boost/tti/has_type.hpp
// (C) Copyright Edward Diener 2011,2012,2013 // Use, modification and distribution are subject to the Boost Software License, // Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at // http://www.boost.org/LICENSE_1_0.txt). #if !defined(BOOST_TTI_HAS_TYPE_HPP) #define BOOST_TTI_HAS_TYPE_HPP #include <boost/mpl/bool.hpp> #include <boost/preprocessor/cat.hpp> #include <boost/tti/gen/has_type_gen.hpp> #include <boost/tti/gen/namespace_gen.hpp> #include <boost/tti/detail/dtype.hpp> #include <boost/tti/detail/ddeftype.hpp> /* The succeeding comments in this file are in doxygen format. */ /** \file */ /** 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. */ #define BOOST_TTI_TRAIT_HAS_TYPE(trait,name) \ BOOST_TTI_DETAIL_TRAIT_HAS_TYPE(trait,name) \ template \ < \ class BOOST_TTI_TP_T, \ class BOOST_TTI_TP_U = BOOST_TTI_NAMESPACE::detail::deftype \ > \ struct trait : \ BOOST_PP_CAT(trait,_detail_type) \ < \ BOOST_TTI_TP_T, \ BOOST_TTI_TP_U, \ typename BOOST_PP_CAT(trait,_detail_type_mpl)<BOOST_TTI_TP_T>::type \ > \ { \ }; \ /**/ /** 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. name = the name of the inner 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'. 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_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. */ #define BOOST_TTI_HAS_TYPE(name) \ BOOST_TTI_TRAIT_HAS_TYPE \ ( \ BOOST_TTI_HAS_TYPE_GEN(name), \ name \ ) \ /**/ #endif // BOOST_TTI_HAS_TYPE_HPP