Metafunction Class

Description

A metafunction class is a certain form of metafunction representation that enables higher-order metaprogramming. In particular, a non-nullary metafunction class is a type with a nested class template member apply. A nullary metafunction class has the form of a nullary metafunction. A metafunction class invocation is defined as invocation of its nested apply metafunction.

Valid expressions

Expression	Expression type
typename f::type	A type
typename f::template apply<a1,..,an>::type	A type

Expression semantics

Expression	Complexity	Precondition	Semantics	Postcondition
typename f::type	unspecified	f is a nullary metafunction class; f::type is a type-name	f::type is the result of the metafunction class invocation
typename f::template apply<a1,..,an>::type	unspecified	f is an n-ary metafunction class; apply is a metafunction	typename f::template apply<a1,..,an>::type is the result of the metafunction class invocation with the actual arguments a1,..,an

Example

// nullary metafunction class
struct always_true { typedef true_ type; };

template< long N > struct le
{
    template< typename M > struct apply
    {
        typedef bool_< (M::value < N) > type;
    };
};

// unary metafunction class
typedef le<5> less_than_5;

// binary metafunction class
struct less_than
{
    template< typename N1, typename N2 > struct apply
    {
        typedef bool_< (N1::value < N2::value) > type;
    };
};

// invocations
typedef always_true::type t1;
typedef less_than_5::apply< int_<7> >::type t2;
typedef less_than::apply< int_<5>,int_<7> >::type t3;

// results checks
BOOST_STATIC_ASSERT(t1::value);
BOOST_STATIC_ASSERT(!t2::value);
BOOST_STATIC_ASSERT(t3::value);

See also

Metafunctions, Metafunction