## __iterator_category

    Iterator primitive

- Summary
- Data Type and Member Function Indexes
- Synopsis
- Description
- Tag Types
- See Also

### Summary

Determines the category to which an iterator belongs. This function is now obsolete. It is included for backward compatibility and to support compilers that do not include partial specialization.

### Data Type and Member Function Indexes
(exclusive of constructors and destructors)

None

### Synopsis

    #include <iterator>
    template <class Category, class T, class Distance,
              class Pointer, class Reference>
    inline Category __iterator_category
      (const iterator<Category, T, Distance, Pointer,
       Reference>&);
    template <class T>
    inline random_access_iterator_tag __iterator_category
      (const T*)

### Description

The __iterator_category family of function templates allows you to determine the category to which any iterator belongs. The first function takes an iterator of a specific type and returns the tag for that type. The last takes a `T*` and returns `random_access_iterator_tag`.

### Tag Types

    input_iterator_tag
    output_iterator_tag
    forward_iterator_tag
    bidirectional_iterator_tag
    random_access_iterator_tag

The __iterator_category function is particularly useful for improving the efficiency of algorithms. An algorithm can use this function to select the most efficient implementation an iterator is capable of handling without sacrificing the ability to work with a wide range of iterator types. For instance, both the `advance` and `distance` primitives use __iterator_category to maximize their efficiency by using the tag returned from iterator_category to select from one of several different auxiliary functions. Because this is a compile time selection, use of this primitive incurs no significant runtime overhead.

__iterator_category is typically used like this:

    template <class Iterator>
    void foo(Iterator first, Iterator last)
    {
      __foo(begin,end,__iterator_category(first));
    }

    template <class Iterator>
    void __foo(Iterator first, Iterator last,
               input_iterator_tag>
    {
      // Most general implementation
    }

    template <class Iterator>
    void __foo(Iterator first, Iterator last,
               bidirectional_iterator_tag>
    {
      // Implementation takes advantage of bi-directional
      // capability of the iterators
    }

    ...etc.

See the iterator section for a description of iterators and the capabilities associated with each type of iterator tag.

### See Also

Other iterator primitives: __value_type, __distance_type, distance, advance, iterator

Copyright (c) 1998, Rogue Wave Software, Inc.
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OEM Release, June 1998