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