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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_template"></a><a class="link" href="tti_detail_has_template.html" title="Introspecting an inner class template">Introspecting |
| an inner class template</a> |
| </h2></div></div></div> |
| <div class="toc"><dl class="toc"> |
| <dt><span class="section"><a href="tti_detail_has_template.html#the_type_traits_introspection_library.tti_detail_has_template.tti_detail_has_template_macro">Using |
| the BOOST_TTI_HAS_TEMPLATE macro</a></span></dt> |
| <dt><span class="section"><a href="tti_detail_has_template/tti_detail_has_template_metafunction.html">Using |
| the has_template_(xxx) metafunction</a></span></dt> |
| </dl></div> |
| <div class="section"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="the_type_traits_introspection_library.tti_detail_has_template.tti_detail_has_template_macro"></a><a class="link" href="tti_detail_has_template.html#the_type_traits_introspection_library.tti_detail_has_template.tti_detail_has_template_macro" title="Using the BOOST_TTI_HAS_TEMPLATE macro">Using |
| the BOOST_TTI_HAS_TEMPLATE macro</a> |
| </h3></div></div></div> |
| <p> |
| The TTI macro <code class="computeroutput"><a class="link" href="../BOOST_TTI_HAS_TEMPLATE.html" title="Macro BOOST_TTI_HAS_TEMPLATE">BOOST_TTI_HAS_TEMPLATE</a></code> |
| introspects an inner class template of a class. The macro must specify, at |
| the least, the name of the class template to introspect. |
| </p> |
| <h5> |
| <a name="the_type_traits_introspection_library.tti_detail_has_template.tti_detail_has_template_macro.h0"></a> |
| <span class="phrase"><a name="the_type_traits_introspection_library.tti_detail_has_template.tti_detail_has_template_macro.two_forms_of_introspection"></a></span><a class="link" href="tti_detail_has_template.html#the_type_traits_introspection_library.tti_detail_has_template.tti_detail_has_template_macro.two_forms_of_introspection">Two |
| forms of introspection</a> |
| </h5> |
| <p> |
| There are two general forms of template introspection which can be used. |
| The first is to find a class template with any number of only template type |
| parameters ( template parameters starting with <code class="computeroutput"><span class="keyword">class</span></code> |
| or <code class="computeroutput"><span class="keyword">typename</span></code> ). In this form |
| only the name of the class template needs to be specified when invoking the |
| macro. We will call this form of the macro the <code class="computeroutput"><span class="keyword">template</span> |
| <span class="identifier">type</span> <span class="identifier">parameters</span></code> |
| form. An example of a class template of this form which could be successfully |
| introspected would be: |
| </p> |
| <pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">X</span><span class="special">,</span><span class="keyword">typename</span> <span class="identifier">Y</span><span class="special">,</span><span class="keyword">class</span> <span class="identifier">Z</span><span class="special">,</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">class</span> <span class="identifier">AClassTemplate</span> <span class="special">{</span> <span class="comment">/* etc. */</span> <span class="special">};</span> |
| </pre> |
| <p> |
| The second is to find a class template with specific template parameters. |
| In this form both the name of the class template and the template parameters |
| are passed to the macro. |
| </p> |
| <p> |
| We will call this form of the macro the <code class="computeroutput"><span class="identifier">specific</span> |
| <span class="identifier">parameters</span></code> form. An example of |
| a class template of this form which could be successfully introspected would |
| be: |
| </p> |
| <pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">X</span><span class="special">,</span> <span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span><span class="special">></span> <span class="keyword">class</span> <span class="identifier">Y</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">Z</span><span class="special">></span> <span class="identifier">BClassTemplate</span> <span class="special">{</span> <span class="comment">/* etc. */</span> <span class="special">};</span> |
| </pre> |
| <p> |
| When using the specific form of the macro, there are two things which need |
| to be understood when passing the template parameters to the macro. First, |
| the actual names of the template parameters passed are irrelevant. They can |
| be left out completely or be different from the names in the nested class |
| template itself. Second, the use of 'typename' or 'class', when referring |
| to a template type parameter, is completely interchangeable, as it is in |
