std::forward_iterator – cppreference.com

From cppreference.com

This concept refines std::input_iterator by requiring that I also model std::incrementable (thereby making it suitable for multi-pass algorithms), and guaranteeing that two iterators to the same range can be compared against each other.

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Iterator concept determination

Definition of this concept is specified via an exposition-only alias template /*ITER_CONCEPT*/.

In order to determine /*ITER_CONCEPT*/<I>, let ITER_TRAITS<I> denote I if the specialization std::iterator_traits<I> is generated from the primary template, or std::iterator_traits<I> otherwise:

• If

  ITER_TRAITS

  <

  I

  >

  ::

  iterator_concept

  is valid and names a type,

  /*ITER_CONCEPT*/

  <

  I

  >

  denotes the type.

• Otherwise, if

  ITER_TRAITS

  <

  I

  >

  ::

  iterator_category

  is valid and names a type,

  /*ITER_CONCEPT*/

  <

  I

  >

  denotes the type.

• Otherwise, if

  std::

  iterator_traits

  <

  I

  >

  /*ITER_CONCEPT*/

  <

  I

  >

  denotes std::random_access_iterator_tag

• Otherwise,

  /*ITER_CONCEPT*/

  <

  I

  >

  does not denote a type and results in a substitution failure.

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Semantic requirements

I models std::forward_iterator if, and only if I models all the concepts it subsumes, and given objects i and j of type I:

• Comparison between iterators i and j has a defined result if

• i and j are iterators to the same underlying sequence, or
• both i and j are value-initialized, in which case they compare equal.

• Pointers and references obtained from a forward iterator into a range remain valid while the range exists.
• If i and j are dereferenceable, they offer the multi-pass guarantee, that is:

• i == j implies ++i == ++j, and

• (

  (

  void

  )

  [

  ]

  (

  auto

  x

  )

  {

  ++

  x

  ;

  }

  (

  i

  )

  ,

  *

  i

  )

  is equivalent to *i.

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Notes

Unlike the LegacyForwardIterator requirements, the forward_iterator concept does not require dereference to return an lvalue.

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See also

input_iterator

(C++20)

specifies that a type is an input iterator, that is, its referenced values can be read and it can be both pre- and post-incremented

(concept)

bidirectional_iterator

(C++20)

specifies that a forward_iterator is a bidirectional iterator, supporting movement backwards

(concept)