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<a name="boost.proto.is_callable"></a><div class="titlepage"></div>
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<h2><span class="refentrytitle">Struct template is_callable</span></h2>
<p>boost::proto::is_callable — Boolean metafunction which tells whether a type is a callable
<a class="link" href="../../PolymorphicFunctionObject.html" title="Concept PolymorphicFunctionObject">PolymorphicFunctionObject</a> or not.</p>
</div>
<h2 xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="refsynopsisdiv-title">Synopsis</h2>
<div xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="refsynopsisdiv"><pre class="synopsis"><span class="comment">// In header: <<a class="link" href="../../proto/reference.html#header.boost.proto.traits_hpp" title="Header <boost/proto/traits.hpp>">boost/proto/traits.hpp</a>>
</span><span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> T<span class="special">></span>
<span class="keyword">struct</span> <a class="link" href="is_callable.html" title="Struct template is_callable">is_callable</a> <span class="special">:</span> <span class="keyword"></span> mpl::bool_<<em class="replaceable"><code>true-or-false</code></em>> <span class="special">{</span>
<span class="special">}</span><span class="special">;</span></pre></div>
<div class="refsect1">
<a name="idm45555179445008"></a><h2>Description</h2>
<p>
<code class="computeroutput">proto::is_callable<></code> is used by the
<code class="computeroutput"><a class="link" href="when.html" title="Struct template when">proto::when<></a></code>
transform to determine whether a function type <code class="computeroutput">R(A<sub>1</sub>,...A<sub>n</sub>)</code>
is a <a class="link" href="../../CallableTransform.html" title="Concept CallableTransform">CallableTransform</a> or an <a class="link" href="../../ObjectTransform.html" title="Concept ObjectTransform">ObjectTransform</a>.
The former are evaluated using <code class="computeroutput"><a class="link" href="call.html" title="Struct template call">proto::call<></a></code>
and the later with <code class="computeroutput"><a class="link" href="make.html" title="Struct template make">proto::make<></a></code>.
If <code class="computeroutput">proto::is_callable<R>::value</code> is <code class="computeroutput">true</code>,
the function type is a <a class="link" href="../../CallableTransform.html" title="Concept CallableTransform">CallableTransform</a>; otherwise, it is an <a class="link" href="../../ObjectTransform.html" title="Concept ObjectTransform">ObjectTransform</a>.
</p>
<p>
Unless specialized for a type
<code class="computeroutput">T</code>, <code class="computeroutput">proto::is_callable<T>::value</code>
is computed as follows:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem"><p>
If <code class="computeroutput">T</code> is a template type
<code class="computeroutput">X<Y<sub>0</sub>,...Y<sub>n</sub>></code>,
where all <code class="computeroutput">Y<sub>x</sub></code> are types for
<code class="computeroutput">x</code> in <code class="computeroutput">[0,n]</code>,
<code class="computeroutput">proto::is_callable<T>::value</code> is
<code class="computeroutput">boost::is_same<Y<sub>n</sub>, <a class="link" href="callable.html" title="Struct callable">proto::callable</a>>::value</code>.
</p></li>
<li class="listitem"><p>
If <code class="computeroutput">T</code> is derived from <code class="computeroutput"><a class="link" href="callable.html" title="Struct callable">proto::callable</a></code>,
<code class="computeroutput">proto::is_callable<T>::value</code> is <code class="computeroutput">true</code>.
</p></li>
<li class="listitem"><p>
Otherwise, <code class="computeroutput">proto::is_callable<T>::value</code>
is <code class="computeroutput">false</code>.
</p></li>
</ul></div>
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<td align="right"><div class="copyright-footer">Copyright © 2008 Eric Niebler<p>
Distributed under the Boost Software License, Version 1.0. (See accompanying
file LICENSE_1_0.txt or 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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