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# BOOST_MPL_HAS_XXX_TRAIT_NAMED_DEF

### Synopsis

    #define BOOST_MPL_HAS_XXX_TRAIT_NAMED_DEF(trait, name, default_) \
        unspecified token sequence \
    /**/

### Description

Expands into a definition of a boolean unary Metafunction `trait` such that for any type `x` `trait<x>::value == true` if and only if `x` is a class type and has a nested type memeber `x::name`.

On the deficient compilers not capabale of performing the detection, `trait<x>::value` always returns a fallback value `default_`. A boolean configuraion macro, BOOST_MPL_CFG_NO_HAS_XXX, is provided to signal or override the "deficient" status of a particular compiler. [Note: The fallback value can also be provided at the point of the metafunction invocation; see the Expression semantics section for details — end note]

### Header

    #include <boost/mpl/has_xxx.hpp>

### Parameters

Parameter | Requirement | Description
--- | --- | ---
`trait` | A legal identifier token | A name of the metafunction to be generated.
`name` | A legal identifier token | A name of the member being detected.
`default_` | An boolean constant | A fallback value for the deficient compilers.

### Expression semantics

For any legal C++ identifiers `trait` and `name`, boolean constant expression `c1`, boolean Integral Constant `c2`, and arbitrary type `x`:

    BOOST_MPL_HAS_XXX_TRAIT_NAMED_DEF(trait, name, c1)

Precondition: | Appears at namespace scope.
Return type: | None.
Semantics: | Expands into an equivalent of the following class template definition
    template< typename X, typename fallback = boost::mpl::bool_<c1> >
    struct trait
    {
        // unspecified
        // ...
    };
where `trait` is a boolean Metafunction with the following semantics:
    typedef trait<x>::type r;
Return type: | Integral Constant.
Semantics: | If BOOST_MPL_CFG_NO_HAS_XXX is defined, `r::value == c1`; otherwise, `r::value == true` if and only if `x` is a class type that has a nested type memeber `x::name`.
    typedef trait< x,c2 >::type r;
Return type: | Integral Constant.
Semantics: | If BOOST_MPL_CFG_NO_HAS_XXX is defined, `r::value == c2::value`; otherwise, equivalent to
    typedef trait<x>::type r;

### Example

    BOOST_MPL_HAS_XXX_TRAIT_NAMED_DEF(has_xxx, xxx, false)

    struct test1 {};
    struct test2 { void xxx(); };
    struct test3 { int xxx; };
    struct test4 { static int xxx(); };
    struct test5 { template< typename T > struct xxx {}; };
    struct test6 { typedef int xxx; };
    struct test7 { struct xxx; };
    struct test8 { typedef void (*xxx)(); };
    struct test9 { typedef void (xxx)(); };

    BOOST_MPL_ASSERT_NOT(( has_xxx<test1> ));
    BOOST_MPL_ASSERT_NOT(( has_xxx<test2> ));
    BOOST_MPL_ASSERT_NOT(( has_xxx<test3> ));
    BOOST_MPL_ASSERT_NOT(( has_xxx<test4> ));
    BOOST_MPL_ASSERT_NOT(( has_xxx<test5> ));

    #if !defined(BOOST_MPL_CFG_NO_HAS_XXX)
    BOOST_MPL_ASSERT(( has_xxx<test6> ));
    BOOST_MPL_ASSERT(( has_xxx<test7> ));
    BOOST_MPL_ASSERT(( has_xxx<test8> ));
    BOOST_MPL_ASSERT(( has_xxx<test9> ));
    #endif

    BOOST_MPL_ASSERT(( has_xxx<test6,true_> ));
    BOOST_MPL_ASSERT(( has_xxx<test7,true_> ));
    BOOST_MPL_ASSERT(( has_xxx<test8,true_> ));
    BOOST_MPL_ASSERT(( has_xxx<test9,true_> ));

### See also

Macros, BOOST_MPL_HAS_XXX_TRAIT_DEF, BOOST_MPL_CFG_NO_HAS_XXX