span class="identifier">bind</span> <span class="identifier">offset</span><span class="special">,</span> <span class="keyword">const</span> <span class="identifier">T</span> <span class="identifier">x</span><span class="special">)</span> <span class="special">{</span> <span class="special">\</span>
- <span class="keyword">return</span> <span class="identifier">x</span> <span class="special">+</span> <span class="identifier">offset</span><span class="special">;</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">inc</span><span class="special">)</span> <span class="special">\</span>
- <span class="special">\</span>
- <span class="identifier">T</span> <span class="identifier">BOOST_LOCAL_FUNCTION_ID_TPL</span><span class="special">(</span><span class="identifier">BOOST_PP_CAT</span><span class="special">(</span><span class="identifier">dec</span><span class="special">,</span> <span class="identifier">__LINE__</span><span class="special">),</span> <span class="special">\</span>
- <span class="keyword">const</span> <span class="identifier">bind</span> <span class="identifier">offset</span><span class="special">,</span> <span class="keyword">const</span> <span class="identifier">T</span> <span class="identifier">x</span><span class="special">)</span> <span class="special">{</span> <span class="special">\</span>
- <span class="keyword">return</span> <span class="identifier">x</span> <span class="special">-</span> <span class="identifier">offset</span><span class="special">;</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">dec</span><span class="special">)</span>
- <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="keyword">void</span> <span class="identifier">f</span><span class="special">(</span><span class="identifier">T</span><span class="special">&</span> <span class="identifier">delta</span><span class="special">)</span> <span class="special">{</span>
- <span class="identifier">LOCAL_INC_DEC_TPL</span><span class="special">(</span><span class="identifier">delta</span><span class="special">)</span> <span class="comment">// Multiple local functions on same line.</span>
- <span class="identifier">BOOST_TEST</span><span class="special">(</span><span class="identifier">dec</span><span class="special">(</span><span class="identifier">inc</span><span class="special">(</span><span class="number">123</span><span class="special">))</span> <span class="special">==</span> <span class="number">123</span><span class="special">);</span>
- <span class="special">}</span>
- <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="keyword">int</span> <span class="identifier">delta</span> <span class="special">=</span> <span class="number">10</span><span class="special">;</span>
- <span class="identifier">LOCAL_INC_DEC</span><span class="special">(</span><span class="identifier">delta</span><span class="special">)</span> <span class="comment">// Multiple local functions on same line.</span>
- <span class="identifier">BOOST_TEST</span><span class="special">(</span><span class="identifier">dec</span><span class="special">(</span><span class="identifier">inc</span><span class="special">(</span><span class="number">123</span><span class="special">))</span> <span class="special">==</span> <span class="number">123</span><span class="special">);</span>
- <span class="identifier">f</span><span class="special">(</span><span class="identifier">delta</span><span class="special">);</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>
- <p>
- As shown by the example above, the <code class="computeroutput">BOOST_LOCAL_FUNCTION_ID</code>
- and <code class="computeroutput">BOOST_LOCAL_FUNCTION_ID_TPL</code>
- macros are especially useful when it is necessary to invoke them multiple
- times within a user-defined macro (because the preprocessor expands all nested
- macros on the same line).
- </p>
- </div>
- <div class="section">
- <div class="titlepage"><div><div><h3 class="title">
- <a name="boost_localfunction.advanced_topics.limitations__operators__etc_"></a><a class="link" href="advanced_topics.html#boost_localfunction.advanced_topics.limitations__operators__etc_" title="Limitations (operators, etc)">Limitations
- (operators, etc)</a>
- </h3></div></div></div>
- <p>
- The following table summarizes all C++ function features indicating those
- features that are not supported by this library for local functions.
- </p>
- <div class="informaltable"><table class="table">
- <colgroup>
- <col>
- <col>
- <col>
- </colgroup>
- <thead><tr>
- <th>
- <p>
- C++ Function Feature
- </p>
- </th>
- <th>
- <p>
- Local Function Support
- </p>
- </th>
- <th>
- <p>
- Comment
- </p>
- </th>
- </tr></thead>
- <tbody>
- <tr>
- <td>
- <p>
- <code class="computeroutput"><span class="keyword">export</span></code>
- </p>
- </td>
- <td>
- <p>
- No.
- </p>
- </td>
- <td>
- <p>
- This is not supported because local functions cannot be templates
- (plus most C++ compilers do not implement <code class="computeroutput"><span class="keyword">export</span></code>
- at all).
