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
 
C++
 
Iterator library
Iterator primitives
input_iterator_tagoutput_iterator_tagforward_iterator_tagbidirectional_iterator_tagrandom_access_iterator_tag
Iterator adaptors
Stream iterators
Iterator operations
(C++11)
(C++11)
Range access
(C++11)(C++14)
(C++11)(C++14)
(C++14)(C++14)
(C++14)(C++14)
Container access
(C++17)(C++20)
(C++17)
(C++17)
 
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 { };
 (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:

(since C++20)

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 used to indicate conformance to the iterator concepts.

(since C++20)

Notes

There is no separate tag for LegacyContiguousIterator.

(since C++17)
(until 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>
 
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";
}
 
template< class Iter >
void alg(Iter first, Iter last)
{
    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

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