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<title>Introspecting member 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_member_function"></a><a class="link" href="tti_detail_has_member_function.html" title="Introspecting member function">Introspecting
    member function</a>
</h2></div></div></div>
<p>
      The TTI macro <code class="computeroutput"><a class="link" href="../BOOST_TT_idm46187185705488.html" title="Macro BOOST_TTI_HAS_MEMBER_FUNCTION">BOOST_TTI_HAS_MEMBER_FUNCTION</a></code>
      introspects a member function of a class.
    </p>
<p>
      BOOST_TTI_HAS_MEMBER_FUNCTION takes a single parameter which is the name of
      an inner member function whose existence the programmer wants to check. The
      macro generates a metafunction called "has_member_function_'name_of_inner_member_function'".
    </p>
<p>
      The metafunction can be invoked in two different ways.
    </p>
<p>
      The first way of invoking the metafunction is by passing it the enclosing type
      to introspect and a signature for the member function as a series of separate
      template arguments. The signature for the member function consists of the template
      arguments of a return type, of optional parameter types in the form of a boost::mpl
      forward sequence of types, and of an optional Boost FunctionTypes tag type.
      A typical boost::mpl forward sequence of types is a boost::mpl::vector&lt;&gt;.
    </p>
<p>
      The optional Boost FunctionTypes tag type may be used to specify cv-qualification.
      This means you can add 'const', 'volatile', or both by specifying an appropriate
      tag type. An alternate to using the tag type is to specify the enclosing type
      as 'const', 'volatile', or both. As an example if you specify the tag type
      as 'boost::function_types::const_qualified' or if you specify the enclosing
      type as 'const T', the member function which you are introspecting must be
      a const function.
    </p>
<p>
      The second way of invoking the metafunction is by passing it a single parameter,
      which is a pointer to member function. This type has the form of:
    </p>
<pre class="programlisting"><span class="identifier">Return_Type</span> <span class="special">(</span> <span class="identifier">Enclosing_Type</span><span class="special">::*</span> <span class="special">)</span> <span class="special">(</span> <span class="identifier">Parameter_Types</span> <span class="special">)</span> <span class="identifier">cv_qualifier</span><span class="special">(</span><span class="identifier">s</span><span class="special">)</span>
</pre>
<p>
      where the Parameter_Types may be empty, or a comma-separated list of parameter
      types if there are more than one parameter type. The cv-qualifier may be 'const',
      'volatile', or 'const volatile'.
    </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 'value' is true or false
      depending on whether the inner member function, of the specified signature,
      exists or not.
    </p>
<h4>
<a name="the_type_traits_introspection_library.tti_detail_has_member_function.h0"></a>
      <span class="phrase"><a name="the_type_traits_introspection_library.tti_detail_has_member_function.generating_the_metafunction"></a></span><a class="link" href="tti_detail_has_member_function.html#the_type_traits_introspection_library.tti_detail_has_member_function.generating_the_metafunction">Generating
      the metafunction</a>
    </h4>
<p>
      You generate the metafunction by invoking the macro with the name of an inner
      member function:
    </p>
<pre class="programlisting"><span class="identifier">BOOST_TTI_HAS_MEMBER_FUNCTION</span><span class="special">(</span><span class="identifier">AMemberFunction</span><span class="special">)</span>
</pre>
<p>
      generates a metafunction called 'has_member_function_AMemberFunction' in the
      current scope.
    </p>
<h4>
<a name="the_type_traits_introspection_library.tti_detail_has_member_function.h1"></a>
      <span class="phrase"><a name="the_type_traits_introspection_library.tti_detail_has_member_function.invoking_the_metafunction"></a></span><a class="link" href="tti_detail_has_member_function.html#the_type_traits_introspection_library.tti_detail_has_member_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 member function as a series of
      template parameters. Alternatively you can invoke the metafunction by passing
      it a single type which is a pointer to member function.
    </p>
<p>
      A return value called 'value' is a compile time bool constant.
    </p>
<pre class="programlisting"><span class="identifier">has_member_function_AMemberFunction</span>
  <span class="special">&lt;</span>
  <span class="identifier">Enclosing_Type</span><span class="special">,</span>
  <span class="identifier">MemberFunction_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">MemberFunction_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>

