ceph/src/boost/libs/mpl/doc/src/refmanual/eval_if.rst

   1 .. Metafunctions/Type Selection//eval_if |30
   2
   3 eval_if
   4 =======
   5
   6 Synopsis
   7 --------
   8
   9 .. parsed-literal::
  10
  11     template<
  12           typename C
  13         , typename F1
  14         , typename F2
  15         >
  16     struct eval_if
  17     {
  18         typedef |unspecified| type;
  19     };
  20
  21
  22
  23 Description
  24 -----------
  25
  26 Evaluates one of its two |nullary metafunction| arguments, ``F1`` or ``F2``, depending
  27 on the value ``C``.
  28
  29
  30 Header
  31 ------
  32
  33 .. parsed-literal::
  34
  35     #include <boost/mpl/eval_if.hpp>
  36
  37
  38 Parameters
  39 ----------
  40
  41 +---------------+-----------------------------------+-----------------------------------------------+
  42 | Parameter     | Requirement                       | Description                                   |
  43 +===============+===================================+===============================================+
  44 | ``C``         | |Integral Constant|               | An evaluation condition.                      |
  45 +---------------+-----------------------------------+-----------------------------------------------+
  46 | ``F1``, ``F2``| Nullary |Metafunction|            | Metafunctions to select for evaluation from.  |
  47 +---------------+-----------------------------------+-----------------------------------------------+
  48
  49
  50 Expression semantics
  51 --------------------
  52
  53 For any |Integral Constant| ``c`` and nullary |Metafunction|\ s ``f1``, ``f2``:
  54
  55
  56 .. parsed-literal::
  57
  58     typedef eval_if<c,f1,f2>::type t;
  59
  60 :Return type:
  61     Any type.
  62
  63 :Semantics:
  64     If ``c::value == true``, ``t`` is identical to ``f1::type``; otherwise ``t`` is
  65     identical to ``f2::type``.
  66
  67
  68 Example
  69 -------
  70
  71 .. parsed-literal::
  72
  73     typedef eval_if< true\_, identity<char>, identity<long> >::type t1;
  74     typedef eval_if< false\_, identity<char>, identity<long> >::type t2;
  75
  76     BOOST_MPL_ASSERT(( is_same<t1,char> ));
  77     BOOST_MPL_ASSERT(( is_same<t2,long> ));
  78
  79
  80 See also
  81 --------
  82
  83 |Metafunctions|, |Integral Constant|, |eval_if_c|, |if_|
  84
  85
  86 .. copyright:: Copyright ©  2001-2009 Aleksey Gurtovoy and David Abrahams
  87    Distributed under the Boost Software License, Version 1.0. (See accompanying
  88    file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)