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

   1 .. Sequences/Intrinsic Metafunctions//at
   2
   3 at
   4 ==
   5
   6 Synopsis
   7 --------
   8
   9 .. parsed-literal::
  10
  11     template<
  12           typename Sequence
  13         , typename N
  14         >
  15     struct at
  16     {
  17         typedef |unspecified| type;
  18     };
  19
  20     template<
  21           typename AssocSeq
  22         , typename Key
  23         , typename Default = |unspecified|
  24         >
  25     struct at
  26     {
  27         typedef |unspecified| type;
  28     };
  29
  30
  31 Description
  32 -----------
  33
  34 ``at`` is an |overloaded name|:
  35
  36 * ``at<Sequence,N>`` returns the ``N``-th element from the beginning of the
  37   |Forward Sequence| ``Sequence``.
  38
  39 * ``at<AssocSeq,Key,Default>`` returns the first element associated with ``Key``
  40   in the |Associative Sequence| ``AssocSeq``, or ``Default`` if no such element
  41   exists.
  42
  43
  44 Header
  45 ------
  46
  47 .. parsed-literal::
  48
  49     #include <boost/mpl/at.hpp>
  50
  51
  52 Model of
  53 --------
  54
  55 |Tag Dispatched Metafunction|
  56
  57
  58 Parameters
  59 ----------
  60
  61 +---------------+---------------------------+-----------------------------------------------+
  62 | Parameter     | Requirement               | Description                                   |
  63 +===============+===========================+===============================================+
  64 | ``Sequence``  | |Forward Sequence|        | A sequence to be examined.                    |
  65 +---------------+---------------------------+-----------------------------------------------+
  66 | ``AssocSeq``  | |Associative Sequence|    | A sequence to be examined.                    |
  67 +---------------+---------------------------+-----------------------------------------------+
  68 | ``N``         | |Integral Constant|       | An offset from the beginning of the sequence  |
  69 |               |                           | specifying the element to be retrieved.       |
  70 +---------------+---------------------------+-----------------------------------------------+
  71 | ``Key``       | Any type                  | A key for the element to be retrieved.        |
  72 +---------------+---------------------------+-----------------------------------------------+
  73 | ``Default``   | Any type                  | A default value to return if the element is   |
  74 |               |                           | not found.                                    |
  75 +---------------+---------------------------+-----------------------------------------------+
  76
  77
  78 Expression semantics
  79 --------------------
  80
  81 .. compound::
  82     :class: expression-semantics
  83
  84     For any |Forward Sequence| ``s``, and |Integral Constant| ``n``:
  85
  86     .. parsed-literal::
  87
  88         typedef at<s,n>::type t;
  89
  90     :Return type:
  91         A type.
  92
  93     :Precondition:
  94         ``0 <= n::value < size<s>::value``.
  95
  96     :Semantics:
  97         Equivalent to
  98
  99         .. parsed-literal::
 100
 101            typedef deref< advance< begin<s>::type,n >::type >::type t;
 102
 103
 104 .. compound::
 105     :class: expression-semantics
 106
 107     For any |Associative Sequence| ``s``, and arbitrary types ``key`` and ``x``:
 108
 109     .. parsed-literal::
 110
 111         typedef at<s,key,x>::type t;
 112
 113     :Return type:
 114         A type.
 115
 116     :Semantics:
 117         If ``has_key<s,key>::value == true``, ``t`` is the value type associated with ``key``;
 118         otherwise ``t`` is identical to ``x``.
 119
 120
 121     .. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 122
 123     .. parsed-literal::
 124
 125         typedef at<s,key>::type t;
 126
 127     :Return type:
 128         A type.
 129
 130     :Semantics:
 131         Equivalent to
 132
 133         .. parsed-literal::
 134
 135            typedef at<s,key,void\_>::type t;
 136
 137
 138 Complexity
 139 ----------
 140
 141 +-------------------------------+-----------------------------------+
 142 | Sequence archetype            | Complexity                        |
 143 +===============================+===================================+
 144 | |Forward Sequence|            | Linear.                           |
 145 +-------------------------------+-----------------------------------+
 146 | |Random Access Sequence|      | Amortized constant time.          |
 147 +-------------------------------+-----------------------------------+
 148 | |Associative Sequence|        | Amortized constant time.          |
 149 +-------------------------------+-----------------------------------+
 150
 151 Example
 152 -------
 153
 154 .. parsed-literal::
 155
 156     typedef range_c<long,10,50> range;
 157     BOOST_MPL_ASSERT_RELATION( (at< range, int_<0> >::value), ==, 10 );
 158     BOOST_MPL_ASSERT_RELATION( (at< range, int_<10> >::value), ==, 20 );
 159     BOOST_MPL_ASSERT_RELATION( (at< range, int_<40> >::value), ==, 50 );
 160
 161
 162 .. parsed-literal::
 163
 164     typedef set< int const,long*,double > s;
 165
 166     BOOST_MPL_ASSERT(( is_same< at<s,char>::type, void\_ > ));
 167     BOOST_MPL_ASSERT(( is_same< at<s,int>::type, int > ));
 168
 169
 170 See also
 171 --------
 172
 173 |Forward Sequence|, |Random Access Sequence|, |Associative Sequence|, |at_c|, |front|, |back|
 174
 175
 176 .. copyright:: Copyright ©  2001-2009 Aleksey Gurtovoy and David Abrahams
 177    Distributed under the Boost Software License, Version 1.0. (See accompanying
 178    file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)