feedc0de/boost_iterator
1
0
Fork 0
forked from boostorg/iterator
Code Pull Requests Activity

some minor edits

Browse Source 
[SVN r1180]
This commit is contained in:
Jeremy Siek
2003-04-25 02:29:28 +00:00
parent aca64eb5b4
commit 80b70a675d
1 changed files with 54 additions and 52 deletions
Download Patch File Download Diff File Expand all files Collapse all files

106
doc/new-iter-concepts.rst
View File

@@ -13,7 +13,7 @@
.. _`Open Systems Lab`: http://www.osl.iu.edu
.. _`Institute for Transport Railway Operation and Construction`: http://www.ive.uni-hannover.de
:abstract: We propose a new system of iterator concepts that treat
:Abstract: We propose a new system of iterator concepts that treat
access and positioning independently. This allows the
concepts to more closely match the requirements
of algorithms and provides better categorizations
@@ -30,11 +30,14 @@
The standard iterator categories and requirements are flawed because
they use a single hierarchy of concepts to address two orthogonal
issues: *iterator traversal* and *value access*. The current iterator
concept hierarchy is geared towards iterator traversal (hence the
category names), while requirements that address value access sneak in
at various places. The following table gives a summary of the current
value access requirements in the iterator categories.
issues: *iterator traversal* and *value access*. As a result
algorithms with type requirements expressed by the iterator
requirements are too strict. Also many concrete iterators can not be
accurately categorized. The current iterator concept hierarchy is
geared towards iterator traversal (hence the category names), while
requirements that address value access sneak in at various places. The
following table gives a summary of the current value access
requirements in the iterator categories.
+------------------------+-------------------------------------------------------------------------+
| Output Iterator | ``*i = a`` |
@@ -57,25 +60,26 @@ single hierarchy, many useful iterators can not be appropriately
categorized. For example, ``vector<bool>::iterator`` is almost a
random access iterator, but the return type is not ``bool&`` (see
`issue 96`_ and Herb Sutter's paper J16/99-0008 = WG21
N1185). Therefore, its iterators only meet the requirements of input
iterator and output iterator. This is so nonintuitive that at least
one implementation erroneously assigns ``random_access_iterator_tag``
as its ``iterator_category``. Also, ``vector<bool>`` is not the only
example of useful iterators that do not return true references: there
is the often cited example of disk-based collections.
N1185). Therefore, the iterators of ``vector<bool>`` only meet the
requirements of input iterator and output iterator. This is so
nonintuitive that at least one implementation erroneously assigns
``random_access_iterator_tag`` as its ``iterator_category``. Also,
``vector<bool>`` is not the only example of useful iterators that do
not return true references: there is the often cited example of
disk-based collections.
.. _issue 96: http://anubis.dkuug.dk/JTC1/SC22/WG21/docs/lwg-active.html#96
Another example is a counting iterator, an iterator the returns a
sequence of integers when incremented and dereferenced (see
counting_iterator_). There are two ways to implement this iterator,
1) return a true reference from ``operator[]`` (a reference to an
integer data member of the counting iterator) or 2) return the
``value_type`` or a proxy from ``operator[]``. Option 1) runs into the
problems discussed in `issue 198`_, the reference will not be valid
after the iterator is destroyed. Option 2) is therefore a better
choice, but then we have a counting iterator that cannot be a random
access iterator.
Another example of a difficult to categorize iterator is a counting
iterator, an iterator the returns a sequence of integers when
incremented and dereferenced (see counting_iterator_). There are two
ways to implement this iterator, 1) return a true reference from
``operator[]`` (a reference to an integer data member of the counting
iterator) or 2) return the ``value_type`` or a proxy from
``operator[]``. Option 1) runs into the problems discussed in `issue
198`_, the reference will not be valid after the iterator is
destroyed. Option 2) is therefore a better choice, but then we have a
counting iterator that cannot be a random access iterator.
.. _counting_iterator: http://www.boost.org/libs/utility/counting_iterator.htm
.. _issue 198: http://anubis.dkuug.dk/JTC1/SC22/WG21/docs/lwg-active.html#198
@@ -99,7 +103,7 @@ iterator category is ``input_iterator_tag``, which means that,
strictly speaking, you could not use these iterators with algorithms
like ``min_element()``. As a temporary solution, the concept
`Multi-Pass Input Iterator`_ was introduced to describe the vertex and
edge descriptors, but as the design notes for concept suggest, a
edge descriptors, but as the design notes for the concept suggest, a
better solution is needed.
.. _Boost Graph Library: http://www.boost.org/libs/graph/doc/table_of_contents.html
@@ -120,15 +124,18 @@ things happen:
========================
The new iterator concepts are backwards compatible with the old
iterator requirements. Iterators that satisfy the old requirements