| the actual class template itself. |
| </p> |
| <h5> |
| <a name="the_type_traits_introspection_library.tti_detail_has_template.tti_detail_has_template_macro.h1"></a> |
| <span class="phrase"><a name="the_type_traits_introspection_library.tti_detail_has_template.tti_detail_has_template_macro.variadic_and_non_variadic_macro_usage"></a></span><a class="link" href="tti_detail_has_template.html#the_type_traits_introspection_library.tti_detail_has_template.tti_detail_has_template_macro.variadic_and_non_variadic_macro_usage">Variadic |
| and non-variadic macro usage</a> |
| </h5> |
| <p> |
| When using the BOOST_TTI_HAS_TEMPLATE macro we distinguish between compilers |
| supporting variadic macros or not supporting variadic macros. |
| </p> |
| <p> |
| The programmer can always tell whether or not the compiler supports variadic |
| macros by checking the value of the macro BOOST_PP_VARIADIC after including |
| the necessary header file <code class="computeroutput"><span class="identifier">boost</span><span class="special">/</span><span class="identifier">tti</span><span class="special">/</span><span class="identifier">has_template</span><span class="special">.</span><span class="identifier">hpp</span></code> in order to use the BOOST_TTI_TEMPLATE |
| macro. A value of 1 indicates the compiler supports variadic macros while |
| a value of 0 indicates the compiler does not support variadic macros. |
| </p> |
| <p> |
| Modern C++ compilers, in supporting the latest C++11 standard, normally support |
| variadic macros. Even before the latest C++11 standard a number of C++ compilers |
| already supported variadic macros. If you feel your compiler supports variadic |
| macros and BOOST_PP_VARIADIC is 0 even after including <code class="computeroutput"><span class="identifier">boost</span><span class="special">/</span><span class="identifier">tti</span><span class="special">/</span><span class="identifier">has_template</span><span class="special">.</span><span class="identifier">hpp</span></code>, you can predefine BOOST_PP_VARIADIC |
| to 1 before including <code class="computeroutput"><span class="identifier">boost</span><span class="special">/</span><span class="identifier">tti</span><span class="special">/</span><span class="identifier">has_template</span><span class="special">.</span><span class="identifier">hpp</span></code>. |
| </p> |
| <h5> |
| <a name="the_type_traits_introspection_library.tti_detail_has_template.tti_detail_has_template_macro.h2"></a> |
| <span class="phrase"><a name="the_type_traits_introspection_library.tti_detail_has_template.tti_detail_has_template_macro.non_variadic_macro_usage"></a></span><a class="link" href="tti_detail_has_template.html#the_type_traits_introspection_library.tti_detail_has_template.tti_detail_has_template_macro.non_variadic_macro_usage">Non-variadic |
| macro usage</a> |
| </h5> |
| <p> |
| We start with syntax for compilers not supporting variadic macros since this |
| syntax can also be used by compilers which do support variadic macros. The |
| form for non-variadic macros always takes two macro parameters. The first |
| macro parameter is always the name of the class template you are trying to |
| introspect. |
| </p> |
| <p> |
| The second macro parameter, when using the <code class="computeroutput"><span class="identifier">specific</span> |
| <span class="identifier">parameters</span></code> form of the macro, is |
| the template parameters in the form of a Boost preprocessor library array |
| data type. When using the <code class="computeroutput"><span class="keyword">template</span> |
| <span class="identifier">type</span> <span class="identifier">parameters</span></code> |
| form of the macro the second macro parameter is BOOST_PP_NIL. If the second |
| parameter is neither a Boost preprocessor library array data type or BOOS_PP_NIL |
| you will get a compiler error if your compiler only supports non-variadic |
| macros. |
| </p> |
| <p> |
| The non-variadic macro form for introspecting the class templates above using |
| the <code class="computeroutput"><span class="keyword">template</span> <span class="identifier">type</span> |
| <span class="identifier">parameters</span></code> form would be: |
| </p> |
| <pre class="programlisting"><span class="identifier">BOOST_TTI_TEMPLATE</span><span class="special">(</span><span class="identifier">AClassTemplate</span><span class="special">,</span><span class="identifier">BOOST_PP_NIL</span><span class="special">)</span> |
| <span class="identifier">BOOST_TTI_TEMPLATE</span><span class="special">(</span><span class="identifier">BClassTemplate</span><span class="special">,</span><span class="identifier">BOOST_PP_NIL</span><span class="special">)</span> |
| </pre> |
| <p> |
| Invoking the metafunction in the second case would always fail since the |
