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## Macro BOOST_TTI_TRAIT_HAS_TYPE

BOOST_TTI_TRAIT_HAS_TYPE

## Synopsis

    // In header: <boost/tti/has_type.hpp>

    BOOST_TTI_TRAIT_HAS_TYPE(trait, name)

## Description

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.

trait = the name of the metafunction within the tti namespace.

name = the name of the inner type.

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.

Example usage:

BOOST_TTI_TRAIT_HAS_TYPE(LookFor,MyType) generates the metafunction LookFor in the current scope to look for an inner type called MyType.

LookFor<EnclosingType>::value is true if MyType is an inner type of EnclosingType, otherwise false.

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.

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:

LookFor<EnclosingType,boost::is_same<_,SomeOtherType> >::value is true if MyType is an inner type of EnclosingType and is the same type as SomeOtherType.