std::input_iterator_tag

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 { };

Defines the category of an iterator. Each tag is an empty type and corresponds to one of the five iterator categories:

input_iterator_tag corresponds to InputIterator.
output_iterator_tag corresponds to OutputIterator.
forward_iterator_tag corresponds to ForwardIterator.
bidirectional_iterator_tag corresponds to BidirectionalIterator.
random_access_iterator_tag corresponds to RandomAccessIterator.

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, a typedef std::iterator_traits<Iterator>::iterator_category is available, which is an alias to one of these five tag types.

Notes

There is no separate tag for ContiguousIterator.

(since C++17)

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>
 
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

Example

Algorithm selection based on iterator category tags using std::enable_if.

#include <iostream>
#include <vector>
#include <list>
#include <iterator>
 
template<typename Condition, typename T = void>
using EnableIf_t = typename std::enable_if<Condition::value, T>::type;
 
template<typename Iterator, typename IteratorTag>
using IsSameIteratorCond = 
    std::is_same<IteratorTag, 
    typename std::iterator_traits<Iterator>::iterator_category>;
 
template<
    typename BDIter, 
    typename = EnableIf_t<IsSameIteratorCond<BDIter, std::bidirectional_iterator_tag>>>
void alg(BDIter, BDIter)
{
    std::cout << "alg() called for bidirectional iterator\n";
}
 
template<
    typename RAIter, 
    typename = EnableIf_t<IsSameIteratorCond<RAIter, std::random_access_iterator_tag>>, 
    typename = void> // dummy value to avoid template re-definition error
void alg(RAIter, RAIter)
{
    std::cout << "alg() called for random-access iterator\n";
}
 
int main()
{
    std::vector<int> v;
    alg(v.begin(), v.end());
 
    std::list<int> l;
    alg(l.begin(), l.end());
}

Output:

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

iterator	the basic iterator (class template)
iterator_traits	provides uniform interface to the properties of an iterator (class template)

std::input_iterator_tag

Notes

Example

Example

See also