[section:archetypes Iterator Archetypes]

The `iterator_archetype` class constructs a minimal implementation of
one of the iterator access concepts and one of the iterator traversal concepts.
This is used for doing a compile-time check to see if a the type requirements
of a template are really enough to cover the implementation of the template.
For further information see the documentation for the |concepts|_ library.

[h2 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;
    };

[h3 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

[h3 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


[h2 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.

[h2 Concepts]

`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.

[endsect]
