NAME¶

std::input_iterator_tag,std::output_iterator_tag,std::forward_iterator_tag, - std::input_iterator_tag,std::output_iterator_tag,std::forward_iterator_tag,

Synopsis¶

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 (5)
bidirectional_iterator_tag { };
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.

1. input_iterator_tag corresponds to LegacyInputIterator.
2. output_iterator_tag corresponds to LegacyOutputIterator.
3. forward_iterator_tag corresponds to LegacyForwardIterator.
4. bidirectional_iterator_tag corresponds to LegacyBidirectionalIterator.
5. 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 concept

For every input_iterator type It, either It::iterator_concept (if
std::iterator_traits<It> is generated from primary template) or
std::iterator_traits<It>::iterator_concept (if
std::iterator_traits<It> is specialized) may be declared as an alias
to one of these tags, to indicate the strongest iterator concept that
It intends to model.

1. input_iterator_tag corresponds to input_iterator.
2. forward_iterator_tag corresponds to forward_iterator. (since C++20)
3. bidirectional_iterator_tag corresponds to bidirectional_iterator.
4. random_access_iterator_tag corresponds to random_access_iterator.
5. contiguous_iterator_tag corresponds to contiguous_iterator.

If iterator_concept is not provided, iterator_category is used as a
fallback. If iterator_category is not provided either (i.e. It is not
a LegacyIterator), and std::iterator_traits<It> is not specialized,
random_access_iterator_tag is assumed.

In any case, each concept is not satisfied if the required operations
are not supported, regardless of the tag.

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)

// Run this code

#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¶

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