++++++++++++++++++++
 Iterator Archetype
++++++++++++++++++++

:Author: David Abrahams, Jeremy Siek, Thomas Witt
:Contact: dave@boost-consulting.com, jsiek@osl.iu.edu, witt@styleadvisor.com
:organization: `Boost Consulting`_, Indiana University `Open Systems
               Lab`_, `Zephyr Associates, Inc.`_
:date: $Date: 2004/01/27 04:06:21 $
:copyright: Copyright David Abrahams, Jeremy Siek, and Thomas Witt 2004. All rights reserved

.. _`Boost Consulting`: http://www.boost-consulting.com
.. _`Open Systems Lab`: http://www.osl.iu.edu
.. _`Zephyr Associates, Inc.`: http://www.styleadvisor.com

:abstract: iterator archetypes provide a means to check the compile time requirements of a generic component on its iterator arguments.

.. contents:: Table of Contents

Reference
=========

``iterator_archetype`` Synopsis
...............................

::

    namespace iterator_archetypes
    {
        // Access categories

        typedef /*implementation  defined*/ readable_iterator_t;
        typedef /*implementation  defined*/ writable_iterator_t;
        typedef /*implementation  defined*/ readable_writable_iterator_t;
        typedef /*implementation  defined*/ readable_lvalue_iterator_t;
        typedef /*implementation  defined*/ writable_lvalue_iterator_t;

    }

    template <
        class Value
      , class AccessCategory
      , class TraversalCategory
    >
    class iterator_archetype
    {
        typedef /* see below */ value_type;
        typedef /* see below */ reference;
        typedef /* see below */ pointer;
        typedef /* see below */ difference_type;
        typedef /* see below */ iterator_category;
    };

``Access Category Tags``
........................

The access category types provided correspond to the following
standard iterator access concept combinations:

::

    readable_iterator_t :=
  
      Readable Iterator

    writable_iterator_t :=
  
      Writeable Iterator

    readable_writable_iterator_t :=
  
      Readable Iterator & Writeable Iterator & Swappable Iterator

    readable_lvalue_iterator_t :=
  
      Readable Iterator & Lvalue Iterator

    writeable_lvalue_iterator_t :=
  
      Readable Iterator & Writeable Iterator & Swappable Iterator & Lvalue Iterator

``iterator_archetype`` Requirements
...................................

The ``AccessCategory`` argument must be one of the predefined access
category tags. The ``TraversalCategory`` must be one of the standard
traversal tags. The ``Value`` type must satisfy the requirements of
the iterator concept specified by ``AccessCategory`` and
``TraversalCategory`` as implied by the nested traits types.

``iterator_archetype`` Models
.............................

``iterator_archetype`` models the iterator concepts specified by the
``AccessCategory`` and ``TraversalCategory``
arguments. ``iterator_archetype`` does not model any other access
concepts or any more derived traversal concepts.

``Traits``
..........

The nested trait types are defined as follows:

::

   if (AccessCategory == readable_iterator_t)
     
     value_type = Value
     reference  = Value
     pointer    = Value*

   else if (AccessCategory == writable_iterator_t)
 
     value_type = void
     reference  = void
     pointer    = void

   else if (AccessCategory == readable_writable_iterator_t)
 
     value_type = Value

     reference :=

       A type X that is convertible to Value for which the following
       expression is valid. Given an object x of type X and v of type 
       Value.

       x = v

     pointer    = Value*

   else if (AccessCategory == readable_lvalue_iterator_t)
     
     value_type = Value
     reference  = Value const&
     pointer    = Value const*

   else if (AccessCategory == writable_lvalue_iterator_t)
     
     value_type = Value
     reference  = Value&
     pointer    = Value*

   if ( TraversalCategory is convertible to forward_traversal_tag )

     difference_type := ptrdiff_t

   else
  
     difference_type := unspecified type

   
   iterator_category := 

     A type X satisfying the following two constraints:

        1. X is convertible to X1, and not to any more-derived
           type, where X1 is defined by:

             if (reference is a reference type
                 && TraversalCategory is convertible to forward_traversal_tag)
             {
                 if (TraversalCategory is convertible to random_access_traversal_tag)
                     X1 = random_access_iterator_tag
                 else if (TraversalCategory is convertible to bidirectional_traversal_tag)
                     X1 = bidirectional_iterator_tag
                 else
                     X1 = forward_iterator_tag
             }
             else
             {
                 if (TraversalCategory is convertible to single_pass_traversal_tag
                     && reference != void)
                     X1 = input_iterator_tag
                 else
                     X1 = output_iterator_tag
             }

        2. X is convertible to TraversalCategory

        