projects / ceph.git / blame
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|---|---|---|
| 7c673cae FG |
1 | .. Iterators/Concepts//Bidirectional Iterator |20 |
| 2 | ||
| 3 | Bidirectional Iterator | |
| 4 | ====================== | |
| 5 | ||
| 6 | Description | |
| 7 | ----------- | |
| 8 | ||
| 9 | A |Bidirectional Iterator| is a |Forward Iterator| that provides a way to | |
| 10 | obtain an iterator to the previous element in a sequence. | |
| 11 | ||
| 12 | Refinement of | |
| 13 | ------------- | |
| 14 | ||
| 15 | |Forward Iterator| | |
| 16 | ||
| 17 | Definitions | |
| 18 | ----------- | |
| 19 | ||
| 20 | * a bidirectional iterator ``i`` is `decrementable` if there is a "previous" | |
| 21 | iterator, that is, if ``prior<i>::type`` expression is well-defined; | |
| 22 | iterators pointing to the first element of the sequence are not | |
| 23 | decrementable. | |
| 24 | ||
| 25 | ||
| 26 | Expression requirements | |
| 27 | ----------------------- | |
| 28 | ||
| 29 | In addition to the requirements defined in |Forward Iterator|, | |
| 30 | the following requirements must be met. | |
| 31 | ||
| 32 | +-----------------------+-------------------------------------------+---------------------------+ | |
| 33 | | Expression | Type | Complexity | | |
| 34 | +=======================+===========================================+===========================+ | |
| 35 | | ``next<i>::type`` | |Bidirectional Iterator| | Amortized constant time | | |
| 36 | +-----------------------+-------------------------------------------+---------------------------+ | |
| 37 | | ``prior<i>::type`` | |Bidirectional Iterator| | Amortized constant time | | |
| 38 | +-----------------------+-------------------------------------------+---------------------------+ | |
| 39 | | ``i::category`` | |Integral Constant|, convertible | Constant time | | |
| 40 | | | to ``bidirectional_iterator_tag`` | | | |
| 41 | +-----------------------+-------------------------------------------+---------------------------+ | |
| 42 | ||
| 43 | ||
| 44 | Expression semantics | |
| 45 | -------------------- | |
| 46 | ||
| 47 | .. parsed-literal:: | |
| 48 | ||
| 49 | typedef prior<i>::type j; | |
| 50 | ||
| 51 | :Precondition: | |
| 52 | ``i`` is decrementable | |
| 53 | ||
| 54 | :Semantics: | |
| 55 | ``j`` is an iterator pointing to the previous element of the | |
| 56 | sequence | |
| 57 | ||
| 58 | :Postcondition: | |
| 59 | ``j`` is dereferenceable and incrementable | |
| 60 | ||
| 61 | ||
| 62 | Invariants | |
| 63 | ---------- | |
| 64 | ||
| 65 | For any bidirectional iterators ``i`` and ``j`` the following invariants | |
| 66 | always hold: | |
| 67 | ||
| 68 | * If ``i`` is incrementable, then ``prior< next<i>::type >::type`` is a null | |
| 69 | operation; similarly, if ``i`` is decrementable, ``next< prior<i>::type >::type`` | |
| 70 | is a null operation. | |
| 71 | ||
| 72 | ||
| 73 | See also | |
| 74 | -------- | |
| 75 | ||
| 76 | |Iterators|, |Forward Iterator|, |Random Access Iterator|, |Bidirectional Sequence|, |prior| | |
| 77 | ||
| 78 | ||
| 79 |