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<title>Introspecting an inner function</title>
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<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="the_type_traits_introspection_library.tti_detail_has_function"></a><a class="link" href="tti_detail_has_function.html" title="Introspecting an inner function">Introspecting
    an inner function</a>
</h2></div></div></div>
<p>
      The TTI macro <code class="computeroutput"><a class="link" href="../BOOST_TTI_HAS_FUNCTION.html" title="Macro BOOST_TTI_HAS_FUNCTION">BOOST_TTI_HAS_FUNCTION</a></code>
      introspects an inner function of a class. The function can be either a member
      function or a static member function.
    </p>
<p>
      BOOST_TTI_HAS_FUNCTION takes a single parameter which is the name of an inner
      function whose existence the programmer wants to check. The macro generates
      a metafunction called "has_function_'name_of_inner_function'".
    </p>
<p>
      The metafunction can be invoked by passing it the enclosing type to introspect
      and a signature for the function as separate template arguments. The signature
      for the function consists of a return type, optional parameter types in the
      form of a boost::mpl forward sequence of types, and an optional Boost FunctionTypes
      tag type. A typical boost::mpl forward sequence of types is a boost::mpl::vector&lt;&gt;.
    </p>
<p>
      The metafunction returns a single type called 'type', which is a boost::mpl::bool_.
      As a convenience the metafunction returns the value of this type directly as
      a compile time bool constant called 'value'. This is true or false depending
      on whether the inner function, of the specified signature, exists or not.
    </p>
<h4>
<a name="the_type_traits_introspection_library.tti_detail_has_function.h0"></a>
      <span class="phrase"><a name="the_type_traits_introspection_library.tti_detail_has_function.generating_the_metafunction"></a></span><a class="link" href="tti_detail_has_function.html#the_type_traits_introspection_library.tti_detail_has_function.generating_the_metafunction">Generating
      the metafunction</a>
    </h4>
<p>
      You generate the metafunction by invoking the macro with the name of an inner
      function:
    </p>
<pre class="programlisting"><span class="identifier">BOOST_TTI_HAS_FUNCTION</span><span class="special">(</span><span class="identifier">AnInnerFunction</span><span class="special">)</span>
</pre>
<p>
      generates a metafunction called 'has_function_AnInnerFunction' in the current
      scope.
    </p>
<h4>
<a name="the_type_traits_introspection_library.tti_detail_has_function.h1"></a>
      <span class="phrase"><a name="the_type_traits_introspection_library.tti_detail_has_function.invoking_the_metafunction"></a></span><a class="link" href="tti_detail_has_function.html#the_type_traits_introspection_library.tti_detail_has_function.invoking_the_metafunction">Invoking
      the metafunction</a>
    </h4>
<p>
      You invoke the metafunction by instantiating the template with an enclosing
      type to introspect and the signature of the function as a series of template
      parameters.
    </p>
<p>
      A return value called 'value' is a compile time bool constant.
    </p>
<pre class="programlisting"><span class="identifier">has_function_AnInnerFunction</span>
  <span class="special">&lt;</span>
  <span class="identifier">Enclosing_Type</span><span class="special">,</span>
  <span class="identifier">Function_ReturnType</span><span class="special">,</span>
  <span class="identifier">boost</span><span class="special">::</span><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="identifier">Function_ParameterTypes</span><span class="special">&gt;,</span> <span class="comment">// optional, can be any mpl forward sequence</span>
  <span class="identifier">boost</span><span class="special">::</span><span class="identifier">function_types</span><span class="special">::</span><span class="identifier">SomeTagType</span>           <span class="comment">// optional, can be any FunctionTypes tag type</span>
  <span class="special">&gt;::</span><span class="identifier">value</span>
</pre>
<h4>
<a name="the_type_traits_introspection_library.tti_detail_has_function.h2"></a>
      <span class="phrase"><a name="the_type_traits_introspection_library.tti_detail_has_function.examples"></a></span><a class="link" href="tti_detail_has_function.html#the_type_traits_introspection_library.tti_detail_has_function.examples">Examples</a>
    </h4>
<p>
      First we generate metafunctions for various inner function names:
    </p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">tti</span><span class="special">/</span><span class="identifier">has_function</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>

