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| <h2><span class="refentrytitle">Macro BOOST_TTI_TRAIT_HAS_TYPE</span></h2> |
| <p>BOOST_TTI_TRAIT_HAS_TYPE</p> |
| </div> |
| <h2 xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="refsynopsisdiv-title">Synopsis</h2> |
| <div xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="refsynopsisdiv"><pre class="synopsis"><span class="comment">// In header: <<a class="link" href="header/boost/tti/has_type_hpp.html" title="Header <boost/tti/has_type.hpp>">boost/tti/has_type.hpp</a>> |
| |
| </span>BOOST_TTI_TRAIT_HAS_TYPE(trait, name)</pre></div> |
| <div class="refsect1"> |
| <a name="idp103096816"></a><h2>Description</h2> |
| <p>BOOST_TTI_TRAIT_HAS_TYPE is a macro which expands to a metafunction. The metafunction tests whether an inner type with a particular name exists and, optionally, whether a lambda expression invoked with the inner type is true or not.</p> |
| <p>trait = the name of the metafunction within the tti namespace.</p> |
| <p>name = the name of the inner type.</p> |
| <p>generates a metafunction called "trait" where 'trait' is the macro parameter. template<class BOOST_TTI_TP_T,class BOOST_TTI_TP_U> |
| struct trait |
| { |
| static const value = unspecified; |
| typedef mpl::bool_<true-or-false> type; |
| }; |
| |
| The metafunction types and return: |
| |
| BOOST_TTI_TP_T = the enclosing type in which to look for our 'name'. |
| |
| BOOST_TTI_TP_U = (optional) An optional template parameter, defaulting to a marker type. |
| If specified it is an MPL lambda expression which is invoked |
| with the inner type found and must return a constant boolean |
| value. |
| |
| returns = 'value' depends on whether or not the optional BOOST_TTI_TP_U is specified. |
| |
| If BOOST_TTI_TP_U is not specified, then 'value' is true if the 'name' type |
| exists within the enclosing type BOOST_TTI_TP_T; otherwise 'value' is false. |
| |
| If BOOST_TTI_TP_U is specified , then 'value' is true if the 'name' type exists |
| within the enclosing type BOOST_TTI_TP_T and the lambda expression as specified |
| by BOOST_TTI_TP_U, invoked by passing the actual inner type of 'name', returns |
| a 'value' of true; otherwise 'value' is false. |
| |
| The action taken with BOOST_TTI_TP_U occurs only when the 'name' type exists |
| within the enclosing type BOOST_TTI_TP_T. |
| </p> |
| <p>Example usage:</p> |
| <p>BOOST_TTI_TRAIT_HAS_TYPE(LookFor,MyType) generates the metafunction LookFor in the current scope to look for an inner type called MyType.</p> |
| <p>LookFor<EnclosingType>::value is true if MyType is an inner type of EnclosingType, otherwise false.</p> |
| <p>LookFor<EnclosingType,ALambdaExpression>::value is true if MyType is an inner type of EnclosingType and invoking ALambdaExpression with the inner type returns a value of true, otherwise false.</p> |
| <p>A popular use of the optional MPL lambda expression is to check whether the type found is the same as another type, when the type found is a typedef. In that case our example would be:</p> |
| <p>LookFor<EnclosingType,boost::is_same<_,SomeOtherType> >::value is true if MyType is an inner type of EnclosingType and is the same type as SomeOtherType. </p> |
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| <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>) |
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