]>
Commit | Line | Data |
---|---|---|
7c673cae FG |
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 |