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

   1 .. Metafunctions/Type Selection//eval_if_c |40
   2
   3 eval_if_c
   4 =========
   5
   6 Synopsis
   7 --------
   8
   9 .. parsed-literal::
  10
  11     template<
  12           bool c
  13         , typename F1
  14         , typename F2
  15         >
  16     struct eval_if_c
  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 of integral constant ``c``. ``eval_if_c<c,f1,f2>::type`` is a shorcut
  28 notation for ``eval_if< bool_<c>,f1,f2 >::type``.
  29
  30
  31 Header
  32 ------
  33
  34 .. parsed-literal::
  35
  36     #include <boost/mpl/eval_if.hpp>
  37
  38
  39 Parameters
  40 ----------
  41
  42 +---------------+-----------------------------------+-----------------------------------------------+
  43 | Parameter     | Requirement                       | Description                                   |
  44 +===============+===================================+===============================================+
  45 | ``c``         | An integral constant              | An evaluation condition.                      |
  46 +---------------+-----------------------------------+-----------------------------------------------+
  47 | ``F1``, ``F2``| Nullary |Metafunction|            | Metafunctions to select for evaluation from.  |
  48 +---------------+-----------------------------------+-----------------------------------------------+
  49
  50
  51 Expression semantics
  52 --------------------
  53
  54 For any integral constant ``c`` and nullary |Metafunction|\ s ``f1``, ``f2``:
  55
  56
  57 .. parsed-literal::
  58
  59     typedef eval_if_c<c,f1,f2>::type t;
  60
  61 :Return type:
  62     Any type.
  63
  64 :Semantics:
  65     Equivalent to ``typedef eval_if< bool_<c>,f1,f2 >::type t;``
  66
  67
  68 Example
  69 -------
  70
  71 .. parsed-literal::
  72
  73     typedef eval_if_c< true, identity<char>, identity<long> >::type t1;
  74     typedef eval_if_c< 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|, |if_|, |bool_|
  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)