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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="the_type_traits_introspection_library.tti_detail_has_type"></a><a class="link" href="tti_detail_has_type.html" title="Introspecting an inner type">Introspecting
    an inner type</a>
</h2></div></div></div>
<p>
      The TTI macro <code class="computeroutput"><a class="link" href="../BOOST_TTI_HAS_TYPE.html" title="Macro BOOST_TTI_HAS_TYPE">BOOST_TTI_HAS_TYPE</a></code>
      introspects a nested type of a class.
    </p>
<p>
      The BOOST_TTI_HAS_TYPE macro takes a single parameter which is the name of
      an inner type whose existence the programmer wants to check. The macro generates
      a metafunction called "has_type_'name_of_inner_type'".
    </p>
<p>
      The main purpose of the generated metafunction is to check for the existence
      by name of the inner type. The metafunction can also be used to invoke an MPL
      lambda expression which is passed the inner type. One of the most common usages
      of the added functionality is to check whether or not the inner type is a typedef
      for another type.
    </p>
<p>
      The metafunction is invoked by passing it the enclosing type to introspect.
      A second type may be passed to the metafunction, an MPL lambda expression taking
      the inner type and returning a boolean constant.
    </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 type exists or not.
    </p>
<p>
      If a second optional type is passed, this type must be an MPL lambda expression
      and the expression will be invoked only if the inner type exists. In that case
      the metafunction returns true or false depending on whether the lambda expression
      returns true or false. If the inner type does not exist, the lambda expression,
      even if specified, is never invoked and the metafunction returns false.
    </p>
<h4>
<a name="the_type_traits_introspection_library.tti_detail_has_type.h0"></a>
      <span class="phrase"><a name="the_type_traits_introspection_library.tti_detail_has_type.generating_the_metafunction"></a></span><a class="link" href="tti_detail_has_type.html#the_type_traits_introspection_library.tti_detail_has_type.generating_the_metafunction">Generating
      the metafunction</a>
    </h4>
<p>
      You generate the metafunction by invoking the macro with the name of an inner
      type:
    </p>
<pre class="programlisting"><span class="identifier">BOOST_TTI_HAS_TYPE</span><span class="special">(</span><span class="identifier">AType</span><span class="special">)</span>
</pre>
<p>
      generates a metafunction called 'has_type_AType' in the current scope.
    </p>
<h4>
<a name="the_type_traits_introspection_library.tti_detail_has_type.h1"></a>
      <span class="phrase"><a name="the_type_traits_introspection_library.tti_detail_has_type.invoking_the_metafunction"></a></span><a class="link" href="tti_detail_has_type.html#the_type_traits_introspection_library.tti_detail_has_type.invoking_the_metafunction">Invoking
      the metafunction</a>
    </h4>
<p>
      You invoke the metafunction by instantiating the template with an enclosing
      type to introspect and, optionally, an MPL lambda expression. A return value
      called 'value' is a compile time bool constant.
    </p>
<pre class="programlisting"><span class="identifier">has_type_AType</span><span class="special">&lt;</span><span class="identifier">Enclosing_Type</span><span class="special">&gt;::</span><span class="identifier">value</span>
<span class="identifier">has_type_AType</span><span class="special">&lt;</span><span class="identifier">Enclosing_Type</span><span class="special">,</span><span class="identifier">ALambdaExpression</span><span class="special">&gt;::</span><span class="identifier">value</span>
</pre>
<h4>
<a name="the_type_traits_introspection_library.tti_detail_has_type.h2"></a>
      <span class="phrase"><a name="the_type_traits_introspection_library.tti_detail_has_type.examples"></a></span><a class="link" href="tti_detail_has_type.html#the_type_traits_introspection_library.tti_detail_has_type.examples">Examples</a>
    </h4>
<p>
      First we generate metafunctions for various inner type 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_type</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>

