The MPL Reference Manual: 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 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...an>::type t;

Return type:	Any type.
Semantics:	If `n > 0`, equivalent to `typedef f::apply 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 ));
BOOST_MPL_ASSERT(( is_same ));
BOOST_MPL_ASSERT(( is_same ));

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