Книга: Standard Template Library Programmer
Forward Iterator
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Forward Iterator
Category: iterators
Component type: concept
Description
A Forward Iterator is an iterator that corresponds to the usual intuitive notion of a linear sequence of values. It is possible to use Forward Iterators (unlike Input Iterators and Output Iterators) in multipass algorithms. Forward Iterators do not, however, allow stepping backwards through a sequence, but only, as the name suggests, forward.
A type that is a model of Forward Iterator may be either mutable or immutable, as defined in the Trivial Iterators requirements.
Refinement of
Input Iterator, Output Iterator
Associated types
The same as for Input Iterator
Notation
X A type that is a model of Forward Iterator
T The value type of X
i, j Object of type X
t Object of type T
Valid expressions
Forward Iterator does not define any new expressions beyond those defined in Input Iterator. However, some of the restrictions described in Input Iterator are relaxed.
| Name | Expression | Return type |
|---|---|---|
| Preincrement | ++i |
X& |
| Postincrement | i++ |
X |
Expression semantics
Forward Iterator does not define any new expressions beyond those defined in Input Iterator. However, some of the restrictions described in Input Iterator are relaxed.
| Name | Expression | Precondition | Semantics | Postcondition |
|---|---|---|---|---|
| Preincrement | ++i |
i is dereferenceable | i points to the next value | i is dereferenceable or past-the-end. &i == &++i . If i == j , then ++i == ++j. [1] |
| Postincrement | i++ |
i is dereferenceable | Equivalent to {X tmp = i; ++i; return tmp;} | i is dereferenceable or past-the-end. [1] |
Complexity guarantees
The complexity of operations on Forward Iterators is guaranteed to be amortized constant time.
Models
• T*
• hash_set<T>::iterator
Notes
[1] The restrictions described in Input Iterator have been removed. Incrementing a forward iterator does not invalidate copies of the old value and it is guaranteed that, if i and j are dereferenceable and i == j, then ++i == ++j. As a consequence of these two facts, it is possible to pass through the same Forward Iterator twice.
See also
Input Iterator, Output Iterator, Bidirectional Iterator, Random Access Iterator, Iterator overview
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