- </p>
- </td>
- </tr>
- <tr>
- <td>
- <p>
- <code class="computeroutput"><span class="keyword">template</span><span class="special"><</span></code><code class="literal"><span class="emphasis"><em>template-parameter-list</em></span></code><code class="computeroutput"><span class="special">></span></code>
- </p>
- </td>
- <td>
- <p>
- No.
- </p>
- </td>
- <td>
- <p>
- This is not supported because local functions are implemented using
- local classes and <a href="http://www.open-std.org/JTC1/SC22/WG21/docs/standards" target="_top">C++03</a>
- local classes cannot be templates.
- </p>
- </td>
- </tr>
- <tr>
- <td>
- <p>
- <code class="computeroutput"><span class="keyword">explicit</span></code>
- </p>
- </td>
- <td>
- <p>
- No.
- </p>
- </td>
- <td>
- <p>
- This is not supported because local functions are not constructors.
- </p>
- </td>
- </tr>
- <tr>
- <td>
- <p>
- <code class="computeroutput"><span class="keyword">inline</span></code>
- </p>
- </td>
- <td>
- <p>
- Yes.
- </p>
- </td>
- <td>
- <p>
- Local functions can be specified <code class="computeroutput"><span class="keyword">inline</span></code>
- to improve the chances that <a href="http://www.open-std.org/JTC1/SC22/WG21/docs/standards" target="_top">C++03</a>
- compilers can optimize the local function call run-time (but <code class="computeroutput"><span class="keyword">inline</span></code> local functions cannot be
- passed as template parameters on <a href="http://www.open-std.org/JTC1/SC22/WG21/docs/standards" target="_top">C++03</a>
- compilers, see the <a class="link" href="advanced_topics.html" title="Advanced Topics">Advanced
- Topics</a> section).
- </p>
- </td>
- </tr>
- <tr>
- <td>
- <p>
- <code class="computeroutput"><span class="keyword">extern</span></code>
- </p>
- </td>
- <td>
- <p>
- No.
- </p>
- </td>
- <td>
- <p>
- This is not supported because local functions are always defined
- locally within the enclosing scope and together with their declarations.
- </p>
- </td>
- </tr>
- <tr>
- <td>
- <p>
- <code class="computeroutput"><span class="keyword">static</span></code>
- </p>
- </td>
- <td>
- <p>
- No.
- </p>
- </td>
- <td>
- <p>
- This is not supported because local functions are not member functions.
- </p>
- </td>
- </tr>
- <tr>
- <td>
- <p>
- <code class="computeroutput"><span class="keyword">virtual</span></code>
- </p>
- </td>
- <td>
- <p>
- No.
- </p>
- </td>
- <td>
- <p>
- This is not supported because local functions are not member functions.
- <a href="#ftn.boost_localfunction.advanced_topics.limitations__operators__etc_.f0" class="footnote" name="boost_localfunction.advanced_topics.limitations__operators__etc_.f0"><sup class="footnote">[a]</sup></a>
- </p>
- </td>
- </tr>
- <tr>
- <td>
- <p>
- <code class="literal"><span class="emphasis"><em>result-type</em></span></code>
- </p>
- </td>
- <td>
- <p>
- Yes.
- </p>
- </td>
- <td>
- <p>
- This is supported (see the <a class="link" href="tutorial.html" title="Tutorial">Tutorial</a>
- section).
- </p>
- </td>
- </tr>
- <tr>
- <td>
- <p>
- <code class="literal"><span class="emphasis"><em>function-name</em></span></code>
- </p>
- </td>
- <td>
- <p>
- Yes.
- </p>
- </td>
- <td>
- <p>
- Local functions are named and they can call themselves recursively
- but they cannot be operators (see the <a class="link" href="tutorial.html" title="Tutorial">Tutorial</a>
- and <a class="link" href="advanced_topics.html" title="Advanced Topics">Advanced
- Topics</a> sections).
- </p>
- </td>
- </tr>
- <tr>
- <td>
- <p>
- <code class="literal"><span class="emphasis"><em>parameter-list</em></span></code>
- </p>
- </td>
- <td>
- <p>
- Yes.
