boost/tti/has_union.hpp
// (C) Copyright Edward Diener 2019 // 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_UNION_HPP) #define BOOST_TTI_HAS_UNION_HPP #include <boost/config.hpp> #include <boost/preprocessor/cat.hpp> #include <boost/tti/gen/has_union_gen.hpp> #include <boost/tti/gen/namespace_gen.hpp> #include <boost/tti/detail/dunion.hpp> #include <boost/tti/detail/ddeftype.hpp> /* The succeeding comments in this file are in doxygen format. */ /** \file */ /// A macro which expands to a metafunction which tests whether an inner union with a particular name exists. /** BOOST_TTI_TRAIT_HAS_UNION is a macro which expands to a metafunction. The metafunction tests whether an inner union with a particular name exists and, optionally, whether an MPL lambda expression invoked with the inner union is true or not. The macro takes the form of BOOST_TTI_TRAIT_HAS_UNION(trait,name) where trait = the name of the metafunction <br/> name = the name of the inner union. BOOST_TTI_TRAIT_HAS_UNION generates a metafunction called "trait" where 'trait' is the macro parameter. @code 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'. 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 union 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' union 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' union 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 union of 'name', returns a 'value' of true; otherwise 'value' is false. The action taken with BOOST_TTI_TP_U occurs only when the 'name' union exists within the enclosing type BOOST_TTI_TP_T. @endcode Example usage: @code BOOST_TTI_TRAIT_HAS_UNION(LookFor,MyType) generates the metafunction LookFor in the current scope to look for an inner union called MyType. LookFor<EnclosingType>::value is true if MyType is an inner union of EnclosingType, otherwise false. LookFor<EnclosingType,ALambdaExpression>::value is true if MyType is an inner union of EnclosingType and invoking ALambdaExpression with the inner union returns a value of true, otherwise false. A popular use of the optional MPL lambda expression is to check whether the union found is the same as another type, when the union 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 union of EnclosingType and is the same type as SomeOtherType. @endcode */ #define BOOST_TTI_TRAIT_HAS_UNION(trait,name) \ BOOST_TTI_DETAIL_TRAIT_HAS_UNION(trait,name) \ template \ < \ class BOOST_TTI_TP_T, \ class BOOST_TTI_TP_U = BOOST_TTI_NAMESPACE::detail::deftype \ > \ struct trait \ { \ typedef typename \ BOOST_PP_CAT(trait,_detail_union)<BOOST_TTI_TP_T,BOOST_TTI_TP_U>::type type; \ BOOST_STATIC_CONSTANT(bool,value=type::value); \ }; \ /**/ /// A macro which expands to a metafunction which tests whether an inner union with a particular name exists. /** BOOST_TTI_HAS_UNION is a macro which expands to a metafunction. The metafunction tests whether an inner union with a particular name exists and, optionally, whether an MPL lambda expression invoked with the inner union is true or not. The macro takes the form of BOOST_TTI_HAS_UNION(name) where name = the name of the inner union. BOOST_TTI_HAS_UNION generates a metafunction called "has_union_'name'" where 'name' is the macro parameter. @code template<class BOOST_TTI_TP_T,class BOOST_TTI_TP_U> struct has_union_'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 union 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' union 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' union 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 union of 'name', returns a 'value' of true; otherwise 'value' is false. The action taken with BOOST_TTI_TP_U occurs only when the 'name' union exists within the enclosing type BOOST_TTI_TP_T. @endcode Example usage: @code BOOST_TTI_HAS_UNION(MyType) generates the metafunction has_union_MyType in the current scope to look for an inner union called MyType. has_union_MyType<EnclosingType>::value is true if MyType is an inner union of EnclosingType, otherwise false. has_class_MyType<EnclosingType,ALambdaExpression>::value is true if MyType is an inner union of EnclosingType and invoking ALambdaExpression with the inner union returns a value of true, otherwise false. A popular use of the optional MPL lambda expression is to check whether the union found is the same as another type, when the union found is a typedef. In that case our example would be: has_union_MyType<EnclosingType,boost::is_same<_,SomeOtherType> >::value is true if MyType is an inner union of EnclosingType and is the same type as SomeOtherType. @endcode */ #define BOOST_TTI_HAS_UNION(name) \ BOOST_TTI_TRAIT_HAS_UNION \ ( \ BOOST_TTI_HAS_UNION_GEN(name), \ name \ ) \ /**/ #endif // BOOST_TTI_HAS_UNION_HPP