| .. Metafunctions/Type Selection//eval_if |30 |
| |
| eval_if |
| ======= |
| |
| Synopsis |
| -------- |
| |
| .. parsed-literal:: |
| |
| template< |
| typename C |
| , typename F1 |
| , typename F2 |
| > |
| struct eval_if |
| { |
| typedef |unspecified| type; |
| }; |
| |
| |
| |
| Description |
| ----------- |
| |
| Evaluates one of its two |nullary metafunction| arguments, ``F1`` or ``F2``, depending |
| on the value ``C``. |
| |
| |
| Header |
| ------ |
| |
| .. parsed-literal:: |
| |
| #include <boost/mpl/eval_if.hpp> |
| |
| |
| Parameters |
| ---------- |
| |
| +---------------+-----------------------------------+-----------------------------------------------+ |
| | Parameter | Requirement | Description | |
| +===============+===================================+===============================================+ |
| | ``C`` | |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,f1,f2>::type t; |
| |
| :Return type: |
| Any type. |
| |
| :Semantics: |
| If ``c::value == true``, ``t`` is identical to ``f1::type``; otherwise ``t`` is |
| identical to ``f2::type``. |
| |
| |
| Example |
| ------- |
| |
| .. parsed-literal:: |
| |
| typedef eval_if< true\_, identity<char>, identity<long> >::type t1; |
| typedef eval_if< 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_c|, |if_| |
| |
| |
| .. 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) |