- </p>
- </td>
- <td>
- <p>
- This is supported and it also supports the <code class="computeroutput"><span class="keyword">auto</span></code>
- and <code class="computeroutput"><span class="keyword">register</span></code> storage
- classifiers, default parameters, and binding of variables in scope
- (see the <a class="link" href="tutorial.html" title="Tutorial">Tutorial</a>
- and <a class="link" href="advanced_topics.html" title="Advanced Topics">Advanced
- Topics</a> sections).
- </p>
- </td>
- </tr>
- <tr>
- <td>
- <p>
- Trailing <code class="computeroutput"><span class="keyword">const</span></code> qualifier
- </p>
- </td>
- <td>
- <p>
- No.
- </p>
- </td>
- <td>
- <p>
- This is not supported because local functions are not member functions.
- </p>
- </td>
- </tr>
- <tr>
- <td>
- <p>
- Trailing <code class="computeroutput"><span class="keyword">volatile</span></code>
- qualifier
- </p>
- </td>
- <td>
- <p>
- No.
- </p>
- </td>
- <td>
- <p>
- This is not supported because local functions are not member functions.
- </p>
- </td>
- </tr>
- </tbody>
- <tbody class="footnotes"><tr><td colspan="3"><div id="ftn.boost_localfunction.advanced_topics.limitations__operators__etc_.f0" class="footnote"><p><a href="#boost_localfunction.advanced_topics.limitations__operators__etc_.f0" class="para"><sup class="para">[a] </sup></a>
- <span class="bold"><strong>Rationale.</strong></span> It would be possible
- to make a local function class inherit from another local function
- class. However, this "inheritance" feature is not implemented
- because it seemed of <a href="http://lists.boost.org/Archives/boost/2010/09/170895.php" target="_top">no
- use</a> given that local functions can be bound to one another
- thus they can simply call each other directly without recurring
- to dynamic binding or base function calls.
- </p></div></td></tr></tbody>
- </table></div>
- <h5>
- <a name="boost_localfunction.advanced_topics.limitations__operators__etc_.h0"></a>
- <span class="phrase"><a name="boost_localfunction.advanced_topics.limitations__operators__etc_.operators"></a></span><a class="link" href="advanced_topics.html#boost_localfunction.advanced_topics.limitations__operators__etc_.operators">Operators</a>
- </h5>
- <p>
- Local functions cannot be operators. Naming a local function <code class="computeroutput"><span class="keyword">operator</span><span class="special">...</span></code>
- will generate a compile-time error. <a href="#ftn.boost_localfunction.advanced_topics.limitations__operators__etc_.f1" class="footnote" name="boost_localfunction.advanced_topics.limitations__operators__etc_.f1"><sup class="footnote">[28]</sup></a>
- </p>
- <p>
- For example, the following code does not compile (see also <a href="../../../test/operator_error.cpp" target="_top"><code class="literal">operator_error.cpp</code></a>):
- </p>
- <p>
- </p>
- <pre class="programlisting"><span class="keyword">bool</span> <span class="identifier">BOOST_LOCAL_FUNCTION</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">point</span><span class="special">&</span> <span class="identifier">p</span><span class="special">,</span> <span class="keyword">const</span> <span class="identifier">point</span><span class="special">&</span> <span class="identifier">q</span><span class="special">)</span> <span class="special">{</span>
- <span class="keyword">return</span> <span class="identifier">p</span><span class="special">.</span><span class="identifier">x</span> <span class="special">==</span> <span class="identifier">q</span><span class="special">.</span><span class="identifier">x</span> <span class="special">&&</span> <span class="identifier">p</span><span class="special">.</span><span class="identifier">y</span> <span class="special">==</span> <span class="identifier">q</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="keyword">operator</span><span class="special">==)</span> <span class="comment">// Error: Cannot use `operator...`.</span>
- </pre>
- <p>
- </p>
- <h5>
- <a name="boost_localfunction.advanced_topics.limitations__operators__etc_.h1"></a>
- <span class="phrase"><a name="boost_localfunction.advanced_topics.limitations__operators__etc_.goto"></a></span><a class="link" href="advanced_topics.html#boost_localfunction.advanced_topics.limitations__operators__etc_.goto">Goto</a>
- </h5>
- <p>
- It is possible to jump with a <code class="computeroutput"><span class="keyword">goto</span></code>
- within the local function body. For example, the following compiles (see
- also <a href="../../../test/goto.cpp" target="_top"><code class="literal">goto.cpp</code></a>):
- </p>
- <p>
- </p>
- <pre class="programlisting"><span class="keyword">int</span> <span class="identifier">error</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="keyword">int</span> <span class="identifier">BOOST_LOCAL_FUNCTION</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">z</span><span class="special">)</span> <span class="special">{</span>
- <span class="keyword">if</span><span class="special">(</span><span class="identifier">z</span> <span class="special">></span> <span class="number">0</span><span class="special">)</span> <span class="keyword">goto</span> <span class="identifier">success</span><span class="special">;</span> <span class="comment">// OK: Can jump within local function.</span>