<span class="identifier">BOOST_TTI_HAS_TYPE</span><span class="special">(</span><span class="identifier">MyTypeDef</span><span class="special">)</span>
<span class="identifier">BOOST_TTI_HAS_TYPE</span><span class="special">(</span><span class="identifier">AType</span><span class="special">)</span>
<span class="identifier">BOOST_TTI_HAS_TYPE</span><span class="special">(</span><span class="identifier">ATypeDef</span><span class="special">)</span>
<span class="identifier">BOOST_TTI_HAS_TYPE</span><span class="special">(</span><span class="identifier">MyType</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">Top</span>
  <span class="special">{</span>
  <span class="keyword">typedef</span> <span class="keyword">int</span> <span class="identifier">MyTypeDef</span><span class="special">;</span>
  <span class="keyword">struct</span> <span class="identifier">AType</span> <span class="special">{</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">typedef</span> <span class="keyword">long</span> <span class="identifier">ATypeDef</span><span class="special">;</span>
  <span class="keyword">struct</span> <span class="identifier">MyType</span> <span class="special">{</span> <span class="special">};</span>
  <span class="special">};</span>
</pre>
<p>
      Finally we invoke our metafunction and return our value.
    </p>
<pre class="programlisting"><span class="identifier">has_type_MyTypeDef</span><span class="special">&lt;</span><span class="identifier">Top</span><span class="special">&gt;::</span><span class="identifier">value</span><span class="special">;</span>  <span class="comment">// true</span>
<span class="identifier">has_type_MyTypeDef</span><span class="special">&lt;</span><span class="identifier">Top2</span><span class="special">&gt;::</span><span class="identifier">value</span><span class="special">;</span> <span class="comment">// false</span>

<span class="identifier">has_type_AType</span><span class="special">&lt;</span><span class="identifier">Top</span><span class="special">&gt;::</span><span class="identifier">value</span><span class="special">;</span>  <span class="comment">// true</span>
<span class="identifier">has_type_AType</span><span class="special">&lt;</span><span class="identifier">Top2</span><span class="special">&gt;::</span><span class="identifier">value</span><span class="special">;</span> <span class="comment">// false</span>

<span class="identifier">has_type_ATypeDef</span><span class="special">&lt;</span><span class="identifier">Top</span><span class="special">&gt;::</span><span class="identifier">value</span><span class="special">;</span>  <span class="comment">// false</span>
<span class="identifier">has_type_ATypeDef</span><span class="special">&lt;</span><span class="identifier">Top2</span><span class="special">&gt;::</span><span class="identifier">value</span><span class="special">;</span> <span class="comment">// true</span>

<span class="identifier">has_type_MyType</span><span class="special">&lt;</span><span class="identifier">Top</span><span class="special">&gt;::</span><span class="identifier">value</span><span class="special">;</span>  <span class="comment">// false</span>
<span class="identifier">has_type_MyType</span><span class="special">&lt;</span><span class="identifier">Top2</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_type.h3"></a>
      <span class="phrase"><a name="the_type_traits_introspection_library.tti_detail_has_type.examples___using_lambda_expressions"></a></span><a class="link" href="tti_detail_has_type.html#the_type_traits_introspection_library.tti_detail_has_type.examples___using_lambda_expressions">Examples
      - using lambda expressions</a>
    </h4>
<p>
      We can further invoke our metafunction with a second type, which is an MPL
      lambda expression.
    </p>
<p>
      An MPL lambda expression, an extremely useful technique in template metaprogramming,
      allows us to pass a metafunction to other metafunctions. The metafunction we
      pass can be in the form of a placeholder expression or a metafunction class.
      In our case the metafunction passed to our has_type_'name_of_inner_type' metafunction
      as a lambda expression must return a boolean constant expression.
    </p>
<h4>
<a name="the_type_traits_introspection_library.tti_detail_has_type.h4"></a>
      <span class="phrase"><a name="the_type_traits_introspection_library.tti_detail_has_type.example___using_a_lambda_expression_with_a_placeholder_expression"></a></span><a class="link" href="tti_detail_has_type.html#the_type_traits_introspection_library.tti_detail_has_type.example___using_a_lambda_expression_with_a_placeholder_expression">Example
      - using a lambda expression with a placeholder expression</a>
    </h4>
<p>
      We will first illustrate the use of a lambda expression in the form of a placeholder
      expression being passed as the second template parameter to our has_type_'name_of_inner_type'
      metafunction. A popular and simple placeholder expression we can use is 'boost::is_same&lt;_1,SomeType&gt;'
      to check if the inner type found is a particular type. This is particularly
      useful when the inner type is a typedef for some other type.
    </p>
<p>
      First we include some more header files and a using declaration for convenience.
    </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">mpl</span><span class="special">/</span><span class="identifier">placeholders</span><span class="special">.</span><span class="identifier">hpp</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">type_traits</span><span class="special">/</span><span class="identifier">is_same</span><span class="special">.</span><span class="identifier">hpp</span>
<span class="keyword">using</span> <span class="keyword">namespace</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">placeholders</span><span class="special">;</span>
</pre>
<p>
      Next we invoke our metafunction:
    </p>
<pre class="programlisting"><span class="identifier">has_type_MyTypeDef</span><span class="special">&lt;</span><span class="identifier">Top</span><span class="special">,</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">is_same</span><span class="special">&lt;</span><span class="identifier">_1</span><span class="special">,</span><span class="keyword">int</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_type_MyTypeDef</span><span class="special">&lt;</span><span class="identifier">Top</span><span class="special">,</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">is_same</span><span class="special">&lt;</span><span class="identifier">_1</span><span class="special">,</span><span class="keyword">long</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_type_ATypeDef</span><span class="special">&lt;</span><span class="identifier">Top2</span><span class="special">,</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">is_same</span><span class="special">&lt;</span><span class="identifier">_1</span><span class="special">,</span><span class="keyword">int</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_type_ATypeDef</span><span class="special">&lt;</span><span class="identifier">Top2</span><span class="special">,</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">is_same</span><span class="special">&lt;</span><span class="identifier">_1</span><span class="special">,</span><span class="keyword">long</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>
</pre>
<h4>
<a name="the_type_traits_introspection_library.tti_detail_has_type.h5"></a>
      <span class="phrase"><a name="the_type_traits_introspection_library.tti_detail_has_type.example___using_a_lambda_expression_with_a_metafunction_class"></a></span><a class="link" href="tti_detail_has_type.html#the_type_traits_introspection_library.tti_detail_has_type.example___using_a_lambda_expression_with_a_metafunction_class">Example
      - using a lambda expression with a metafunction class</a>
    </h4>
<p>
      We will next illustrate the use of a lambda expression in the form of a metafunction
      class being passed as the second template parameter to our has_type_'name_of_inner_type'
      metafunction.
    </p>
<p>
      A metafunction class is a type which has a nested class template called 'apply'.
      For our metafunction class example we will check if the inner type is a built-in
      integer type. First let us write out metafunction class:
    </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">type_traits</span><span class="special">/</span><span class="identifier">is_integral</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>

