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

   1 .. Metafunctions/Miscellaneous//identity |10
   2
   3 identity
   4 ========
   5
   6 Synopsis
   7 --------
   8
   9 .. parsed-literal::
  10
  11     template<
  12           typename X
  13         >
  14     struct identity
  15     {
  16         typedef X type;
  17     };
  18
  19
  20 Description
  21 -----------
  22
  23 The `identity`__ metafunction. Returns ``X`` unchanged.
  24
  25 __ http://mathworld.wolfram.com/IdentityFunction.html
  26
  27
  28 Header
  29 ------
  30
  31 .. parsed-literal::
  32
  33     #include <boost/mpl/identity.hpp>
  34
  35
  36 Model of
  37 --------
  38
  39 |Metafunction|
  40
  41
  42 Parameters
  43 ----------
  44
  45 +---------------+-------------------+-----------------------------------+
  46 | Parameter     | Requirement       | Description                       |
  47 +===============+===================+===================================+
  48 | ``X``         | Any type          | An argument to be returned.       |
  49 +---------------+-------------------+-----------------------------------+
  50
  51
  52 Expression semantics
  53 --------------------
  54
  55 For an arbitrary type ``x``:
  56
  57
  58 .. parsed-literal::
  59
  60     typedef identity<x>::type r;
  61
  62 :Return type:
  63     A type.
  64
  65 :Semantics:
  66     Equivalent to
  67
  68     .. parsed-literal::
  69
  70         typedef x r;
  71
  72
  73 :Postcondition:
  74     ``is_same<r,x>::value == true``.
  75
  76
  77
  78 Example
  79 -------
  80
  81 .. parsed-literal::
  82
  83     typedef apply< identity<_1>, char >::type t1;
  84     typedef apply< identity<_2>, char,int >::type t2;
  85
  86     BOOST_MPL_ASSERT(( is_same< t1, char > ));
  87     BOOST_MPL_ASSERT(( is_same< t2, int > ));
  88
  89
  90 See also
  91 --------
  92
  93 |Metafunctions|, |Placeholders|, |Trivial Metafunctions|, |always|, |apply|
  94
  95
  96 .. copyright:: Copyright ©  2001-2009 Aleksey Gurtovoy and David Abrahams
  97    Distributed under the Boost Software License, Version 1.0. (See accompanying
  98    file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)