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

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 { };

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:

• input_iterator_tag corresponds to LegacyInputIterator.
• output_iterator_tag corresponds to LegacyOutputIterator.
• forward_iterator_tag corresponds to LegacyForwardIterator.
• bidirectional_iterator_tag corresponds to LegacyBidirectionalIterator.
• 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.

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.

Example

#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
}

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

See also