- <span class="keyword">return</span> <span class="special">-</span><span class="number">1</span><span class="special">;</span>
- <span class="identifier">success</span><span class="special">:</span>
- <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
- <span class="special">}</span> <span class="identifier">BOOST_LOCAL_FUNCTION_NAME</span><span class="special">(</span><span class="identifier">validate</span><span class="special">)</span>
- <span class="keyword">return</span> <span class="identifier">validate</span><span class="special">(</span><span class="identifier">x</span> <span class="special">+</span> <span class="identifier">y</span><span class="special">);</span>
- <span class="special">}</span>
- </pre>
- <p>
- </p>
- <p>
- However, it is not possible to jump with a <code class="computeroutput"><span class="keyword">goto</span></code>
- from within the local function body to to a label defined in the enclosing
- scope. For example, the following does not compile (see also <a href="../../../test/goto_error.cpp" target="_top"><code class="literal">goto_error.cpp</code></a>):
- </p>
- <p>
- </p>
- <pre class="programlisting"><span class="keyword">int</span> <span class="identifier">error</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="keyword">int</span> <span class="identifier">BOOST_LOCAL_FUNCTION</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">z</span><span class="special">)</span> <span class="special">{</span>
- <span class="keyword">if</span><span class="special">(</span><span class="identifier">z</span> <span class="special"><=</span> <span class="number">0</span><span class="special">)</span> <span class="keyword">goto</span> <span class="identifier">failure</span><span class="special">;</span> <span class="comment">// Error: Cannot jump to enclosing scope.</span>
- <span class="keyword">else</span> <span class="keyword">goto</span> <span class="identifier">success</span><span class="special">;</span> <span class="comment">// OK: Can jump within local function.</span>
- <span class="identifier">success</span><span class="special">:</span>
- <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
- <span class="special">}</span> <span class="identifier">BOOST_LOCAL_FUNCTION_NAME</span><span class="special">(</span><span class="identifier">validate</span><span class="special">)</span>
- <span class="keyword">return</span> <span class="identifier">validate</span><span class="special">(</span><span class="identifier">x</span> <span class="special">+</span> <span class="identifier">y</span><span class="special">);</span>
- <span class="identifier">failure</span><span class="special">:</span>
- <span class="keyword">return</span> <span class="special">-</span><span class="number">1</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.advanced_topics.default_parameters.f0" class="footnote"><p><a href="#boost_localfunction.advanced_topics.default_parameters.f0" class="para"><sup class="para">[17] </sup></a>
- <span class="bold"><strong>Rationale.</strong></span> The assignment symbol <code class="computeroutput"><span class="special">=</span></code> cannot be used to specify default parameter
- values because default values are not part of the parameter type so they
- cannot be handled using template meta-programming. Default parameter values
- need to be separated from the rest of the parameter declaration using the
- preprocessor. Specifically, this library needs to use preprocessor meta-programming
- to remove default values when constructing the local function type and
- also to count the number of default values to provide the correct set of
- call operators for the local functor. Therefore, the symbol <code class="computeroutput"><span class="special">=</span></code> cannot be used because it cannot be handled
- by preprocessor meta-programming (non-alphanumeric symbols cannot be detected
- by preprocessor meta-programming because they cannot be concatenated by
- the preprocessor).
- </p></div>
- <div id="ftn.boost_localfunction.advanced_topics.default_parameters.f1" class="footnote"><p><a href="#boost_localfunction.advanced_topics.default_parameters.f1" class="para"><sup class="para">[18] </sup></a>
- The authors do not personally find the use of the <code class="computeroutput"><span class="identifier">WITH_DEFAULT</span></code>
- macro more readable and they prefer to use the <code class="computeroutput"><span class="keyword">default</span></code>
- keyword directly. Furthermore, <code class="computeroutput"><span class="identifier">WITH_DEFAULT</span></code>
- needs to be defined differently for compilers without variadic macros
- <code class="computeroutput"><span class="preprocessor">#define</span> <span class="identifier">WITH_DEFAULT</span>
- <span class="special">(</span><span class="keyword">default</span><span class="special">)</span></code> so it can only be defined by programmers
- based on the syntax they decide to use (see the <a class="link" href="no_variadic_macros.html" title="Annex: No Variadic Macros">No
- Variadic Macros</a> section).