<span class="keyword">class</span> <span class="identifier">OurMetafunctionClass</span>
  <span class="special">{</span>
  <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">&gt;</span> <span class="keyword">struct</span> <span class="identifier">apply</span> <span class="special">:</span>
      <span class="identifier">boost</span><span class="special">::</span><span class="identifier">is_integral</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span>
      <span class="special">{</span>
      <span class="special">};</span>
  <span class="special">};</span>
</pre>
<p>
      Now we can invoke our metafunction:
    </p>
<pre class="programlisting"><span class="identifier">has_type_MyTypeDef</span><span class="special">&lt;</span><span class="identifier">Top</span><span class="special">,</span><span class="identifier">OurMetafunctionClass</span><span class="special">&gt;::</span><span class="identifier">value</span><span class="special">;</span> <span class="comment">// true</span>
<span class="identifier">has_type_AType</span><span class="special">&lt;</span><span class="identifier">Top</span><span class="special">,</span><span class="identifier">OurMetafunctionClass</span><span class="special">&gt;::</span><span class="identifier">value</span><span class="special">;</span> <span class="comment">// false</span>

<span class="identifier">has_type_ATypeDef</span><span class="special">&lt;</span><span class="identifier">Top2</span><span class="special">,</span><span class="identifier">OurMetafunctionClass</span><span class="special">&gt;::</span><span class="identifier">value</span><span class="special">;</span> <span class="comment">// true</span>
<span class="identifier">has_type_MyType</span><span class="special">&lt;</span><span class="identifier">Top2</span><span class="special">,</span><span class="identifier">OurMetafunctionClass</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_type.h6"></a>
      <span class="phrase"><a name="the_type_traits_introspection_library.tti_detail_has_type.metafunction_re_use"></a></span><a class="link" href="tti_detail_has_type.html#the_type_traits_introspection_library.tti_detail_has_type.metafunction_re_use">Metafunction
      re-use</a>
    </h4>
<p>
      The macro encodes only the name of the inner type for which we are searching
      and the fact that we are introspecting for an inner type within an enclosing
      type.
    </p>
<p>
      Because of this, once we create our metafunction for introspecting an inner
      type by name, we can reuse the metafunction for introspecting any enclosing
      type, having any inner type, for that name.
    </p>
<p>
      Furthermore since we have only encoded the name of the inner type for which
      we are introspecting, we can not only introspect for that inner type by name
      but add different lambda expressions to inspect that inner type for whatever
      we want to find out about it using the same metafunction.
    </p>
</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
<td align="left"></td>
<td align="right"><div class="copyright-footer">Copyright &#169; 2011-2013 Tropic Software
      East Inc<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>)
      </p>
</div></td>
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