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28 <h2><span class="refentrytitle">Macro BOOST_TTI_TRAIT_HAS_TYPE</span></h2>
29 <p>BOOST_TTI_TRAIT_HAS_TYPE</p>
31 <h2 xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="refsynopsisdiv-title">Synopsis</h2>
32 <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>>
34 </span>BOOST_TTI_TRAIT_HAS_TYPE(trait, name)</pre></div>
35 <div class="refsect1">
36 <a name="idm45456560396352"></a><h2>Description</h2>
37 <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>
38 <p>trait = the name of the metafunction within the tti namespace.</p>
39 <p>name = the name of the inner type.</p>
40 <p>generates a metafunction called "trait" where 'trait' is the macro parameter. template<class BOOST_TTI_TP_T,class BOOST_TTI_TP_U>
43 static const value = unspecified;
44 typedef mpl::bool_<true-or-false> type;
47 The metafunction types and return:
49 BOOST_TTI_TP_T = the enclosing type in which to look for our 'name'.
51 BOOST_TTI_TP_U = (optional) An optional template parameter, defaulting to a marker type.
52 If specified it is an MPL lambda expression which is invoked
53 with the inner type found and must return a constant boolean
56 returns = 'value' depends on whether or not the optional BOOST_TTI_TP_U is specified.
58 If BOOST_TTI_TP_U is not specified, then 'value' is true if the 'name' type
59 exists within the enclosing type BOOST_TTI_TP_T; otherwise 'value' is false.
61 If BOOST_TTI_TP_U is specified , then 'value' is true if the 'name' type exists
62 within the enclosing type BOOST_TTI_TP_T and the lambda expression as specified
63 by BOOST_TTI_TP_U, invoked by passing the actual inner type of 'name', returns
64 a 'value' of true; otherwise 'value' is false.
66 The action taken with BOOST_TTI_TP_U occurs only when the 'name' type exists
67 within the enclosing type BOOST_TTI_TP_T.
70 <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>
71 <p>LookFor<EnclosingType>::value is true if MyType is an inner type of EnclosingType, otherwise false.</p>
72 <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>
73 <p>A popular use of the optional MPL lambda expression is to check whether the type found is the same</p>
74 <p>as another type, when the type found is a typedef. In that case our example would be:</p>
75 <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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