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
• Let T be the value type of It. The type std::iterator_traits<It&tg;::reference must be either
  • T& or T&& (since C++11) if It satisfies LegacyOutputIterator (It is mutable), or
  • const T& or const T&& (since C++11) 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 lvalue 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
i++	It	It ip = i; ++i; return ip;
*i++	reference

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.

Singular iterators (since C++14)

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

Concept (since C++20)

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For the definition of std::iterator_traits, the following exposition-only concept is defined.

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

where the exposition-only concept __LegacyIterator is described in LegacyIterator#Concept.

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 3798	C++20	__LegacyForwardIterator requires std::iter_reference_t<It> to be an lvalue reference type	also allows rvalue reference