| .. Metafunctions/Miscellaneous//identity |10 |
| |
| identity |
| ======== |
| |
| Synopsis |
| -------- |
| |
| .. parsed-literal:: |
| |
| template< |
| typename X |
| > |
| struct identity |
| { |
| typedef X type; |
| }; |
| |
| |
| Description |
| ----------- |
| |
| The `identity`__ metafunction. Returns ``X`` unchanged. |
| |
| __ http://mathworld.wolfram.com/IdentityFunction.html |
| |
| |
| Header |
| ------ |
| |
| .. parsed-literal:: |
| |
| #include <boost/mpl/identity.hpp> |
| |
| |
| Model of |
| -------- |
| |
| |Metafunction| |
| |
| |
| Parameters |
| ---------- |
| |
| +---------------+-------------------+-----------------------------------+ |
| | Parameter | Requirement | Description | |
| +===============+===================+===================================+ |
| | ``X`` | Any type | An argument to be returned. | |
| +---------------+-------------------+-----------------------------------+ |
| |
| |
| Expression semantics |
| -------------------- |
| |
| For an arbitrary type ``x``: |
| |
| |
| .. parsed-literal:: |
| |
| typedef identity<x>::type r; |
| |
| :Return type: |
| A type. |
| |
| :Semantics: |
| Equivalent to |
| |
| .. parsed-literal:: |
| |
| typedef x r; |
| |
| |
| :Postcondition: |
| ``is_same<r,x>::value == true``. |
| |
| |
| |
| Example |
| ------- |
| |
| .. parsed-literal:: |
| |
| typedef apply< identity<_1>, char >::type t1; |
| typedef apply< identity<_2>, char,int >::type t2; |
| |
| BOOST_MPL_ASSERT(( is_same< t1, char > )); |
| BOOST_MPL_ASSERT(( is_same< t2, int > )); |
| |
| |
| See also |
| -------- |
| |
| |Metafunctions|, |Placeholders|, |Trivial Metafunctions|, |always|, |apply| |
| |
| |
| .. 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) |
