C++ concepts: ForwardIterator

A ForwardIterator is an Iterator that can read data from the pointed-to element.

Unlike InputIterator and OutputIterator, it can be used in multipass algorithms.

Requirements

The type It satisfies ForwardIterator if.

• The type It satisfies InputIterator
• The type It satisfies DefaultConstructible
• Objects of the type It provide multipass guarantee described below
• The type std::iterator_traits<It>::reference must be exactly

• T& if It satisfies OutputIterator (It is mutable)
• const T& otherwise (It is constant),

(where T is the type denoted by std::iterator_traits<It>::value_type)

• Equality and inequality comparison is defined over all iterators for the same underlying sequence and the value initialized-iterators (since C++14).

And, given.

• i, dereferenceable iterator of type It
• reference, the type denoted by std::iterator_traits<It>::reference

The following expressions must be valid and have their specified effects.

Expression	Return type	Equivalent expression	Notes
i++	It	It ip=i; ++i; return ip;
*i++	reference

A mutable ForwardIterator is a ForwardIterator that additionally satisfies the OutputIterator requirements.

Multipass guarantee

Given a and b, dereferenceable iterators of type It.

• If a and b compare equal (a == b is contextually convertible to true) then either they are both non-dereferenceable or *a and *b are references bound to the same object
• Assignment through a mutable ForwardIterator iterator cannot invalidate the iterator (implicit due to reference defined as a true reference)
• incrementing a copy of a does not change the value read from a (formally, either It is a raw pointer type or the expression (void)++It(a), *a is equivalent to the expression *a)
• a == b implies ++a == ++b

Singular iterators A value-initialized ForwardIterator behaves like the past-the-end iterator of some unspecified empty container: it compares equal to all value-initialized ForwardIterators of the same type.

(since C++14)

See also

• Iterator library