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

< cpp‎ | iterator
Iterator library
Iterator concepts
Iterator primitives
(deprecated in C++17)
Algorithm concepts and utilities
Indirect callable concepts
Common algorithm requirements
Iterator adaptors
Stream iterators
Iterator customization points
Iterator operations
Range access
Defined in header <iterator>
struct input_iterator_tag { };
struct output_iterator_tag { };
struct forward_iterator_tag : public input_iterator_tag { };
struct bidirectional_iterator_tag : public forward_iterator_tag { };
struct random_access_iterator_tag : public bidirectional_iterator_tag { };
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 and corresponds to one of the five (until C++20)six (since C++20) iterator categories:

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.
6) contiguous_iterator_tag corresponds to LegacyContiguousIterator.

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.

For every iterator type It, a typedef std::iterator_traits<It>::iterator_category is available, which is an alias to one of these five (until C++20)six (since C++20) tag types.

Additionally, std::iterator_traits<It>::iterator_concept may be declared as an alias to one of these tags, to indicate conformance to the iterator concepts (provided that the iterator also satisfies other requirements as declared in the concepts).

(since C++20)


There is no separate tag for LegacyContiguousIterator.

(since C++17)
(until C++20)

[edit] Example

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

#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


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

[edit] See also

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