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Iterator Archetype

Author: David Abrahams, Jeremy Siek, Thomas Witt
Organization: Boost Consulting, Indiana University Open Systems Lab, Zephyr Associates, Inc.
Date: 2004-01-16
Copyright: Copyright David Abrahams, Jeremy Siek, and Thomas Witt 2004. All rights reserved
abstract:iterator archetypes provide a means to check the compile time requirements of a generic component on its iterator arguments.

Table of Contents


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.


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 

    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


  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
                  X1 = forward_iterator_tag
              if (TraversalCategory is convertible to single_pass_traversal_tag
                  && reference != void)
                  X1 = input_iterator_tag
                  X1 = output_iterator_tag

     2. X is convertible to TraversalCategory