projects / ceph.git / blame
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|---|---|---|
| 7c673cae FG |
1 | .. Sequences/Intrinsic Metafunctions//begin |
| 2 | ||
| 3 | begin | |
| 4 | ===== | |
| 5 | ||
| 6 | Synopsis | |
| 7 | -------- | |
| 8 | ||
| 9 | .. parsed-literal:: | |
| 10 | ||
| 11 | template< | |
| 12 | typename X | |
| 13 | > | |
| 14 | struct begin | |
| 15 | { | |
| 16 | typedef |unspecified| type; | |
| 17 | }; | |
| 18 | ||
| 19 | ||
| 20 | ||
| 21 | Description | |
| 22 | ----------- | |
| 23 | ||
| 24 | Returns an iterator that points to the first element of the sequence. If | |
| 25 | the argument is not a |Forward Sequence|, returns |void_|. | |
| 26 | ||
| 27 | ||
| 28 | Header | |
| 29 | ------ | |
| 30 | ||
| 31 | .. parsed-literal:: | |
| 32 | ||
| 33 | #include <boost/mpl/begin_end.hpp> | |
| 34 | ||
| 35 | ||
| 36 | ||
| 37 | Model of | |
| 38 | -------- | |
| 39 | ||
| 40 | |Tag Dispatched Metafunction| | |
| 41 | ||
| 42 | ||
| 43 | Parameters | |
| 44 | ---------- | |
| 45 | ||
| 46 | +---------------+-------------------+---------------------------------------------------+ | |
| 47 | | Parameter | Requirement | Description | | |
| 48 | +===============+===================+===================================================+ | |
| 49 | | ``X`` | Any type | A type whose begin iterator, if any, will be | | |
| 50 | | | | returned. | | |
| 51 | +---------------+-------------------+---------------------------------------------------+ | |
| 52 | ||
| 53 | ||
| 54 | Expression semantics | |
| 55 | -------------------- | |
| 56 | ||
| 57 | For any arbitrary type ``x``: | |
| 58 | ||
| 59 | .. parsed-literal:: | |
| 60 | ||
| 61 | typedef begin<x>::type first; | |
| 62 | ||
| 63 | :Return type: | |
| 64 | |Forward Iterator| or |void_|. | |
| 65 | ||
| 66 | :Semantics: | |
| 67 | If ``x`` is a |Forward Sequence|, ``first`` is an iterator pointing to the | |
| 68 | first element of ``s``; otherwise ``first`` is |void_|. | |
| 69 | ||
| 70 | :Postcondition: | |
| 71 | If ``first`` is an iterator, it is either dereferenceable or past-the-end; it | |
| 72 | is past-the-end if and only if ``size<x>::value == 0``. | |
| 73 | ||
| 74 | ||
| 75 | Complexity | |
| 76 | ---------- | |
| 77 | ||
| 78 | Amortized constant time. | |
| 79 | ||
| 80 | ||
| 81 | Example | |
| 82 | ------- | |
| 83 | ||
| 84 | .. parsed-literal:: | |
| 85 | ||
| 86 | typedef vector< unsigned char,unsigned short, | |
| 87 | unsigned int,unsigned long > unsigned_types; | |
| 88 | ||
| 89 | typedef begin<unsigned_types>::type iter; | |
| 90 | BOOST_MPL_ASSERT(( is_same< deref<iter>::type, unsigned char > )); | |
| 91 | ||
| 92 | BOOST_MPL_ASSERT(( is_same< begin<int>::type, void\_ > )); | |
| 93 | ||
| 94 | ||
| 95 | See also | |
| 96 | -------- | |
| 97 | ||
| 98 | |Iterators|, |Forward Sequence|, |end|, |size|, |empty| | |
| 99 | ||
| 100 | ||
| 101 |