C++ named requirements: LegacyForwardIterator

A LegacyForwardIterator is a LegacyIterator that can read data from the pointed-to element.

Unlike LegacyInputIterator and LegacyOutputIterator, it can be used in multipass algorithms.

If a LegacyForwardIterator it originates from a Container, then it's value_type is the same as the container's, so dereferencing (*it) obtains the container's value_type.

Requirements

The type It satisfies LegacyForwardIterator if.

• The type It satisfies LegacyInputIterator
• 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 LegacyOutputIterator (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.

A mutable LegacyForwardIterator is a LegacyForwardIterator that additionally satisfies the LegacyOutputIterator 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
• If *a and *b refer to the same object, then a == b.
• 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

template<class I>
concept __LegacyForwardIterator =
  __LegacyInputIterator<I> && std::constructible_from<I> &&
  std::is_lvalue_reference_v<std::iter_reference_t<I>> &&
  std::same_as<
    std::remove_cvref_t<std::iter_reference_t<I>>,
    typename std::indirectly_readable_traits<I>::value_type> &&
  requires(I i) {
    {  i++ } -> std::convertible_to<const I&>;
    { *i++ } -> std::same_as<std::iter_reference_t<I>>;
  };

See also