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
Defined in header <iterator>
|
||
struct input_iterator_tag { }; |
(1) | |
struct output_iterator_tag { }; |
(2) | |
struct forward_iterator_tag : public input_iterator_tag { }; |
(3) | |
struct bidirectional_iterator_tag : public forward_iterator_tag { }; |
(4) | |
struct random_access_iterator_tag : public bidirectional_iterator_tag { }; |
(5) | |
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.
Iterator category
For every LegacyIterator type It
, a typedef
std::iterator_traits<It>::iterator_category must be defined to be an alias to one of these tag types, to indicate the most specific category that It
is in.
-
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.
Iterator conceptFor every
If In any case, each concept is not satisfied if the required operations are not supported, regardless of the tag. |
(since C++20) |
Notes
There is no separate tag for LegacyContiguousIterator. That is, it is not possible to tell a LegacyContiguousIterator based on its iterator_category
. To define specialized algorithm for contiguous iterators, use the contiguous_iterator
concept. (since C++20)
There are no correspondences between output_iterator_tag
and the output_iterator
concept. Setting iterator_concept
to output_iterator_tag
only indicates that the type does not model input_iterator
.
Example
Common technique for algorithm selection based on iterator category tags is to use a dispatcher function (the alternative is std::enable_if). The iterator tag classes are also used in the corresponding concepts definitions to denote the requirements, which can't be expressed in terms of usage patterns alone. (since C++20)
#include <iostream> #include <vector> #include <list> #include <iterator> // Using concepts (tag checking is part of the concepts themselves) template<std::bidirectional_iterator BDIter> void alg(BDIter, BDIter) { std::cout << "1. alg() \t called for bidirectional iterator\n"; } template<std::random_access_iterator RAIter> void alg(RAIter, RAIter) { std::cout << "2. alg() \t called for random-access iterator\n"; } // Legacy, using tag dispatch namespace legacy { // 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 << "3. legacy::alg() called for bidirectional iterator\n"; } template<class RAIter> void alg(RAIter, RAIter, std::random_access_iterator_tag) { std::cout << "4. legacy::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()); } } // namespace legacy int main() { std::list<int> l; alg(l.begin(), l.end()); // 1. legacy::alg(l.begin(), l.end()); // 3. std::vector<int> v; alg(v.begin(), v.end()); // 2. legacy::alg(v.begin(), v.end()); // 4. // std::istreambuf_iterator<char> i1(std::cin), i2; // alg(i1, i2); // compile error: no matching function for call // legacy::alg(i1, i2); // compile error: no matching function for call }
Output:
1. alg() called for bidirectional iterator 3. legacy::alg() called for bidirectional iterator 2. alg() called for random-access iterator 4. legacy::alg() called for random-access 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) |