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<div class="section boost_typetraits_reference_is_function">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_typetraits.reference.is_function"></a><a class="link" href="is_function.html" title="is_function">is_function</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">T</span><span class="special">></span>
<span class="keyword">struct</span> <span class="identifier">is_function</span> <span class="special">:</span> <span class="keyword">public</span> <em class="replaceable"><code><a class="link" href="integral_constant.html" title="integral_constant">true_type</a>-or-<a class="link" href="integral_constant.html" title="integral_constant">false_type</a></code></em> <span class="special">{};</span>
</pre>
<p>
<span class="bold"><strong>Inherits:</strong></span> If T is a (possibly cv-qualified)
function type then inherits from <a class="link" href="integral_constant.html" title="integral_constant">true_type</a>,
otherwise inherits from <a class="link" href="integral_constant.html" title="integral_constant">false_type</a>.
Note that this template does not detect <span class="emphasis"><em>pointers to functions</em></span>,
or <span class="emphasis"><em>references to functions</em></span>, these are detected by <a class="link" href="is_pointer.html" title="is_pointer">is_pointer</a> and <a class="link" href="is_reference.html" title="is_reference">is_reference</a> respectively:
</p>
<pre class="programlisting"><span class="keyword">typedef</span> <span class="keyword">int</span> <span class="identifier">f1</span><span class="special">();</span> <span class="comment">// f1 is of function type.</span>
<span class="keyword">typedef</span> <span class="keyword">int</span> <span class="special">(</span><span class="identifier">f2</span><span class="special">*)();</span> <span class="comment">// f2 is a pointer to a function.</span>
<span class="keyword">typedef</span> <span class="keyword">int</span> <span class="special">(</span><span class="identifier">f3</span><span class="special">&)();</span> <span class="comment">// f3 is a reference to a function.</span>
</pre>
<p>
<span class="bold"><strong>C++ Standard Reference:</strong></span> 3.9.2p1 and 8.3.5.
</p>
<p>
<span class="bold"><strong>Header:</strong></span> <code class="computeroutput"> <span class="preprocessor">#include</span>
<span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">type_traits</span><span class="special">/</span><span class="identifier">is_function</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span></code>
or <code class="computeroutput"> <span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">type_traits</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span></code>
</p>
<p>
<span class="bold"><strong>Examples:</strong></span>
</p>
<div class="blockquote"><blockquote class="blockquote"><p>
<code class="computeroutput"><span class="identifier">is_function</span><span class="special"><</span><span class="keyword">int</span> <span class="special">(</span><span class="keyword">void</span><span class="special">)></span></code> inherits from <code class="computeroutput"><a class="link" href="integral_constant.html" title="integral_constant">true_type</a></code>.
</p></blockquote></div>
<div class="blockquote"><blockquote class="blockquote"><p>
<code class="computeroutput"><span class="identifier">is_function</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">int</span><span class="special">)>::</span><span class="identifier">type</span></code> is the type <code class="computeroutput"><a class="link" href="integral_constant.html" title="integral_constant">true_type</a></code>.
</p></blockquote></div>
<div class="blockquote"><blockquote class="blockquote"><p>
<code class="computeroutput"><span class="identifier">is_function</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">int</span><span class="special">)>::</span><span class="identifier">value</span></code> is an integral constant expression
that evaluates to <span class="emphasis"><em>true</em></span>.
</p></blockquote></div>
<div class="blockquote"><blockquote class="blockquote"><p>
<code class="computeroutput"><span class="identifier">is_function</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">int</span><span class="special">)>::</span><span class="identifier">value</span></code> is an integral constant expression
that evaluates to <span class="emphasis"><em>false</em></span>: the argument in this case
is a pointer type, not a function type.
</p></blockquote></div>
<div class="blockquote"><blockquote class="blockquote"><p>
<code class="computeroutput"><span class="identifier">is_function</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">int</span><span class="special">)>::</span><span class="identifier">value</span></code> is an integral constant expression
that evaluates to <span class="emphasis"><em>false</em></span>: the argument in this case
is a reference to a function, not a function type.
</p></blockquote></div>
<div class="blockquote"><blockquote class="blockquote"><p>
<code class="computeroutput"><span class="identifier">is_function</span><span class="special"><</span><span class="keyword">long</span> <span class="special">(</span><span class="identifier">MyClass</span><span class="special">::*)(</span><span class="keyword">double</span><span class="special">,</span> <span class="keyword">int</span><span class="special">)>::</span><span class="identifier">value</span></code> is an integral constant expression
that evaluates to <span class="emphasis"><em>false</em></span>: the argument in this case
is a pointer to a member function.
</p></blockquote></div>
<div class="blockquote"><blockquote class="blockquote"><p>
<code class="computeroutput"><span class="identifier">is_function</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">value_type</span></code> is the type <code class="computeroutput"><span class="keyword">bool</span></code>.
</p></blockquote></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>
Don't confuse function-types with pointers to functions:
</p>
<p>
<code class="computeroutput"><span class="keyword">typedef</span> <span class="keyword">int</span>
<span class="identifier">f</span><span class="special">(</span><span class="keyword">double</span><span class="special">);</span></code>
</p>
<p>
defines a function type,
</p>
<p>
<code class="computeroutput"><span class="identifier">f</span> <span class="identifier">foo</span><span class="special">;</span></code>
</p>
<p>
declares a prototype for a function of type <code class="computeroutput"><span class="identifier">f</span></code>,
</p>
<p>
<code class="computeroutput"><span class="identifier">f</span><span class="special">*</span>
<span class="identifier">pf</span> <span class="special">=</span>
<span class="identifier">foo</span><span class="special">;</span></code>
</p>
<p>
<code class="computeroutput"><span class="identifier">f</span><span class="special">&</span>
<span class="identifier">fr</span> <span class="special">=</span>
<span class="identifier">foo</span><span class="special">;</span></code>
</p>
<p>
declares a pointer and a reference to the function <code class="computeroutput"><span class="identifier">foo</span></code>.
</p>
<p>
If you want to detect whether some type is a pointer-to-function then use:
</p>
<p>
<code class="computeroutput"><a class="link" href="is_function.html" title="is_function">is_function</a><span class="special"><</span><a class="link" href="remove_pointer.html" title="remove_pointer">remove_pointer</a><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">type</span><span class="special">>::</span><span class="identifier">value</span>
<span class="special">&&</span> <a class="link" href="is_pointer.html" title="is_pointer">is_pointer</a><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">value</span></code>
</p>
<p>
or for pointers to member functions you can just use <a class="link" href="is_member_function_pointer.html" title="is_member_function_pointer">is_member_function_pointer</a>
directly.
</p>
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