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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"><</span><span class="identifier">Enclosing_Type</span><span class="special">>::</span><span class="identifier">value</span> |
| <span class="identifier">has_type_AType</span><span class="special"><</span><span class="identifier">Enclosing_Type</span><span class="special">,</span><span class="identifier">ALambdaExpression</span><span class="special">>::</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"><</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">></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"><</span><span class="identifier">Top</span><span class="special">>::</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"><</span><span class="identifier">Top2</span><span class="special">>::</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"><</span><span class="identifier">Top</span><span class="special">>::</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"><</span><span class="identifier">Top2</span><span class="special">>::</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"><</span><span class="identifier">Top</span><span class="special">>::</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"><</span><span class="identifier">Top2</span><span class="special">>::</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"><</span><span class="identifier">Top</span><span class="special">>::</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"><</span><span class="identifier">Top2</span><span class="special">>::</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<_1,SomeType>' |
| 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"><</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"><</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"><</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"><</span><span class="identifier">_1</span><span class="special">,</span><span class="keyword">int</span><span class="special">></span> <span class="special">>::</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"><</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"><</span><span class="identifier">_1</span><span class="special">,</span><span class="keyword">long</span><span class="special">></span> <span class="special">>::</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"><</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"><</span><span class="identifier">_1</span><span class="special">,</span><span class="keyword">int</span><span class="special">></span> <span class="special">>::</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"><</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"><</span><span class="identifier">_1</span><span class="special">,</span><span class="keyword">long</span><span class="special">></span> <span class="special">>::</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"><</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">></span> |
| |
| <span class="keyword">class</span> <span class="identifier">OurMetafunctionClass</span> |
| <span class="special">{</span> |
| <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">apply</span> <span class="special">:</span> |
| <span class="identifier">boost</span><span class="special">::</span><span class="identifier">is_integral</span><span class="special"><</span><span class="identifier">T</span><span class="special">></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"><</span><span class="identifier">Top</span><span class="special">,</span><span class="identifier">OurMetafunctionClass</span><span class="special">>::</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"><</span><span class="identifier">Top</span><span class="special">,</span><span class="identifier">OurMetafunctionClass</span><span class="special">>::</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"><</span><span class="identifier">Top2</span><span class="special">,</span><span class="identifier">OurMetafunctionClass</span><span class="special">>::</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"><</span><span class="identifier">Top2</span><span class="special">,</span><span class="identifier">OurMetafunctionClass</span><span class="special">>::</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 © 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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