< cpp‎ | iterator
Iterator library
Iterator concepts

Iterator primitives
Algorithm concepts and utilities
Indirect callable concepts
Common algorithm requirements
Iterator adaptors
Range access
Defined in header <iterator>
template< class I >

    concept input_iterator =
        std::input_or_output_iterator<I> &&
        std::indirectly_readable<I> &&
        requires { typename /*ITER_CONCEPT*/<I>; } &&

        std::derived_from</*ITER_CONCEPT*/<I>, std::input_iterator_tag>;
(since C++20)

The input_iterator concept is a refinement of input_or_output_iterator, adding the requirement that the referenced values can be read (via indirectly_readable) and the requirement that the iterator concept tag be present.


[edit] 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> is generated from the primary template, /*ITER_CONCEPT*/<I> denotes std::random_access_iterator_tag.
  • Otherwise, /*ITER_CONCEPT*/<I> does not denote a type and results in a substitution failure.

[edit] Notes

Unlike the LegacyInputIterator requirements, the input_iterator concept does not require equality_comparable, since input iterators are typically compared with sentinels.

[edit] Example

A minimum input iterator.

#include <cstddef>
#include <iterator>
struct SimpleInputIterator
    using difference_type = std::ptrdiff_t;
    using value_type = int;
    int operator*() const;
    SimpleInputIterator& operator++();
    void operator++(int) { ++*this; }

[edit] See also

specifies that objects of a type can be incremented and dereferenced
(concept) [edit]
specifies that an input_iterator is a forward iterator, supporting equality comparison and multi-pass
(concept) [edit]