Standard C++ LibraryCopyright 1996, Rogue Wave Software, Inc.

NAME

  iterator_traits  - Provides basic information about an iterator.

SYNOPSIS

  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;
  };

DESCRIPTION

  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.

TAG TYPES

  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.

WARNING

  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.

SEE ALSO

  value_type, distance_type, iterator_category, distance,advance, iterator

STANDARDS CONFORMANCE

  ANSI X3J16/ISO WG21 Joint C++ Committee



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