Category: iterators
Component type: concept
A Reverse Arbitrary Access Iterator is an iterator that provides both increment and decrement (just like a Bidirectional Iterator), and that also provides constant-time methods for moving backward in arbitrary-sized steps.
Bidirectional Iterator, LessThan Comparable
The same as for Bidirectional Iterator
X | A type that is a model of Reverse Arbitrary Access Iterator |
T | The value type of X |
Distance | The distance type of X |
i, j | Object of type X |
t | Object of type T |
n | Object of type Distance |
In addition to the expressions defined in Bidirectional Iterator, the following expressions must be valid.
| Name | Expression | Type requirements | Return type |
|---|---|---|---|
| Iterator subtraction | i -= n | X& | |
| Iterator subtraction | i - n | X | |
| Difference | i - j | Distance |
Semantics of an expression is defined only where it differs from, or is not defined in, Bidirectional Iterator or LessThan Comparable.
| Name | Expression | Precondition | Semantics | Postcondition |
|---|---|---|---|---|
| Iterator subtraction | i -= n | Including i itself, there must be n dereferenceable or past-the-end iterators preceding or following i, depending on whether n is positive or negative. (n <= 0). | i is dereferenceable or past-the-end. | |
| Iterator subtraction | i - n | Same as for i -= n | Equivalent to { X tmp = i; return tmp -= n; }. | Result is dereferenceable or past-the-end |
| Difference | i - j | Either i is reachable from j or j is reachable from i, or both. | Returns a number n such that i == j + n | |
| Less | i < j | Either i is reachable from j or j is reachable from i, or both. [1] | As described in LessThan Comparable [2] |
All operations on Reverse Arbitrary Access Iterators are amortized constant time. [3]
| Symmetry of addition and subtraction | If i + n is well-defined, then i += n; i -= n; and (i + n) - n are null operations. Similarly, if i - n is well-defined, then i -= n; i += n; and (i - n) + n are null operations. |
| Relation between distance and addition | If i - j is well-defined, then i == j + (i - j). |
| Reachability and distance | If i is reachable from j, then i - j >= 0. |
| Ordering | operator < is a strict weak ordering, as defined in LessThan Comparable. |
T* vector<T>::::iterator vector<T>::::const_iterator deque<T>::::iterator deque<T>::::const_iterator [1] The precondition defined in LessThan Comparable is that i and j be in the domain of operator <. Essentially, then, this is a definition of that domain: it is the set of pairs of iterators such that one iterator is reachable from the other.
[2] All of the other comparison operators have the same domain and are defined in terms of operator <, so they have exactly the same semantics as described in LessThan Comparable.
[3] This complexity guarantee is in fact the only reason why Reverse Arbitrary Iterator exists as a distinct concept. Every operation in iterator arithmetic can be defined for Bidirectional Iterator; in fact, that is exactly what the algorithms advance and distance do. The distinction is simply that the Bidirectional Iterator implementations are linear time, while Reverse Arbitrary Iterators are required to support random access to elements in amortized logarithmic time. This has major implications for the sorts of algorithms that can sensibly be written using the two types of iterators.
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