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64 lines
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64 lines
2.6 KiB
ReStructuredText
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
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Problem with ``reference`` and old/new iterator category correspondance
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.. _N1550: http://www.boost-consulting.com/writing/n1550.html
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.. _N1530: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2003/n1530.html
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:Author: David Abrahams and Jeremy Siek
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:Contact: dave@boost-consulting.com, jsiek@osl.iu.edu
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:Organization: `Boost Consulting`_, Indiana University Bloomington
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:date: $Date$
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:Copyright: Copyright David Abrahams, Jeremy Siek 2003. Use, modification and
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distribution is subject to the Boost Software License,
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Version 1.0. (See accompanying file LICENSE_1_0.txt or copy
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at http://www.boost.org/LICENSE_1_0.txt)
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.. _`Boost Consulting`: http://www.boost-consulting.com
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==============
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Introduction
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==============
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The new iterator categories are intended to correspond to the old
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iterator categories, as specified in a diagram in N1550_. For example,
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an iterator categorized as a mutable Forward Iterator under the old
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scheme is now a Writable, Lvalue, and Foward Traversal iterator.
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However, there is a problem with this correspondance, the new iterator
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categories place requirements on the ``iterator_traits<X>::reference``
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type whereas the standard iterator requirements say nothing about the
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``reference`` type . In particular, the new Readable Iterator
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requirements say that the return type of ``*a`` must be
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``iterator_traits<X>::reference`` and the Lvalue Iterator requirements
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says that ``iterator_traits<X>::reference`` must be ``T&`` or ``const
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T&``.
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====================
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Proposed Resolution
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====================
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Change the standard requirements to match the requirements of the new
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iterators. (more details to come)
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==========
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Rationale
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==========
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The lack of specification in the standard of the ``reference`` type is
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certainly a defect. Without specification, it is entirely useless in a
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generic function. The current practice in the community is generally
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to assume there are requirements on the ``reference`` type, such as
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those proposed in the new iterator categories.
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There is some danger in *adding* requirements to existing concepts.
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This will mean that some existing iterator types will no longer meet
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the iterator requirements. However, we feel that the impact of this is
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small enough to warrant going ahead with this change.
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An alternative solution would be to leave the standard requirements as
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is, and to remove the requirements for the ``reference`` type in the
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new iterator concepts. We are not in favor of this approach because it
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extends what we see as a defect further into the future.
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