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Front Page / Metafunctions / Invocation / apply_wrap

apply_wrap

Synopsis

template<
      typename F
    >
struct apply_wrap0
{
    typedef unspecified type;
};

template<
      typename F, typename A1
    >
struct apply_wrap1
{
    typedef unspecified type;
};

...

template<
      typename F, typename A1,... typename An
    >
struct apply_wrapn
{
    typedef unspecified type;
};

Description

Invokes a Metafunction Class F with arguments A1,... An.

In essence, apply_wrap forms are nothing more than syntactic wrappers around F::apply<A1,... An>::type / F::apply::type expressions (hence the name). They provide a more concise notation and higher portability than their underlaying constructs at the cost of an extra template instantiation.

Header

#include <boost/mpl/apply_wrap.hpp>

Parameters

Parameter Requirement Description
F Metafunction Class A metafunction class to invoke.
A1,... An Any type Invocation arguments.

Expression semantics

For any Metafunction Class f and arbitrary types a1,... an:

typedef apply_wrapn<f,a1,...an>::type t;
Return type:Any type.
Semantics:If n > 0, equivalent to typedef f::apply<a1,... an>::type t;, otherwise equivalent to either typedef f::apply::type t; or typedef f::apply<>::type t; depending on whether f::apply is a class or a class template.

Example

struct f0
{
    template< typename T = int > struct apply
    {
        typedef char type;
    };
};

struct g0
{
    struct apply { typedef char type; };
};

struct f2
{
    template< typename T1, typename T2 > struct apply
    {
        typedef T2 type;
    };
};


typedef apply_wrap0< f0 >::type r1;
typedef apply_wrap0< g0 >::type r2;
typedef apply_wrap2< f2,int,char >::type r3;

BOOST_MPL_ASSERT(( is_same<r1,char> ));
BOOST_MPL_ASSERT(( is_same<r2,char> ));
BOOST_MPL_ASSERT(( is_same<r3,char> ));

See also

Metafunctions, invocation, apply, lambda, quote, bind, protect