Книга: Standard Template Library Programmer
Bidirectional Iterator
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Bidirectional Iterator
Category: iterators
Component type: concept
Description
A Bidirectional Iterator is an iterator that can be both incremented and decremented. The requirement that a Bidirectional Iterator can be decremented is the only thing that distinguishes Bidirectional Iterators from Forward Iterators.
Refinement of
Forward Iterator
Associated types
The same as for Forward Iterator.
Notation
X A type that is a model of Bidirectional Iterator
T The value type of X
i, j Object of type X
t Object of type T
Valid expressions
In addition to the expressions defined in Forward Iterator, the following expressions must be valid.
| Name | Expression | Return type |
|---|---|---|
| Predecrement | --i |
X& |
| Postdecrement | i-- |
X |
Expression Semantics
Semantics of an expression is defined only where it is not defined in Forward Iterator.
| Name | Expression | Precondition | Semantics | Postcondition |
|---|---|---|---|---|
| Predecrement | --i |
i is dereferenceable or past-the-end. There exists a dereferenceable iterator j such that i == ++j. | i is modified to point to the previous element. | i is dereferenceable. &i = &--i . If i == j , then --i == --j . If j is dereferenceable and i == ++j , then --i == j. |
| Postdecrement | i-- |
i is dereferenceable or past-the-end. There exists a dereferenceable iterator j such that i == ++j. | Equivalent to { X tmp = i; --i; return tmp; } |
Complexity guarantees
The complexity of operations on bidirectional iterators is guaranteed to be amortized constant time.
Invariants
| Symmetry of increment and decrement | If i is dereferenceable, then ++i; --i; is a null operation. Similarly, --i; ++i; is a null operation. |
Models
• T*
• list<T>::iterator
See also
Input Iterator, Output Iterator, Forward Iterator, Random Access Iterator, Iterator overview
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