The MPL Reference Manual: BOOST_MPL_HAS_XXX_TRAIT_NAMED_DEF

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::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::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_ >
struct trait
{
    // unspecified
    // ...
};

where trait is a boolean Metafunction with the following semantics:

typedef trait::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::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 ));
BOOST_MPL_ASSERT_NOT(( has_xxx ));
BOOST_MPL_ASSERT_NOT(( has_xxx ));
BOOST_MPL_ASSERT_NOT(( has_xxx ));
BOOST_MPL_ASSERT_NOT(( has_xxx ));

#if !defined(BOOST_MPL_CFG_NO_HAS_XXX)
BOOST_MPL_ASSERT(( has_xxx ));
BOOST_MPL_ASSERT(( has_xxx ));
BOOST_MPL_ASSERT(( has_xxx ));
BOOST_MPL_ASSERT(( has_xxx ));
#endif

BOOST_MPL_ASSERT(( has_xxxtrue_> ));
BOOST_MPL_ASSERT(( has_xxxtrue_> ));
BOOST_MPL_ASSERT(( has_xxxtrue_> ));
BOOST_MPL_ASSERT(( has_xxxtrue_> ));

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