projects / ceph.git / blame
| Commit | Line | Data |
|---|---|---|
| 7c673cae FG |
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 |