| BClassTemplate does not have all template type parameters. |
| </p> |
| <p> |
| The non-variadic macro form for introspecting the class templates above using |
| the <code class="computeroutput"><span class="identifier">specific</span> <span class="identifier">parameters</span></code> |
| form would be: |
| </p> |
| <pre class="programlisting"><span class="identifier">BOOST_TTI_TEMPLATE</span><span class="special">(</span><span class="identifier">AClassTemplate</span><span class="special">,(</span><span class="number">4</span><span class="special">,(</span><span class="keyword">class</span><span class="special">,</span><span class="keyword">typename</span><span class="special">,</span><span class="keyword">class</span><span class="special">,</span><span class="keyword">typename</span><span class="special">)))</span> |
| <span class="identifier">BOOST_TTI_TEMPLATE</span><span class="special">(</span><span class="identifier">BClassTemplate</span><span class="special">,(</span><span class="number">3</span><span class="special">,(</span><span class="keyword">class</span><span class="special">,</span> <span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span><span class="special">></span> <span class="keyword">class</span><span class="special">,</span> <span class="keyword">int</span><span class="special">)))</span> |
| </pre> |
| <p> |
| You need to be careful using the non-variadic <code class="computeroutput"><span class="identifier">specific</span> |
| <span class="identifier">parameters</span></code> form to specify the |
| correct number of array parameters. This can sometimes be tricky if you have |
| a template template parameter, or a non-type template parameter which has |
| parentheses surrounding part of the type specification. In the latter case, |
| when parentheses surround a comma ( ',' ), do not count that as creating |
| another Boost PP array token. Two examples: |
| </p> |
| <pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">void</span> <span class="special">(*</span><span class="identifier">FunctionPointer</span><span class="special">)(</span><span class="keyword">int</span><span class="special">,</span><span class="keyword">long</span><span class="special">)></span> <span class="keyword">class</span> <span class="identifier">CClassTemplate</span> <span class="special">{</span> <span class="comment">/* etc. */</span> <span class="special">};</span> |
| <span class="keyword">template</span><span class="special"><</span><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span><span class="special">,</span><span class="keyword">class</span><span class="special">></span> <span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">class</span> <span class="identifier">DClassTemplate</span> <span class="special">{</span> <span class="comment">/* etc. */</span> <span class="special">};</span> |
| |
| <span class="identifier">BOOST_TTI_TEMPLATE</span><span class="special">(</span><span class="identifier">CClassTemplate</span><span class="special">,(</span><span class="number">1</span><span class="special">,(</span><span class="keyword">void</span> <span class="special">(*)(</span><span class="keyword">int</span><span class="special">,</span><span class="keyword">long</span><span class="special">))))</span> |
| <span class="identifier">BOOST_TTI_TEMPLATE</span><span class="special">(</span><span class="identifier">DClassTemplate</span><span class="special">,(</span><span class="number">2</span><span class="special">,(</span><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span><span class="special">,</span><span class="keyword">class</span><span class="special">></span> <span class="keyword">class</span><span class="special">)))</span> |
| </pre> |
| <p> |
| In the case of using the macro to introspect CClassTemplate the number of |
| Boost PP array parameters is 1, even though there is a comma separating the |
| tokens in <code class="computeroutput"><span class="keyword">void</span> <span class="special">(*</span><span class="identifier">FunctionPointer</span><span class="special">)(</span><span class="keyword">int</span><span class="special">,</span><span class="keyword">long</span><span class="special">)</span></code>. This is because the comma is within parentheses. |
| </p> |
| <p> |
| In the case of using the macro to introspect DClassTemplate the number of |
| Boost PP array parameters is 2, because there is a comma separating the tokens |
| in <code class="computeroutput"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span><span class="special">,</span><span class="keyword">class</span><span class="special">></span> <span class="keyword">class</span> <span class="identifier">T</span></code>. |
| </p> |
| <h5> |
| <a name="the_type_traits_introspection_library.tti_detail_has_template.tti_detail_has_template_macro.h3"></a> |
