...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
BOOST_TTI_HAS_TYPE — A macro which expands to a metafunction which tests whether an inner type with a particular name exists.
// In header: <boost/tti/has_type.hpp>
BOOST_TTI_HAS_TYPE(name)
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 an MPL lambda expression invoked with the inner type is true or not. The macro takes the form of BOOST_TTI_HAS_TYPE(name) where
name = the name of the inner type.
BOOST_TTI_HAS_TYPE 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'. The enclosing type can be a class, struct, or union. 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 MPL 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_HAS_TYPE(MyType) generates the metafunction has_type_MyType in the current scope to look for an inner type called MyType. has_type_MyType<EnclosingType>::value is true if MyType is an inner type of EnclosingType, otherwise false. 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. 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: 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.