<span class="identifier">BOOST_TTI_HAS_FUNCTION</span><span class="special">(</span><span class="identifier">function1</span><span class="special">)</span>
<span class="identifier">BOOST_TTI_HAS_FUNCTION</span><span class="special">(</span><span class="identifier">function2</span><span class="special">)</span>
<span class="identifier">BOOST_TTI_HAS_FUNCTION</span><span class="special">(</span><span class="identifier">function3</span><span class="special">)</span>
</pre>
<p>
      Next let us create some user-defined types we want to introspect.
    </p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">AClass</span> <span class="special">{</span> <span class="special">};</span>
<span class="keyword">struct</span> <span class="identifier">Top</span>
  <span class="special">{</span>
  <span class="keyword">static</span> <span class="keyword">int</span> <span class="identifier">function1</span><span class="special">();</span>
  <span class="identifier">AClass</span> <span class="identifier">function2</span><span class="special">(</span><span class="keyword">double</span><span class="special">,</span><span class="keyword">short</span> <span class="special">*);</span>
  <span class="special">};</span>
<span class="keyword">struct</span> <span class="identifier">Top2</span>
  <span class="special">{</span>
  <span class="keyword">long</span> <span class="identifier">function2</span><span class="special">(</span><span class="identifier">Top</span> <span class="special">&amp;,</span><span class="keyword">int</span><span class="special">,</span><span class="keyword">bool</span><span class="special">,</span><span class="keyword">short</span><span class="special">,</span><span class="keyword">float</span><span class="special">);</span>
  <span class="keyword">static</span> <span class="identifier">Top</span> <span class="special">*</span> <span class="identifier">function3</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">AClass</span> <span class="special">&amp;);</span>
  <span class="special">};</span>
</pre>
<p>
      Finally we invoke our metafunction and return our value. This all happens at
      compile time, and can be used by programmers doing compile time template metaprogramming.
    </p>
<pre class="programlisting"><span class="identifier">has_function_function1</span><span class="special">&lt;</span><span class="identifier">Top</span><span class="special">,</span><span class="keyword">int</span><span class="special">&gt;::</span><span class="identifier">value</span><span class="special">;</span> <span class="comment">// true</span>
<span class="identifier">has_function_function1</span><span class="special">&lt;</span><span class="identifier">Top2</span><span class="special">,</span><span class="keyword">int</span><span class="special">&gt;::</span><span class="identifier">value</span><span class="special">;</span> <span class="comment">// false</span>

<span class="identifier">has_function_function2</span><span class="special">&lt;</span><span class="identifier">Top</span><span class="special">,</span><span class="identifier">AClass</span><span class="special">,</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">,</span><span class="keyword">short</span> <span class="special">*&gt;</span> <span class="special">&gt;::</span><span class="identifier">value</span><span class="special">;</span> <span class="comment">// true</span>
<span class="identifier">has_function_function2</span><span class="special">&lt;</span><span class="identifier">Top2</span><span class="special">,</span><span class="identifier">AClass</span><span class="special">,</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">,</span><span class="keyword">short</span> <span class="special">*&gt;</span> <span class="special">&gt;::</span><span class="identifier">value</span><span class="special">;</span> <span class="comment">// false</span>

<span class="identifier">has_function_function3</span><span class="special">&lt;</span><span class="identifier">Top2</span><span class="special">,</span><span class="keyword">int</span><span class="special">&gt;::</span><span class="identifier">value</span><span class="special">;</span> <span class="comment">// false</span>
<span class="identifier">has_function_function3</span><span class="special">&lt;</span><span class="identifier">Top2</span><span class="special">,</span><span class="identifier">Top</span> <span class="special">*,</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="keyword">long</span><span class="special">,</span><span class="keyword">int</span><span class="special">,</span><span class="identifier">AClass</span> <span class="special">&amp;&gt;</span> <span class="special">&gt;::</span><span class="identifier">value</span><span class="special">;</span> <span class="comment">// true;</span>
</pre>
<h4>
<a name="the_type_traits_introspection_library.tti_detail_has_function.h3"></a>
      <span class="phrase"><a name="the_type_traits_introspection_library.tti_detail_has_function.metafunction_re_use"></a></span><a class="link" href="tti_detail_has_function.html#the_type_traits_introspection_library.tti_detail_has_function.metafunction_re_use">Metafunction
      re-use</a>
    </h4>
<p>
      The macro encodes only the name of the function for which we are searching
      and the fact that we are introspecting for a function within an enclosing type.
    </p>
<p>
      Because of this, once we create our metafunction for introspecting a function
      by name, we can reuse the metafunction for introspecting any enclosing type,
      having any function, for that name.
    </p>
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        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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