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

   1 .. Sequences/Intrinsic Metafunctions//key_type
   2
   3 key_type
   4 ========
   5
   6 Synopsis
   7 --------
   8
   9 .. parsed-literal::
  10
  11     template<
  12           typename Sequence
  13         , typename X
  14         >
  15     struct key_type
  16     {
  17         typedef |unspecified| type;
  18     };
  19
  20
  21
  22 Description
  23 -----------
  24
  25 Returns the |key| that would be used to identify ``X`` in ``Sequence``.
  26
  27
  28 Header
  29 ------
  30
  31 .. parsed-literal::
  32
  33     #include <boost/mpl/key_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 |key| for.                |
  51 +---------------+---------------------------+-----------------------------------------------+
  52
  53
  54 Expression semantics
  55 --------------------
  56
  57 For any |Associative Sequence| ``s``, iterators ``pos1`` and ``pos2`` in ``s``, and an
  58 artibrary type ``x``:
  59
  60 .. parsed-literal::
  61
  62     typedef key_type<s,x>::type k;
  63
  64 :Return type:
  65     A type.
  66
  67 :Precondition:
  68     ``x`` can be put in ``s``.
  69
  70 :Semantics:
  71     ``k`` is the |key| that would be used to identify ``x`` in ``s``.
  72
  73 :Postcondition:
  74     If ``key_type< s,deref<pos1>::type >::type`` is identical to
  75     ``key_type< s,deref<pos2>::type >::type`` then ``pos1`` is identical to ``pos2``.
  76
  77
  78
  79 Complexity
  80 ----------
  81
  82 Amortized constant time.
  83
  84
  85 Example
  86 -------
  87
  88 .. parsed-literal::
  89
  90     typedef key_type< map<>,pair<int,unsigned> >::type k1;
  91     typedef key_type< set<>,pair<int,unsigned> >::type k2;
  92
  93     BOOST_MPL_ASSERT(( is_same< k1,int > ));
  94     BOOST_MPL_ASSERT(( is_same< k2,pair<int,unsigned> > ));
  95
  96
  97 See also
  98 --------
  99
 100 |Associative Sequence|, |value_type|, |has_key|, |set|, |map|
 101
 102
 103 .. copyright:: Copyright ©  2001-2009 Aleksey Gurtovoy and David Abrahams
 104    Distributed under the Boost Software License, Version 1.0. (See accompanying
 105    file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)