- </p></div>
- <div id="ftn.boost_localfunction.advanced_topics.commas_and_symbols_in_macros.f0" class="footnote"><p><a href="#boost_localfunction.advanced_topics.commas_and_symbols_in_macros.f0" class="para"><sup class="para">[19] </sup></a>
- <span class="bold"><strong>Rationale.</strong></span> This limitation is because
- this library uses preprocessor token concatenation <code class="literal">##</code>
- to inspect the macro parameters (to distinguish between function parameters,
- bound variables, etc) and the C++ preprocessor does not allow to concatenate
- non-alphanumeric tokens.
- </p></div>
- <div id="ftn.boost_localfunction.advanced_topics.commas_and_symbols_in_macros.f1" class="footnote"><p><a href="#boost_localfunction.advanced_topics.commas_and_symbols_in_macros.f1" class="para"><sup class="para">[20] </sup></a>
- The preprocessor always interprets unwrapped commas as separating macro
- parameters. Thus in this case the comma will indicate to the preprocessor
- that the first macro parameter is <code class="computeroutput"><span class="keyword">const</span>
- <span class="identifier">std</span><span class="special">::</span><span class="identifier">map</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">tring</span></code>, the second macro parameter is
- <code class="computeroutput"><span class="identifier">size_t</span><span class="special">>&</span>
- <span class="identifier">m</span></code>, etc instead of passing <code class="computeroutput"><span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">map</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">,</span> <span class="identifier">size_t</span><span class="special">>&</span> <span class="identifier">m</span></code>
- as a single macro parameter.
- </p></div>
- <div id="ftn.boost_localfunction.advanced_topics.accessing_types__concepts__etc_.f0" class="footnote"><p><a href="#boost_localfunction.advanced_topics.accessing_types__concepts__etc_.f0" class="para"><sup class="para">[21] </sup></a>
- <span class="bold"><strong>Rationale.</strong></span> The type names <code class="computeroutput"><span class="identifier">result_type</span></code> and <code class="computeroutput"><span class="identifier">arg</span></code><code class="literal"><span class="emphasis"><em>N</em></span></code><code class="computeroutput"><span class="identifier">_type</span></code> follow the <a href="http://www.boost.org/libs/type_traits" target="_top">Boost.TypeTraits</a>
- naming conventions for function traits.
- </p></div>
- <div id="ftn.boost_localfunction.advanced_topics.specifying_types__no_boost_typeof_.f0" class="footnote"><p><a href="#boost_localfunction.advanced_topics.specifying_types__no_boost_typeof_.f0" class="para"><sup class="para">[22] </sup></a>
- In the examples of this documentation, bound variables, function parameters,
- and the result type are specified in this order because this is the order
- used by <a href="http://en.wikipedia.org/wiki/C%2B%2B0x#Lambda_functions_and_expressions" target="_top">C++11
- lambda functions</a>. However, the library accepts bound variables,
- function parameters, and the result type in any order.
- </p></div>
- <div id="ftn.boost_localfunction.advanced_topics.inlining.f0" class="footnote"><p><a href="#boost_localfunction.advanced_topics.inlining.f0" class="para"><sup class="para">[23] </sup></a>
- <span class="bold"><strong>Rationale.</strong></span> This library uses an indirect
- function call via a function pointer in order to pass the local function
- as a template parameter (see the <a class="link" href="implementation.html" title="Annex: Implementation">Implementation</a>
- section). No compiler has yet been observed to be able to inline function
- calls when they use such indirect function pointer calls. Therefore,
- inline local functions do not use such indirect function pointer call
- (so they are more likely to be optimized) but because of that they
- cannot be passed as template parameters. The indirect function pointer
- call is needed on <a href="http://www.open-std.org/JTC1/SC22/WG21/docs/standards" target="_top">C++03</a>
- but it is not needed on <a href="http://www.open-std.org/JTC1/SC22/WG21/" target="_top">C++11</a>
- (see <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2657.htm" target="_top">[N2657]</a>
- and <a href="http://www.boost.org/libs/chrono" target="_top">Boost.Config</a>'s
- <code class="computeroutput"><span class="identifier">BOOST_NO_CXX11_LOCAL_CLASS_TEMPLATE_PARAMETERS</span></code>)
- thus this library automatically generates local function calls that
- can be inline on <a href="http://www.open-std.org/JTC1/SC22/WG21/" target="_top">C++11</a>
- compilers (even when the local function is not declared inline).
