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Apache C++ Standard Library Reference Guide


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An iterator primitive that determines the category to which an iterator belongs.

NOTE -- This function is now obsolete. It is retained here to support compilers that do not include partial specialization, but will be dropped in a subsequent release.


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


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

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:

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

See Also

Other iterator primitives: __distance_type(), distance(), advance(), iterator

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