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

   1 .. Sequences/Intrinsic Metafunctions//value_type
   2
   3 value_type
   4 ==========
   5
   6 Synopsis
   7 --------
   8
   9 .. parsed-literal::
  10
  11     template<
  12           typename Sequence
  13         , typename X
  14         >
  15     struct value_type
  16     {
  17         typedef |unspecified| type;
  18     };
  19
  20
  21
  22 Description
  23 -----------
  24
  25 Returns the |value| that would be used for element ``X`` in ``Sequence``.
  26
  27
  28 Header
  29 ------
  30
  31 .. parsed-literal::
  32
  33     #include <boost/mpl/value_type.hpp>
  34
  35
  36 Model of
  37 --------
  38
  39 |Tag Dispatched Metafunction|
  40
  41
  42 Parameters
  43 ----------
  44
  45 +---------------+---------------------------+-----------------------------------------------+
  46 | Parameter     | Requirement               | Description                                   |
  47 +===============+===========================+===============================================+
  48 | ``Sequence``  | |Associative Sequence|    | A sequence to query.                          |
  49 +---------------+---------------------------+-----------------------------------------------+
  50 | ``X``         | Any type                  | The type to get the |value| for.              |
  51 +---------------+---------------------------+-----------------------------------------------+
  52
  53
  54 Expression semantics
  55 --------------------
  56
  57 For any |Associative Sequence| ``s``, and an artibrary type ``x``:
  58
  59
  60 .. parsed-literal::
  61
  62     typedef value_type<s,x>::type v;
  63
  64 :Return type:
  65     A type.
  66
  67 :Precondition:
  68     ``x`` can be put in ``s``.
  69
  70 :Semantics:
  71     ``v`` is the |value| that would be used for ``x`` in ``s``.
  72
  73 :Postcondition:
  74     If
  75     .. parsed-literal::
  76
  77        has_key< s,key_type<s,x>::type >::type
  78
  79     then
  80     .. parsed-literal::
  81
  82        at< s,key_type<s,x>::type >::type
  83
  84     is identical to ``value_type<s,x>::type``.
  85
  86
  87
  88 Complexity
  89 ----------
  90
  91 Amortized constant time.
  92
  93
  94 Example
  95 -------
  96
  97 .. parsed-literal::
  98
  99     typedef value_type< map<>,pair<int,unsigned> >::type v1;
 100     typedef value_type< set<>,pair<int,unsigned> >::type v2;
 101
 102     BOOST_MPL_ASSERT(( is_same< v1,unsigned > ));
 103     BOOST_MPL_ASSERT(( is_same< v2,pair<int,unsigned> > ));
 104
 105
 106 See also
 107 --------
 108
 109 |Associative Sequence|, |key_type|, |at|, |set|, |map|
 110
 111
 112 .. copyright:: Copyright ©  2001-2009 Aleksey Gurtovoy and David Abrahams
 113    Distributed under the Boost Software License, Version 1.0. (See accompanying
 114    file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)