- </p></div>
- <div id="ftn.boost_localfunction.advanced_topics.recursion.f0" class="footnote"><p><a href="#boost_localfunction.advanced_topics.recursion.f0" class="para"><sup class="para">[24] </sup></a>
- <span class="bold"><strong>Rationale.</strong></span> This limitation comes from
- the fact that the global functor used to pass the local function as a template
- parameter (and eventually returned outside the declarations scope) does
- not know the local function name so the local function name used for recursive
- call cannot be set in the global functor. This limitation together with
- preventing the possibility for inlining are the reasons why local functions
- are not recursive unless programmers explicitly declare them <code class="computeroutput"><span class="identifier">recursive</span></code>.
- </p></div>
- <div id="ftn.boost_localfunction.advanced_topics.storage_classifiers.f0" class="footnote"><p><a href="#boost_localfunction.advanced_topics.storage_classifiers.f0" class="para"><sup class="para">[25] </sup></a>
- The <code class="computeroutput"><span class="keyword">auto</span></code> storage classifier
- is part of the <a href="http://www.open-std.org/JTC1/SC22/WG21/docs/standards" target="_top">C++03</a>
- standard and therefore supported by this library. However, the meaning
- and usage of the <code class="computeroutput"><span class="keyword">auto</span></code> keyword
- changed in <a href="http://www.open-std.org/JTC1/SC22/WG21/" target="_top">C++11</a>.
- Therefore, use the <code class="computeroutput"><span class="keyword">auto</span></code> storage
- classifier with the usual care in order to avoid writing <a href="http://www.open-std.org/JTC1/SC22/WG21/docs/standards" target="_top">C++03</a>
- code that might not work on <a href="http://www.open-std.org/JTC1/SC22/WG21/" target="_top">C++11</a>.
- </p></div>
- <div id="ftn.boost_localfunction.advanced_topics.same_line_expansions.f0" class="footnote"><p><a href="#boost_localfunction.advanced_topics.same_line_expansions.f0" class="para"><sup class="para">[26] </sup></a>
- <span class="bold"><strong>Rationale.</strong></span> The <code class="computeroutput">BOOST_LOCAL_FUNCTION</code>
- and <code class="computeroutput">BOOST_LOCAL_FUNCTION_TPL</code>
- macros internally use <code class="computeroutput"><span class="identifier">__LINE__</span></code>
- to generate unique identifiers. Therefore, if these macros are expanded
- more than on time on the same line, the generated identifiers will no longer
- be unique and the code will not compile. (This restriction does not apply
- to MSVC and other compilers that provide the non-standard <code class="computeroutput"><span class="identifier">__COUNTER__</span></code> macro.) Note that the <code class="computeroutput">BOOST_LOCAL_FUNCTION_NAME</code> macro
- can always be expanded multiple times on the same line because the unique
- local function name (and not <code class="computeroutput"><span class="identifier">__LINE__</span></code>)
- is used by this macro to generate unique identifiers (so there is no need
- for a <code class="computeroutput"><span class="identifier">BOOST_LOCAL_FUNCTION_NAME_ID</span></code>
- macro).
- </p></div>
- <div id="ftn.boost_localfunction.advanced_topics.same_line_expansions.f1" class="footnote"><p><a href="#boost_localfunction.advanced_topics.same_line_expansions.f1" class="para"><sup class="para">[27] </sup></a>
- Because there are restrictions on the set of tokens that the preprocessor
- can concatenate and because not all compilers correctly implement these
- restrictions, it is in general recommended to specify unique identifiers
- as a combination of alphanumeric tokens.
- </p></div>
- <div id="ftn.boost_localfunction.advanced_topics.limitations__operators__etc_.f1" class="footnote"><p><a href="#boost_localfunction.advanced_topics.limitations__operators__etc_.f1" class="para"><sup class="para">[28] </sup></a>
- <span class="bold"><strong>Rationale.</strong></span> This is the because a local
- function name must be a valid local variable name (the local variable used
- to hold the local functor) and operators cannot be used as local variable
- names.
- </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>)
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