C++ named requirements: LegacyForwardIterator

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C++ named requirements

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

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


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
(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

Expression Return type Equivalent expression Notes
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

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.
(since C++14)


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

template<class I>

concept __LegacyForwardIterator =
  __LegacyInputIterator<I> && std::Constructible<I> &&
  std::is_lvalue_reference_v<std::iter_reference_t<I>> &&
  std::Same<std::remove_cvref_t<std::iter_reference_t<I>>, typename std::readable_traits<I>::value_type> &&
  requires(I i) {
    {  i++ } -> const I&;
    { *i++ } -> std::Same<std::iter_reference_t<I>>;


where the exposition-only concept __LegacyInputIterator<T> is described in LegacyInputIterator#Concept.

(since C++20)

See also

specifies that an InputIterator is a forward iterator, supporting equality comparison and multi-pass