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

   1 .. Metafunctions/Miscellaneous//always |20
   2
   3 always
   4 ======
   5
   6 Synopsis
   7 --------
   8
   9 .. parsed-literal::
  10
  11     template<
  12           typename X
  13         >
  14     struct always
  15     {
  16         // |unspecified|
  17         // |...|
  18     };
  19
  20
  21 Description
  22 -----------
  23
  24 ``always<X>`` specialization is a variadic |Metafunction Class| always returning the
  25 same type, ``X``, regardless of the number and types of passed arguments.
  26
  27
  28 Header
  29 ------
  30
  31 .. parsed-literal::
  32
  33     #include <boost/mpl/always.hpp>
  34
  35 Model of
  36 --------
  37
  38 |Metafunction Class|
  39
  40
  41 Parameters
  42 ----------
  43
  44 +---------------+-------------------+-----------------------------------+
  45 | Parameter     | Requirement       | Description                       |
  46 +===============+===================+===================================+
  47 | ``X``         | Any type          | A type to be returned.            |
  48 +---------------+-------------------+-----------------------------------+
  49
  50
  51 Expression semantics
  52 --------------------
  53
  54 For an arbitrary type ``x``:
  55
  56
  57 .. parsed-literal::
  58
  59     typedef always<x> f;
  60
  61 :Return type:
  62     |Metafunction Class|.
  63
  64 :Semantics:
  65     Equivalent to
  66
  67     .. parsed-literal::
  68
  69         struct f : bind< identity<_1>, x > {};
  70
  71
  72 Example
  73 -------
  74
  75 .. parsed-literal::
  76
  77     typedef always<true\_> always_true;
  78
  79     BOOST_MPL_ASSERT(( apply< always_true,false\_> ));
  80     BOOST_MPL_ASSERT(( apply< always_true,false\_,false\_ > ));
  81     BOOST_MPL_ASSERT(( apply< always_true,false\_,false\_,false\_ > ));
  82
  83
  84 See also
  85 --------
  86
  87 |Metafunctions|, |Metafunction Class|, |identity|, |bind|, |apply|
  88
  89
  90 .. copyright:: Copyright ©  2001-2009 Aleksey Gurtovoy and David Abrahams
  91    Distributed under the Boost Software License, Version 1.0. (See accompanying
  92    file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)