25 Iterators library [iterators]

25.3 Iterator requirements [iterator.requirements]

25.3.5 C++17 iterator requirements [iterator.cpp17] Forward iterators [forward.iterators]

A class or pointer type X meets the Cpp17ForwardIterator requirements if
  • X meets the Cpp17InputIterator requirements ([input.iterators]),
  • X meets the Cpp17DefaultConstructible requirements ([utility.arg.requirements]),
  • 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,
  • the expressions in Table 92 are valid and have the indicated semantics, and
  • objects of type X offer the multi-pass guarantee, described below.
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]
Two dereferenceable iterators a and b of type X offer the multi-pass guarantee if:
  • a == b implies ++a == ++b and
  • X is a pointer type or the expression (void)++X(a), *a is equivalent to the expression *a.
[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 92: Cpp17ForwardIterator requirements (in addition to Cpp17InputIterator) [tab:forwarditerator]
Return type
convertible to const X&
{ X tmp = r;
return tmp; }
If a and b are equal, then either a and b are both dereferenceable or else neither is dereferenceable.
If a and b are both dereferenceable, then a == b if and only if *a and *b are bound to the same object.