The MPL Reference Manual: iterator_category

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# iterator_category

### Synopsis

    template<
          typename Iterator
        >
    struct iterator_category
    {
        typedef typename Iterator::category type;
    };

### Description

Returns one of the following iterator category tags:

- `forward_iterator_tag`
- `bidirectional_iterator_tag`
- `random_access_iterator_tag`

### Header

    #include <boost/mpl/iterator_category.hpp>
    #include <boost/mpl/iterator_tags.hpp>

### Parameters

Parameter | Requirement | Description
--- | --- | ---
`Iterator` | Forward Iterator | The iterator to obtain a category for.

### Expression semantics

For any Forward Iterators `iter`:

    typedef iterator_category<iter>::type tag;

Return type: | Integral Constant.
Semantics: | `tag` is `forward_iterator_tag` if `iter` is a model of Forward Iterator, `bidirectional_iterator_tag` if `iter` is a model of Bidirectional Iterator, or `random_access_iterator_tag` if `iter` is a model of Random Access Iterator;
Postcondition: | `forward_iterator_tag::value < bidirectional_iterator_tag::value`, `bidirectional_iterator_tag::value < random_access_iterator_tag::value`.

### Complexity

Amortized constant time.

### Example

    template< typename Tag, typename Iterator >
    struct algorithm_impl
    {
        // O(n) implementation
    };

    template< typename Iterator >
    struct algorithm_impl<random_access_iterator_tag,Iterator>
    {
        // O(1) implementation
    };

    template< typename Iterator >
    struct algorithm
        : algorithm_impl<
              iterator_category<Iterator>::type
            , Iterator
            >
    {
    };

### See also

Iterators, begin / end, advance, distance, next