text: 
  //  (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
  