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## Macro BOOST_TTI_TRAIT_MEMBER_TYPE

BOOST_TTI_TRAIT_MEMBER_TYPE — A macro expands to a metafunction whose typedef 'type' is either the named type or a marker type.

## Synopsis

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

    BOOST_TTI_TRAIT_MEMBER_TYPE(trait, name)

## Description

BOOST_TTI_TRAIT_MEMBER_TYPE is a macro which expands to a metafunction. The metafunction tests whether an inner type with a particular name exists by returning the inner type or a marker type. The macro takes the form of BOOST_TTI_TRAIT_MEMBER_TYPE(trait,name) where

trait = the name of the metafunction
name = the name of the inner type.

BOOST_TTI_TRAIT_MEMBER_TYPE generates a metafunction called "trait" where 'trait' is the macro parameter.

    template<class BOOST_TTI_TP_T,class BOOST_TTI_TP_MARKER_TYPE = boost::tti::detail::notype>
    struct trait
      {
      typedef unspecified type;
      typedef BOOST_TTI_TP_MARKER_TYPE boost_tti_marker_type;
      };

    The metafunction types and return:

      BOOST_TTI_TP_T           = the enclosing type.
                                 The enclosing type can be a class, struct, or union.

      BOOST_TTI_TP_MARKER_TYPE = (optional) a type to use as the marker type.
                                 defaults to the internal boost::tti::detail::notype.

      returns                  = 'type' is the inner type of 'name' if the inner type exists
                                 within the enclosing type, else 'type' is a marker type.
                                 if the end-user does not specify a marker type then
                                 an internal boost::tti::detail::notype marker type,
                                 which is empty, is used.

      The metafunction also encapsulates the type of the marker type as
      a nested 'boost_tti_marker_type'.

The purpose of this macro is to encapsulate the 'name' type as the typedef 'type' of a metafunction, but only if it exists within the enclosing type. This allows for an evaluation of inner type existence, without generating a compiler error, which can be used by other metafunctions in this library.