also satisfy appropriate concepts in the new system, and iterators
modeling the new concepts will automatically satisfy the appropriate
old requirements.
iterator requirements, and old iterators are forward compatible with
the new iterator concepts. That is to say, iterators that satisfy the
old requirements also satisfy appropriate concepts in the new system,
and iterators modeling the new concepts will automatically satisfy the
appropriate old requirements.
The algorithms in the standard library benefit from the new iterator
concepts because the new concepts provide a more accurate way to
express their type requirements. The result is algorithms that are
usable in more situations and have fewer type requirements.
usable in more situations and have fewer type requirements. The
following lists the proposed changes to the type requirements of
algorithms.
Forward Iterator -> Forward Traversal Iterator and Readable Iterator
``find_end, find_first_of, adjacent_find, search, search_n, rotate_copy, lower_bound, upper_bound, equal_range, binary_search, min_element, max_element``
@@ -216,14 +223,14 @@ Like the old iterator requirements, we provide tags for purposes of
dispatching. There are two hierarchies of tags, one for the access
concepts and one for the traversal concepts. We provide an access
mechanism for mapping iterator types to these new tags. Our design
opts to reuse ``iterator_traits<Iter>::iterator_category`` as the
access mechanism. To enable this, a pair of access and traversal tags
are combined using the new `iterator_tag` class.
reuses ``iterator_traits<Iter>::iterator_category`` as the access
mechanism. To enable this, a pair of access and traversal tags are
combined into a single type using the following `iterator_tag` class.
::
template <class AccessTag, class TraversalTag>
struct iterator_tag : <appropriate old category>
struct iterator_tag : appropriate old category
{
typedef AccessTag access;
typedef TraversalTag traversal;
@@ -234,11 +241,11 @@ iterator tag or tags from the old requirements based on the new-style
tags passed as template parameters. The algorithm for determining the
old tag or tags from the new tags picks the least-refined old concepts
that include all of the requirements of the access and traversal
concepts, if any such category exists. For example, a the category
tag for a Readable Single Pass Iterator will 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 ``writable_iterator_tag``.
concepts (that is, the closest fit), if any such category exists. For
example, a the category tag for a Readable Single Pass Iterator will
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 two helper classes that make it convenient to obtain
the access and traversal tags of an iterator. These helper classes
@@ -523,7 +530,7 @@ Addition to [lib.iterator.synopsis]
template <class Iterator> struct traversal_category;
template <class AccessTag, class TraversalTag>
struct iterator_tag : <appropriate old category> {
struct iterator_tag : appropriate old category {
typedef AccessTag access;
typedef TraversalTag traversal;
};
@@ -558,7 +565,7 @@ pseudo-code.
::
inherit-category(access-tag, traversal-tag) {
inherit-category(access-tag, traversal-tag) =
if (access-tag is convertible to readable_lvalue_iterator_tag
or access-tag is convertible to writable_lvalue_iterator_tag) {
if (traversal-tag is convertible to random_access_traversal_tag)
@@ -579,8 +586,7 @@ pseudo-code.
and traversal-tag is convertible to incrementable_iterator_tag)
return output_iterator_tag;
else
return null_category_tag
}
return null_category_tag;
The ``access_category`` and ``traversal_category`` class templates are
@@ -590,32 +596,28 @@ the ``access_category`` and ``traversal_category`` traits access the
``access`` and ``traversal`` member types within ``iterator_tag``.
For iterators whose ``iterator_traits<Iter>::iterator_category`` type
is not ``iterator_tag`` and instead is a tag convertible to one of the
original tags, an appropriate traversal and access tags are deduced.
original tags, the appropriate traversal and access tags is deduced.
The following pseudo-code describes the algorithm.
::
template <class Iterator>
struct access_category {
// pseudo code
access-category(Iterator) =
cat = iterator_traits<Iterator>::iterator_category;
if (cat == iterator_tag<Access,Traversal>)
return Access;
else if (cat is convertible to forward_iterator_tag)
else if (cat is convertible to forward_iterator_tag) {
if (iterator_traits<Iterator>::reference is a const reference)
return readable_lvalue_iterator_tag;
else
return writable_lvalue_iterator_tag;
else if (cat is convertible to input_iterator_tag)
} else if (cat is convertible to input_iterator_tag)
return readable_iterator_tag;
else if (cat is convertible to output_iterator_tag)
return writable_iterator_tag;
else
return null_category_tag;
};
template <class Iterator>
struct traversal_category {
// pseudo code
traversal-category(Iterator) =
cat = iterator_traits<Iterator>::iterator_category;
if (cat == iterator_tag<Access,Traversal>)
return Traversal;
@@ -634,7 +636,7 @@ original tags, an appropriate traversal and access tags are deduced.
};
The following specializations provide the access and traversal
categories for pointer types.
category tags for pointer types.
::