| .. Metafunctions/Invocation//apply_wrap |20 |
| |
| apply_wrap |
| ========== |
| |
| Synopsis |
| -------- |
| |
| .. parsed-literal:: |
| |
| 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_wrap\ *n* |
| { |
| 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 |
| ------ |
| |
| .. parsed-literal:: |
| |
| #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|: |
| |
| |
| .. parsed-literal:: |
| |
| typedef apply_wrap\ *n*\ <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 |
| ------- |
| |
| .. parsed-literal:: |
| |
| 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_wrap\ ``0``\ < f0 >::type r1; |
| typedef apply_wrap\ ``0``\ < g0 >::type r2; |
| typedef apply_wrap\ ``2``\ < 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| |
| |
| |
| .. copyright:: Copyright © 2001-2009 Aleksey Gurtovoy and David Abrahams |
| Distributed under the Boost Software License, Version 1.0. (See accompanying |
| file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) |