<span class="identifier">OR</span>

<span class="identifier">has_member_function_AMemberFunction</span>
  <span class="special">&lt;</span>
  <span class="identifier">MemberFunction_ReturnType</span> <span class="special">(</span><span class="identifier">Enclosing_Type</span><span class="special">::*)</span> <span class="special">(</span><span class="identifier">MemberFunction_ParameterTypes</span><span class="special">)</span> <span class="identifier">optional_cv_qualification</span>
  <span class="special">&gt;::</span><span class="identifier">value</span>
</pre>
<h4>
<a name="the_type_traits_introspection_library.tti_detail_has_member_function.h2"></a>
      <span class="phrase"><a name="the_type_traits_introspection_library.tti_detail_has_member_function.examples"></a></span><a class="link" href="tti_detail_has_member_function.html#the_type_traits_introspection_library.tti_detail_has_member_function.examples">Examples</a>
    </h4>
<p>
      First we generate metafunctions for various inner member 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_member_function</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>

<span class="identifier">BOOST_TTI_HAS_MEMBER_FUNCTION</span><span class="special">(</span><span class="identifier">function1</span><span class="special">)</span>
<span class="identifier">BOOST_TTI_HAS_MEMBER_FUNCTION</span><span class="special">(</span><span class="identifier">function2</span><span class="special">)</span>
<span class="identifier">BOOST_TTI_HAS_MEMBER_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">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="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>
<p>
      We will show both forms in the following examples. Both forms are completely
      interchangeable as to the result desired.
    </p>
<pre class="programlisting"><span class="identifier">has_member_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_member_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">,</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;&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_member_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_member_function_function2</span><span class="special">&lt;</span><span class="identifier">AClass</span> <span class="special">(</span><span class="identifier">Top</span><span class="special">::*)</span> <span class="special">(</span><span class="keyword">double</span><span class="special">,</span><span class="keyword">short</span> <span class="special">*)&gt;::</span><span class="identifier">value</span><span class="special">;</span> <span class="comment">// true</span>
<span class="identifier">has_member_function_function2</span><span class="special">&lt;</span><span class="identifier">AClass</span> <span class="special">(</span><span class="identifier">Top2</span><span class="special">::*)</span> <span class="special">(</span><span class="keyword">double</span><span class="special">,</span><span class="keyword">short</span> <span class="special">*)&gt;::</span><span class="identifier">value</span><span class="special">;</span> <span class="comment">// false</span>
<span class="identifier">has_member_function_function2</span><span class="special">&lt;</span><span class="keyword">long</span> <span class="special">(</span><span class="identifier">Top2</span><span class="special">::*)</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">)&gt;::</span><span class="identifier">value</span><span class="special">;</span> <span class="comment">// true</span>

<span class="identifier">has_member_function_function3</span><span class="special">&lt;</span><span class="keyword">int</span> <span class="special">(</span><span class="identifier">Top2</span><span class="special">::*)</span> <span class="special">()&gt;::</span><span class="identifier">value</span><span class="special">;</span> <span class="comment">// false</span>
<span class="identifier">has_member_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_member_function.h3"></a>
      <span class="phrase"><a name="the_type_traits_introspection_library.tti_detail_has_member_function.metafunction_re_use"></a></span><a class="link" href="tti_detail_has_member_function.html#the_type_traits_introspection_library.tti_detail_has_member_function.metafunction_re_use">Metafunction
      re-use</a>
    </h4>
<p>
      The macro encodes only the name of the member function for which we are searching
      and the fact that we are introspecting for a member function within an enclosing
      type.
    </p>
<p>
      Because of this, once we create our metafunction for introspecting a member
      function by name, we can reuse the metafunction for introspecting any enclosing
      type, having any member function, for that name.
    </p>
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