C++ named requirements: LegacyForwardIterator

< cpp‎ | named req
C++ named requirements
Type properties
Container Elements

Stream I/O
Random Numbers


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.


[edit] Requirements

The type It satisfies LegacyForwardIterator if

  • 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.

[edit] 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 It>

concept __LegacyForwardIterator =
  __LegacyInputIterator<It> && std::constructible_from<It> &&
  std::is_reference_v<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 __LegacyInputIterator<T> is described in LegacyInputIterator#Concept.

(since C++20)

[edit] 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

[edit] See also

specifies that an input_iterator is a forward iterator, supporting equality comparison and multi-pass
(concept) [edit]
Iterator library provides definitions for iterators, iterator traits, adaptors, and utility functions