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added more design rationale, and moved some text

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[SVN r20158]
This commit is contained in:
Jeremy Siek
2003-09-22 16:28:27 +00:00
parent a3856b5c60
commit d85b8db0aa
2 changed files with 42 additions and 28 deletions
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29
doc/new-iter-concepts.rst
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@ -251,10 +251,10 @@ inheritance so that a tag is convertible to another tag if the concept
associated with the first tag is a refinement of the second tag.
Since the access concepts are not related via refinement, but instead
cover orthogonal issues, we do not use tags for the access concepts,
but instead use the equivalent of a bitfield.
but instead use the equivalent of a bit field.
We provide an access mechanism for mapping iterator types to the new
traversal tags and access bitfield. Our design reuses
traversal tags and access bit field. Our design reuses
``iterator_traits<Iter>::iterator_category`` as the access
mechanism. To that end, the access and traversal information is
bundled into a single type using the following `iterator_tag` class.
@ -266,10 +266,22 @@ bundled into a single type using the following `iterator_tag` class.
template <unsigned int access_bits, class TraversalTag>
struct iterator_tag : /* appropriate old category or categories */ {
static const iterator_access access = (iterator_access)access_bits;
static const iterator_access access =
(iterator_access)access_bits &
(readable_iterator | writable_iterator | swappable_iterator);
typedef TraversalTag traversal;
};
The ``access_bits`` argument is declared to be ``unsigned int``
instead of the enum ``iterator_access`` for convenience of use. For
example, the expression ``(readable_iterator | writable_iterator)``
produces an unsigned int, not an ``iterator_access``. The purpose of
the ``lvalue_iterator`` part of the ``iterator_access`` enum is to
communicate to ``iterator_tag`` whether the reference type is an
lvalue so that the appropriate old category can be chosen for the base
class. The ``lvalue_iterator`` bit is not recorded in the
``iterator_tag::access`` data member.
The ``iterator_tag`` class template is derived from the appropriate
iterator tag or tags from the old requirements based on the new-style
tags passed as template parameters. The algorithm for determining the
@ -281,6 +293,7 @@ always be derived from ``input_iterator_tag``, while the category tag
for a Single Pass Iterator that is both Readable and Writable will be
derived from both ``input_iterator_tag`` and ``output_iterator_tag``.
We also provide several helper classes that make it convenient to
obtain the access and traversal characteristics of an iterator. These
helper classes work both for iterators whose ``iterator_category`` is
@ -698,12 +711,8 @@ pseudo-code.
else
return null_category_tag;
The access argument is declared to be ``unsigned int`` instead of the
enum ``iterator_access`` for convenience of use. For example, the
expression ``(readable_iterator | writable_iterator)`` produces an
unsigned int, not ``iterator_access``. If the argument for
``TraversalTag`` is not convertible to ``incrementable_iterator_tag``
then the programm is ill-formed.
If the argument for ``TraversalTag`` is not convertible to
``incrementable_iterator_tag`` then the program is ill-formed.
The ``is_readable``, ``is_writable``, ``is_swappable``, and
``traversal_category`` class templates are traits classes. For
@ -800,4 +809,4 @@ category tags for pointer types.
LocalWords: ForwardTraversalIterator BidirectionalTraversalIterator lvalue
LocalWords: RandomAccessTraversalIterator dereferenceable Incrementable tmp
LocalWords: incrementable xxx min prev inplace png oldeqnew AccessTag struct
LocalWords: TraversalTag typename lvalues DWA Hmm JGS
LocalWords: TraversalTag typename lvalues DWA Hmm JGS mis enum