std::input_iterator_tag, std::output_iterator_tag, std::forward_iterator_tag, std::bidirectional_iterator_tag, std::random_access_iterator_tag
From cppreference.com
| 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_tagcorresponds toInputIterator. -
output_iterator_tagcorresponds toOutputIterator. -
forward_iterator_tagcorresponds toForwardIterator. -
bidirectional_iterator_tagcorresponds toBidirectionalIterator. -
random_access_iterator_tagcorresponds toRandomAccessIterator.
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.
NotesThere is no separate tag for |
(since C++17) |
[edit] 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
[edit] Example
Algorithm selection based on iterator category tags using std::enable_if
Run this code
#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
[edit] See also
| the basic iterator (class template) | |
| provides uniform interface to the properties of an iterator (class template) |