iterator_traits

Iterator traits class

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

Provides basic information about an iterator.

None

template <class Iterator> struct iterator_traits
{
   typedef Iterator::value_type value_type;
   typedef Iterator::distance_type distance_type;
   typedef Iterator::iterator::category iterator_category;
};

// Specialization
template <class T> struct iterator_traits<T*>
{
   typedef T value_type;
   typedef Distance ptrdiff_t;
   typedef Category random_access_iterator_tag;
};

The iterator_traits template and specialization provides a uniform way for algorithms to access information about a particular iterator. The template depends on an iterator providing a basic interface consisting of the types value_type, distance_type, and iterator_category, or on there being a specialization for the iterator. The library provides one specialization (partial) to handle all pointer iterator types.

iterator_traits are used within algorithms to provide local variables of the type pointed to by the iterator, or of the iterator's distance type. The traits are is also used to improve the efficiency of algorithms by making use of knowledge about basic iterator categories provided by the iterator_category member. An algorithm can use this "tag" 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 to select from one of several different auxiliary functions. The iterator_category must therefore be one of the iterator tags provided by the library.

input_iterator_tag
output_iterator_tag
forward_iterator_tag
bidirectional_iterator_tag
random_access_iterator_tag

iterator_traits::iterator_category is typically used like this:

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

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-diretional
  // capability of the iterators
}

_etc.

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

If your compiler does not support partial specialization then this template and specialization will not be available to you. Instead you will need to use the distance_type, value_type, and iterator_category families of function templates. The Rogue Wave Standard C++ Library also provides alternate implementations of the distance, advance, and count functions when partial specialization is not supported by a particular compiler.

value_type, distance_type, iterator_category, distance, advance, iterator

©Copyright 1996, Rogue Wave Software, Inc.