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C++ named requirements: LegacyForwardIterator

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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.

Contents

[edit] Requirements

The type It satisfies LegacyForwardIterator if

  • a reference to T if It satisfies LegacyOutputIterator (It is mutable), or
  • a reference to 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

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)

Concept

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_lvalue_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 __LegacyInputIterator<T> is described in LegacyInputIterator#Concept.

Note that whilst LegacyForwardIterator allows for iterators to have their reference types being rvalue reference types, this concept does not. And so it is important for such iterators (e.g. std::move_iterator) to explicitly specify their It::iterator_category to be std::forward_iterator_tag (or stronger iterator category tags), as std::iterator_traits<It>::iterator_category would be incorrectly deduced as std::input_iterator_tag otherwise.

(since C++20)

[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