This concept refines std::input_iterator by requiring that I also model std::incrementable (thereby making it suitable for multi-pass algorithms), and guaranteeing that two iterators to the same range can be compared against each other.

Iterator concept determination

Definition of this concept is specified via an exposition-only alias template /*ITER_CONCEPT*/.

In order to determine /*ITER_CONCEPT*/<I>, let ITER_TRAITS<I> denote I if the specialization std::iterator_traits<I> is generated from the primary template, or std::iterator_traits<I> otherwise:

• If ITER_TRAITS<I>::iterator_concept is valid and names a type, /*ITER_CONCEPT*/<I> denotes the type.
• Otherwise, if ITER_TRAITS<I>::iterator_category is valid and names a type, /*ITER_CONCEPT*/<I> denotes the type.
• Otherwise, if std::iterator_traits<I> is generated from the primary template, /*ITER_CONCEPT*/<I> denotes std::random_access_iterator_tag.
  (That is, std::derived_from</*ITER_CONCEPT*/<I>, std::forward_iterator_tag> is assumed to be true.)
• Otherwise, /*ITER_CONCEPT*/<I> does not denote a type and results in a substitution failure.

Semantic requirements

I models std::forward_iterator if, and only if I models all the concepts it subsumes, and given objects i and j of type I:

• Comparison between iterators i and j has a defined result if

• i and j are iterators to the same underlying sequence, or
• both i and j are value-initialized, in which case they compare equal.

• Pointers and references obtained from a forward iterator into a range remain valid while the range exists.
• If i and j are dereferenceable, they offer the multi-pass guarantee, that is:

• i == j implies ++i == ++j, and
• ((void)[](auto x){ ++x; }(i), *i) is equivalent to *i.

Notes

Unlike the LegacyForwardIterator requirements, the forward_iterator concept does not require dereference to return a reference.

Example

A minimum forward iterator.

#include <cstddef>
#include <iterator>

struct SimpleForwardIterator
{
    using difference_type = std::ptrdiff_t;
    using value_type = int;

    int operator*() const;

    SimpleForwardIterator& operator++();

    SimpleForwardIterator operator++(int)
    {
        auto tmp = *this;
        ++*this;
        return tmp;
    }

    bool operator==(const SimpleForwardIterator&) const;
};

static_assert(std::forward_iterator<SimpleForwardIterator>);

See also