std::forward_iterator

Since C++20

Header: <iterator>

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.

# Declarations

template< class I >
concept forward_iterator =
std::input_iterator<I> &&
std::derived_from</*ITER_CONCEPT*/<I>, std::forward_iterator_tag> &&
std::incrementable<I> &&
std::sentinel_for<I, I>;

(since C++20)

# Notes

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

# Example

#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