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

Definitions
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

Type requirements

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 pasttheend. &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 pasttheend. [1]

Complexity guarantees
The complexity of operations on Forward Iterators is
guaranteed to be amortized constant time.
Invariants
Models
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