std::forward_iterator(3)

NAME
std::forward_iterator - std::forward_iterator

Synopsis
   Defined in header <iterator>
   template< class I >
       concept forward_iterator =
           std::input_iterator<I> &&
           std::derived_from</*ITER_CONCEPT*/<I>, (since C++20)
   std::forward_iterator_tag> &&
           std::incrementable<I> &&
           std::sentinel_for<I, I>;
   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.

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
                        specifies that a type is an input iterator, that is, its
   input_iterator referenced values can be read and it can be both pre- and
   (C++20) post-incremented
                        (concept)
   bidirectional_iterator specifies that a forward_iterator is a bidirectional
   (C++20) iterator, supporting movement backwards
                        (concept)