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C++ Libraries
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— Herb Sutter and Andrei
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Coding Standards
| Front Page / Metafunctions / Invocation / apply_wrap |
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;
};
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.
#include <boost/mpl/apply_wrap.hpp>
| Parameter | Requirement | Description |
|---|---|---|
| F | Metafunction Class | A metafunction class to invoke. |
| A1,... An | Any type | Invocation arguments. |
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. |
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> ));