25 Iterators library [iterators]

25.3 Iterator requirements [iterator.requirements]

25.3.5 C++17 iterator requirements [iterator.cpp17]

25.3.5.5 Forward iterators [forward.iterators]

1
#
A class or pointer type X meets the requirements of a forward iterator if
  • (1.1)
    X meets the Cpp17InputIterator requirements ([input.iterators]),
  • (1.2)
    X meets the Cpp17DefaultConstructible requirements ([utility.arg.requirements]),
  • (1.3)
    if X is a mutable iterator, reference is a reference to T; if X is a constant iterator, reference is a reference to const T,
  • (1.4)
    the expressions in Table 89 are valid and have the indicated semantics, and
  • (1.5)
    objects of type X offer the multi-pass guarantee, described below.
2
#
The domain of == for forward iterators is that of iterators over the same underlying sequence.
However, value-initialized iterators may be compared and shall compare equal to other value-initialized iterators of the same type.
[Note 1: 
Value-initialized iterators behave as if they refer past the end of the same empty sequence.
— end note]
3
#
Two dereferenceable iterators a and b of type X offer the multi-pass guarantee if:
  • (3.1)
    a == b implies ++a == ++b and
  • (3.2)
    X is a pointer type or the expression (void)++X(a), *a is equivalent to the expression *a.
4
#
[Note 2: 
The requirement that a == b implies ++a == ++b (which is not true for input and output iterators) and the removal of the restrictions on the number of the assignments through a mutable iterator (which applies to output iterators) allows the use of multi-pass one-directional algorithms with forward iterators.
— end note]
Table 89: Cpp17ForwardIterator requirements (in addition to Cpp17InputIterator) [tab:forwarditerator]
Expression
Return type
Operational
Assertion/note
semantics
pre-/post-condition
r++
convertible to const X&
{ X tmp = r;
++r;
return tmp; }
*r++
reference
5
#
If a and b are equal, then either a and b are both dereferenceable or else neither is dereferenceable.
6
#
If a and b are both dereferenceable, then a == b if and only if *a and *b are bound to the same object.