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| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="boost_typetraits.reference.function_traits"></a><a class="link" href="function_traits.html" title="function_traits"> function_traits</a> |
| </h3></div></div></div> |
| <pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">F</span><span class="special">></span> |
| <span class="keyword">struct</span> <span class="identifier">function_traits</span> |
| <span class="special">{</span> |
| <span class="keyword">static</span> <span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">arity</span> <span class="special">=</span> <em class="replaceable"><code>see-below</code></em><span class="special">;</span> |
| <span class="keyword">typedef</span> <em class="replaceable"><code>see-below</code></em> <span class="identifier">result_type</span><span class="special">;</span> |
| <span class="keyword">typedef</span> <em class="replaceable"><code>see-below</code></em> arg<em class="replaceable"><code>N</code></em>_type<span class="special">;</span> |
| <span class="special">};</span> |
| </pre> |
| <p> |
| The class template function_traits will only compile if: |
| </p> |
| <div class="itemizedlist"><ul type="disc"> |
| <li> |
| The compiler supports partial specialization of class templates. |
| </li> |
| <li> |
| The template argument <code class="computeroutput"><span class="identifier">F</span></code> |
| is a <span class="emphasis"><em>function type</em></span>, note that this <span class="emphasis"><em><span class="bold"><strong>is not</strong></span></em></span> the same thing as a <span class="emphasis"><em>pointer |
| to a function</em></span>. |
| </li> |
| </ul></div> |
| <div class="tip"><table border="0" summary="Tip"> |
| <tr> |
| <td rowspan="2" align="center" valign="top" width="25"><img alt="[Tip]" src="../../../../../../doc/src/images/tip.png"></td> |
| <th align="left">Tip</th> |
| </tr> |
| <tr><td align="left" valign="top"><p> |
| function_traits is intended to introspect only C++ functions of the form |
| R (), R( A1 ), R ( A1, ... etc. ) and not function pointers or class member |
| functions. To convert a function pointer type to a suitable type use <a class="link" href="remove_pointer.html" title="remove_pointer">remove_pointer</a>. |
| </p></td></tr> |
| </table></div> |
| <div class="table"> |
| <a name="id1062849"></a><p class="title"><b>Table 1.14. Function Traits Members</b></p> |
| <div class="table-contents"><table class="table" summary="Function Traits Members"> |
| <colgroup> |
| <col> |
| <col> |
| </colgroup> |
| <thead><tr> |
| <th> |
| <p> |
| Member |
| </p> |
| </th> |
| <th> |
| <p> |
| Description |
| </p> |
| </th> |
| </tr></thead> |
| <tbody> |
| <tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="identifier">function_traits</span><span class="special"><</span><span class="identifier">F</span><span class="special">>::</span><span class="identifier">arity</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| An integral constant expression that gives the number of arguments |
| accepted by the function type <code class="computeroutput"><span class="identifier">F</span></code>. |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="identifier">function_traits</span><span class="special"><</span><span class="identifier">F</span><span class="special">>::</span><span class="identifier">result_type</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| The type returned by function type <code class="computeroutput"><span class="identifier">F</span></code>. |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="identifier">function_traits</span><span class="special"><</span><span class="identifier">F</span><span class="special">>::</span>arg<em class="replaceable"><code>N</code></em>_type</code> |
| </p> |
| </td> |
| <td> |
| <p> |
| The <em class="replaceable"><code>N</code></em>th argument type of function type <code class="computeroutput"><span class="identifier">F</span></code>, |
| where <code class="computeroutput"><span class="number">1</span> <span class="special"><=</span> |
| <span class="identifier">N</span> <span class="special"><=</span> |
| <span class="identifier">arity</span></code> of <code class="computeroutput"><span class="identifier">F</span></code>. |
| </p> |
| </td> |
| </tr> |
| </tbody> |
| </table></div> |
| </div> |
| <br class="table-break"><div class="table"> |
| <a name="id1063066"></a><p class="title"><b>Table 1.15. Examples</b></p> |
| <div class="table-contents"><table class="table" summary="Examples"> |
| <colgroup> |
| <col> |
| <col> |
| </colgroup> |
| <thead><tr> |
| <th> |
| <p> |
| Expression |
| </p> |
| </th> |
| <th> |
| <p> |
| Result |
| </p> |
| </th> |
| </tr></thead> |
| <tbody> |
| <tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="identifier">function_traits</span><span class="special"><</span><span class="keyword">void</span> |
| <span class="special">(</span><span class="keyword">void</span><span class="special">)>::</span><span class="identifier">arity</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| An integral constant expression that has the value 0. |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="identifier">function_traits</span><span class="special"><</span><span class="keyword">long</span> |
| <span class="special">(</span><span class="keyword">int</span><span class="special">)>::</span><span class="identifier">arity</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| An integral constant expression that has the value 1. |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="identifier">function_traits</span><span class="special"><</span><span class="keyword">long</span> |
| <span class="special">(</span><span class="keyword">int</span><span class="special">,</span> <span class="keyword">long</span><span class="special">,</span> <span class="keyword">double</span><span class="special">,</span> <span class="keyword">void</span><span class="special">*)>::</span><span class="identifier">arity</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| An integral constant expression that has the value 4. |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="identifier">function_traits</span><span class="special"><</span><span class="keyword">void</span> |
| <span class="special">(</span><span class="keyword">void</span><span class="special">)>::</span><span class="identifier">result_type</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| The type <code class="computeroutput"><span class="keyword">void</span></code>. |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="identifier">function_traits</span><span class="special"><</span><span class="keyword">long</span> |
| <span class="special">(</span><span class="keyword">int</span><span class="special">)>::</span><span class="identifier">result_type</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| The type <code class="computeroutput"><span class="keyword">long</span></code>. |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="identifier">function_traits</span><span class="special"><</span><span class="keyword">long</span> |
| <span class="special">(</span><span class="keyword">int</span><span class="special">)>::</span><span class="identifier">arg1_type</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| The type <code class="computeroutput"><span class="keyword">int</span></code>. |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="identifier">function_traits</span><span class="special"><</span><span class="keyword">long</span> |
| <span class="special">(</span><span class="keyword">int</span><span class="special">,</span> <span class="keyword">long</span><span class="special">,</span> <span class="keyword">double</span><span class="special">,</span> <span class="keyword">void</span><span class="special">*)>::</span><span class="identifier">arg4_type</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| The type <code class="computeroutput"><span class="keyword">void</span><span class="special">*</span></code>. |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="identifier">function_traits</span><span class="special"><</span><span class="keyword">long</span> |
| <span class="special">(</span><span class="keyword">int</span><span class="special">,</span> <span class="keyword">long</span><span class="special">,</span> <span class="keyword">double</span><span class="special">,</span> <span class="keyword">void</span><span class="special">*)>::</span><span class="identifier">arg5_type</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| A compiler error: there is no <code class="computeroutput"><span class="identifier">arg5_type</span></code> |
| since there are only four arguments. |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="identifier">function_traits</span><span class="special"><</span><span class="keyword">long</span> |
| <span class="special">(*)(</span><span class="keyword">void</span><span class="special">)>::</span><span class="identifier">arity</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| A compiler error: argument type is a <span class="emphasis"><em>function pointer</em></span>, |
| and not a <span class="emphasis"><em>function type</em></span>. |
| </p> |
| </td> |
| </tr> |
| </tbody> |
| </table></div> |
| </div> |
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