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- <h2><span class="refentrytitle">Macro BOOST_TTI_HAS_TYPE</span></h2>
- <p>BOOST_TTI_HAS_TYPE</p>
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- <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_HAS_TYPE(name)</pre></div>
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- <a name="idm45456560383264"></a><h2>Description</h2>
- <p>BOOST_TTI_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>name = the name of the inner type.</p>
- <p>generates a metafunction called "has_type_'name'" where 'name' is the macro parameter. template<class BOOST_TTI_TP_T,class BOOST_TTI_TP_U>
- struct has_type_'name'
- {
- 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_HAS_TYPE(MyType) generates the metafunction has_type_MyType in the current scope to look for an inner type called MyType.</p>
- <p>has_type_MyType<EnclosingType>::value is true if MyType is an inner type of EnclosingType, otherwise false.</p>
- <p>has_type_MyType<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</p>
- <p>as another type, when the type found is a typedef. In that case our example would be:</p>
- <p>has_type_MyType<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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