std::input_iterator_tag, std::output_iterator_tag, std::forward_iterator_tag, std::bidirectional_iterator_tag, std::random_access_iterator_tag, std::contiguous_iterator_tag

 

C++

Compiler support

Freestanding and hosted

Language

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (C++17)

Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

Technical specifications

Symbols index

External libraries

 

Iterator library

Iterator concepts
indirectly_readable (C++20) indirectly_writable (C++20) weakly_incrementable (C++20) incrementable (C++20) input_or_output_iterator (C++20) sentinel_for (C++20) sized_sentinel_for (C++20)	input_iterator (C++20) output_iterator (C++20) forward_iterator (C++20) bidirectional_iterator (C++20) random_access_iterator (C++20) contiguous_iterator (C++20)
Iterator primitives
iterator_traits input_iterator_tagoutput_iterator_tagforward_iterator_tagbidirectional_iterator_tagrandom_access_iterator_tagcontiguous_iterator_tag (C++20) iterator (deprecated in C++17)	incrementable_traits (C++20) indirectly_readable_traits (C++20) iter_value_titer_difference_titer_reference_titer_rvalue_reference_titer_common_reference_t (C++20)(C++20)(C++20)(C++20)(C++20)
Algorithm concepts and utilities
Indirect callable concepts
indirectly_unary_invocableindirectly_regular_unary_invocable (C++20)(C++20) indirect_unary_predicate (C++20)	indirect_binary_predicate (C++20) indirect_equivalence_relation (C++20) indirect_strict_weak_order (C++20)
Common algorithm requirements
indirectly_movable (C++20) indirectly_movable_storable (C++20) indirectly_copyable (C++20) indirectly_copyable_storable (C++20)	indirectly_swappable (C++20) indirectly_comparable (C++20) permutable (C++20) mergeable (C++20) sortable (C++20)
Utilities
indirect_result_t (C++20)	projected (C++20)
Iterator adaptors
reverse_iterator move_iterator (C++11) move_sentinel (C++20) common_iterator (C++20) counted_iterator (C++20) back_insert_iterator front_insert_iterator insert_iterator	make_reverse_iterator (C++14) make_move_iterator (C++11) default_sentinel_tdefault_sentinel (C++20)(C++20) unreachable_sentinel_tunreachable_sentinel (C++20)(C++20) back_inserter front_inserter inserter
Stream iterators
istream_iterator ostream_iterator	istreambuf_iterator ostreambuf_iterator
Iterator customization points
ranges::iter_move (C++20)
ranges::iter_swap (C++20)
Iterator operations
advance distance prev (C++11) next (C++11)	ranges::advance (C++20) ranges::distance (C++20) ranges::prev (C++20) ranges::next (C++20)
Range access
begincbegin (C++11)(C++14) endcend (C++11)(C++14) sizessize (C++17)(C++20)	rbegincrbegin (C++14)(C++14) rendcrend (C++14)(C++14) empty (C++17) data (C++17)

 

Defined in header <iterator>
struct input_iterator_tag { };	(1)
struct output_iterator_tag { };	(2)
struct forward_iterator_tag : public input_iterator_tag { };	(3)
struct bidirectional_iterator_tag : public forward_iterator_tag { };	(4)
struct random_access_iterator_tag : public bidirectional_iterator_tag { };	(5)
struct contiguous_iterator_tag: public random_access_iterator_tag { };	(6)	(since C++20)

Defines the category of an iterator. Each tag is an empty type.

Iterator category

For every LegacyIterator type It, a typedef std::iterator_traits<It>::iterator_category must be defined to be an alias to one of these tag types, to indicate the most specific category that It is in.

1. input_iterator_tag corresponds to LegacyInputIterator.
2. output_iterator_tag corresponds to LegacyOutputIterator.
3. forward_iterator_tag corresponds to LegacyForwardIterator.
4. bidirectional_iterator_tag corresponds to LegacyBidirectionalIterator.
5. random_access_iterator_tag corresponds to LegacyRandomAccessIterator.

Iterator category tags carry information that can be used to select the most efficient algorithms for the specific requirement set that is implied by the category.

Iterator concept For every input_iterator type It, either It::iterator_concept (if std::iterator_traits<It> is generated from primary template) or std::iterator_traits<It>::iterator_concept (if std::iterator_traits<It> is specialized) may be declared as an alias to one of these tags, to indicate the strongest iterator concept that It intends to model. input_iterator_tag corresponds to input_iterator. forward_iterator_tag corresponds to forward_iterator. bidirectional_iterator_tag corresponds to bidirectional_iterator. random_access_iterator_tag corresponds to random_access_iterator. contiguous_iterator_tag corresponds to contiguous_iterator. If iterator_concept is not provided, iterator_category is used as a fallback. If iterator_category is not provided either (i.e. It is not a LegacyIterator), and std::iterator_traits<It> is not specialized, random_access_iterator_tag is assumed. In any case, each concept is not satisfied if the required operations are not supported, regardless of the tag.

(since C++20)

Notes

There is no separate tag for LegacyContiguousIterator. That is, it is not possible to tell a LegacyContiguousIterator based on its iterator_category. To define specialized algorithm for contiguous iterators, use the contiguous_iterator concept. (since C++20)

Example

Common technique for algorithm selection based on iterator category tags is to use a dispatcher function (the alternative is std::enable_if)

Run this code

#include <iostream>
#include <vector>
#include <list>
#include <iterator>
 
// quite often implementation details are hidden in a dedicated namespace
namespace implementation_details {
template<class BDIter>
void alg(BDIter, BDIter, std::bidirectional_iterator_tag)
{
    std::cout << "alg() called for bidirectional iterator\n";
}
 
template<class RAIter>
void alg(RAIter, RAIter, std::random_access_iterator_tag)
{
    std::cout << "alg() called for random-access iterator\n";
}
} // namespace implementation_details
 
template<class Iter>
void alg(Iter first, Iter last)
{
    implementation_details::alg(first, last,
        typename std::iterator_traits<Iter>::iterator_category());
}
 
int main()
{
    std::vector<int> v;
    alg(v.begin(), v.end());
 
    std::list<int> l;
    alg(l.begin(), l.end());
 
//    std::istreambuf_iterator<char> i1(std::cin), i2;
//    alg(i1, i2); // compile error: no matching function for call
}

Output:

alg() called for random-access iterator
alg() called for bidirectional iterator

See also

iterator (deprecated in C++17)	base class to ease the definition of required types for simple iterators (class template)
iterator_traits	provides uniform interface to the properties of an iterator (class template)