| <span class="phrase"><a name="the_type_traits_introspection_library.tti_detail_has_template.tti_detail_has_template_macro.variadic_macro_usage"></a></span><a class="link" href="tti_detail_has_template.html#the_type_traits_introspection_library.tti_detail_has_template.tti_detail_has_template_macro.variadic_macro_usage">Variadic |
| macro usage</a> |
| </h5> |
| <p> |
| Having the ability to use variadic macros makes the syntax for using BOOST_TTI_TEMPLATE |
| easier to specify in both the <code class="computeroutput"><span class="keyword">template</span> |
| <span class="identifier">type</span> <span class="identifier">parameters</span></code> |
| form and the <code class="computeroutput"><span class="identifier">specific</span> <span class="identifier">parameters</span></code> |
| form of using the macro. This is because variadic macros can take a variable |
| number of parameters. When using the variadic macro form the first macro |
| parameter is always the name of the class template you are trying to introspect. |
| You only specify further parameters when using the <code class="computeroutput"><span class="identifier">specific</span> |
| <span class="identifier">parameters</span></code> form of the macro, in |
| which case the further parameters to the macro are the specific template |
| parameters. |
| </p> |
| <p> |
| Introspecting the first class template above using the <code class="computeroutput"><span class="keyword">template</span> |
| <span class="identifier">type</span> <span class="identifier">parameters</span></code> |
| form the variadic macro would be: |
| </p> |
| <pre class="programlisting"><span class="identifier">BOOST_TTI_TEMPLATE</span><span class="special">(</span><span class="identifier">AClassTemplate</span><span class="special">)</span> |
| </pre> |
| <p> |
| Introspecting the other class templates above using the <code class="computeroutput"><span class="identifier">specific</span> |
| <span class="identifier">parameters</span></code> form the variadic macros |
| would be: |
| </p> |
| <pre class="programlisting"><span class="identifier">BOOST_TTI_TEMPLATE</span><span class="special">(</span><span class="identifier">BClassTemplate</span><span class="special">,</span><span class="keyword">class</span><span class="special">,</span><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span><span class="special">></span> <span class="keyword">class</span><span class="special">,</span> <span class="keyword">int</span><span class="special">)</span> |
| <span class="identifier">BOOST_TTI_TEMPLATE</span><span class="special">(</span><span class="identifier">CClassTemplate</span><span class="special">,</span><span class="keyword">void</span> <span class="special">(*)(</span><span class="keyword">int</span><span class="special">,</span><span class="keyword">long</span><span class="special">))</span> |
| <span class="identifier">BOOST_TTI_TEMPLATE</span><span class="special">(</span><span class="identifier">DClassTemplate</span><span class="special">,</span><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span><span class="special">,</span><span class="keyword">class</span><span class="special">></span> <span class="keyword">class</span><span class="special">)</span> |
| </pre> |
| <p> |
| Here we have no problem with counting the number of tuple tokens for the |
| Boost PP array, nor do we have to specify BOOST_PP_NIL if we are using the |
| <code class="computeroutput"><span class="keyword">template</span> <span class="identifier">type</span> |
| <span class="identifier">parameters</span></code> form. Also for the specific |
| parameters form we simply use the template parameters as the remaining tokens |
| of the variadic macro. |
| </p> |
| <h5> |
| <a name="the_type_traits_introspection_library.tti_detail_has_template.tti_detail_has_template_macro.h4"></a> |
| <span class="phrase"><a name="the_type_traits_introspection_library.tti_detail_has_template.tti_detail_has_template_macro.the_resulting_metafunction"></a></span><a class="link" href="tti_detail_has_template.html#the_type_traits_introspection_library.tti_detail_has_template.tti_detail_has_template_macro.the_resulting_metafunction">The |
| resulting metafunction</a> |
| </h5> |
| <p> |
| Using either form of the macro, whether using variadic or non-variadic syntax, |
| the macro generates a metafunction called 'has_template_'name_of_inner_class_template'. |
| </p> |
| <p> |
| The metafunction can be invoked by passing it the enclosing type to introspect. |
| </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 class template exists or not. |
| </p> |
| </div> |
| </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> |
| </tr></table> |
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