# 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

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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.

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. `input_iterator_tag` corresponds to `input_iterator`. `forward_iterator_tag` corresponds to `forward_iterator`. `bidirectional_iterator_tag` corresponds to `bidirectional_iterator`. `random_access_iterator_tag` corresponds to `random_access_iterator`. `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. | (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)

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

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