| .. Metafunctions/Type Selection//eval_if_c |40 |
| |
| eval_if_c |
| ========= |
| |
| Synopsis |
| -------- |
| |
| .. parsed-literal:: |
| |
| template< |
| bool c |
| , typename F1 |
| , typename F2 |
| > |
| struct eval_if_c |
| { |
| typedef |unspecified| type; |
| }; |
| |
| |
| |
| Description |
| ----------- |
| |
| Evaluates one of its two |nullary metafunction| arguments, ``F1`` or ``F2``, depending |
| on the value of integral constant ``c``. ``eval_if_c<c,f1,f2>::type`` is a shorcut |
| notation for ``eval_if< bool_<c>,f1,f2 >::type``. |
| |
| |
| Header |
| ------ |
| |
| .. parsed-literal:: |
| |
| #include <boost/mpl/eval_if.hpp> |
| |
| |
| Parameters |
| ---------- |
| |
| +---------------+-----------------------------------+-----------------------------------------------+ |
| | Parameter | Requirement | Description | |
| +===============+===================================+===============================================+ |
| | ``c`` | An integral constant | An evaluation condition. | |
| +---------------+-----------------------------------+-----------------------------------------------+ |
| | ``F1``, ``F2``| Nullary |Metafunction| | Metafunctions to select for evaluation from. | |
| +---------------+-----------------------------------+-----------------------------------------------+ |
| |
| |
| Expression semantics |
| -------------------- |
| |
| For any integral constant ``c`` and nullary |Metafunction|\ s ``f1``, ``f2``: |
| |
| |
| .. parsed-literal:: |
| |
| typedef eval_if_c<c,f1,f2>::type t; |
| |
| :Return type: |
| Any type. |
| |
| :Semantics: |
| Equivalent to ``typedef eval_if< bool_<c>,f1,f2 >::type t;`` |
| |
| |
| Example |
| ------- |
| |
| .. parsed-literal:: |
| |
| typedef eval_if_c< true, identity<char>, identity<long> >::type t1; |
| typedef eval_if_c< false, identity<char>, identity<long> >::type t2; |
| |
| BOOST_MPL_ASSERT(( is_same<t1,char> )); |
| BOOST_MPL_ASSERT(( is_same<t2,long> )); |
| |
| |
| See also |
| -------- |
| |
| |Metafunctions|, |Integral Constant|, |eval_if|, |if_|, |bool_| |
| |
| |
| .. 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) |