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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_data"></a><a class="link" href="tti_detail_has_member_data.html" title="Introspecting member data">Introspecting |
| member data</a> |
| </h2></div></div></div> |
| <p> |
| The TTI macro <code class="computeroutput"><a class="link" href="../BOOST_TTI_HAS_MEMBER_DATA.html" title="Macro BOOST_TTI_HAS_MEMBER_DATA">BOOST_TTI_HAS_MEMBER_DATA</a></code> |
| introspects member data of a class. |
| </p> |
| <p> |
| BOOST_TTI_HAS_MEMBER_DATA macro takes a single parameter which is the name |
| of an inner member data whose existence the programmer wants to check. The |
| macro generates a metafunction called 'has_member_data_'name_of_inner_member_data'. |
| </p> |
| <p> |
| The metafunction can be invoked in two different ways. |
| </p> |
| <p> |
| The first way is by passing it two parameters. The first parameter is the enclosing |
| type to introspect and the second parameter is the type of the member data. |
| </p> |
| <p> |
| The second way is by passing it a single parameter, which is a pointer to member |
| type. This type has the form of: |
| </p> |
| <pre class="programlisting"><span class="identifier">MemberData_Type</span> <span class="identifier">Enclosing_Type</span><span class="special">::*</span> |
| </pre> |
| <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 data, of the specified type, exists or not. |
| </p> |
| <h4> |
| <a name="the_type_traits_introspection_library.tti_detail_has_member_data.h0"></a> |
| <span class="phrase"><a name="the_type_traits_introspection_library.tti_detail_has_member_data.generating_the_metafunction"></a></span><a class="link" href="tti_detail_has_member_data.html#the_type_traits_introspection_library.tti_detail_has_member_data.generating_the_metafunction">Generating |
| the metafunction</a> |
| </h4> |
| <p> |
| You generate the metafunction by invoking the macro with the name of an inner |
| member data: |
| </p> |
| <pre class="programlisting"><span class="identifier">BOOST_TTI_HAS_MEMBER_DATA</span><span class="special">(</span><span class="identifier">AMemberData</span><span class="special">)</span> |
| </pre> |
| <p> |
| generates a metafunction called 'has_member_data_AMemberData' in the current |
| scope. |
| </p> |
| <h4> |
| <a name="the_type_traits_introspection_library.tti_detail_has_member_data.h1"></a> |
| <span class="phrase"><a name="the_type_traits_introspection_library.tti_detail_has_member_data.invoking_the_metafunction"></a></span><a class="link" href="tti_detail_has_member_data.html#the_type_traits_introspection_library.tti_detail_has_member_data.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 type of the member data, or by instantiating the |
| template with a pointer to member data type. The return value called 'value' |
| is a compile time bool constant telling you whether or not the member data |
| . exists. |
| </p> |
| <pre class="programlisting"><span class="identifier">has_member_data_AMemberData</span><span class="special"><</span><span class="identifier">Enclosing_Type</span><span class="special">,</span><span class="identifier">MemberData_Type</span><span class="special">>::</span><span class="identifier">value</span> |
| |
| <span class="identifier">OR</span> |
| |
| <span class="identifier">has_member_data_AMemberData</span><span class="special"><</span><span class="identifier">MemberData_Type</span> <span class="identifier">Enclosing_Type</span><span class="special">::*>::</span><span class="identifier">value</span> |
| </pre> |
| <h4> |
| <a name="the_type_traits_introspection_library.tti_detail_has_member_data.h2"></a> |
| <span class="phrase"><a name="the_type_traits_introspection_library.tti_detail_has_member_data.examples"></a></span><a class="link" href="tti_detail_has_member_data.html#the_type_traits_introspection_library.tti_detail_has_member_data.examples">Examples</a> |
| </h4> |
| <p> |
| First we generate metafunctions for various inner member data 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_member_data</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> |
| |
| <span class="identifier">BOOST_TTI_HAS_MEMBER_DATA</span><span class="special">(</span><span class="identifier">data1</span><span class="special">)</span> |
| <span class="identifier">BOOST_TTI_HAS_MEMBER_DATA</span><span class="special">(</span><span class="identifier">data2</span><span class="special">)</span> |
| <span class="identifier">BOOST_TTI_HAS_MEMBER_DATA</span><span class="special">(</span><span class="identifier">data3</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">data1</span><span class="special">;</span> |
| <span class="identifier">AClass</span> <span class="special">*</span> <span class="identifier">data2</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">data1</span><span class="special">;</span> |
| <span class="identifier">Top</span> <span class="identifier">data3</span><span class="special">;</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_data_data1</span><span class="special"><</span><span class="identifier">Top</span><span class="special">,</span><span class="keyword">int</span><span class="special">>::</span><span class="identifier">value</span><span class="special">;</span> <span class="comment">// true</span> |
| <span class="identifier">has_member_data_data1</span><span class="special"><</span><span class="identifier">Top</span><span class="special">,</span><span class="keyword">long</span><span class="special">>::</span><span class="identifier">value</span><span class="special">;</span> <span class="comment">// false</span> |
| |
| <span class="identifier">has_member_data_data1</span><span class="special"><</span><span class="identifier">Top2</span><span class="special">,</span><span class="keyword">int</span><span class="special">>::</span><span class="identifier">value</span><span class="special">;</span> <span class="comment">// false</span> |
| <span class="identifier">has_member_data_data1</span><span class="special"><</span><span class="keyword">long</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_member_data_data2</span><span class="special"><</span><span class="identifier">AClass</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_member_data_data2</span><span class="special"><</span><span class="identifier">Top</span><span class="special">,</span><span class="keyword">int</span> <span class="special">*>::</span><span class="identifier">value</span><span class="special">;</span> <span class="comment">// false</span> |
| |
| <span class="identifier">has_member_data_data3</span><span class="special"><</span><span class="keyword">int</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_member_data_data3</span><span class="special"><</span><span class="identifier">Top</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_member_data.h3"></a> |
| <span class="phrase"><a name="the_type_traits_introspection_library.tti_detail_has_member_data.metafunction_re_use"></a></span><a class="link" href="tti_detail_has_member_data.html#the_type_traits_introspection_library.tti_detail_has_member_data.metafunction_re_use">Metafunction |
| re-use</a> |
| </h4> |
| <p> |
| The macro encodes only the name of the member data for which we are searching |
| and the fact that we are introspecting for member data within an enclosing |
| type. |
| </p> |
| <p> |
| Because of this, once we create our metafunction for introspecting an inner |
| member data by name, we can reuse the metafunction for introspecting any enclosing |
| type, having any inner member data type, for that name. |
| </p> |
| </div> |
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| 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>) |
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