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357 lines
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357 lines
10 KiB
HTML
Executable File
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<meta name="author" content="David Abrahams and Jeremy Siek" />
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<meta name="date" content="2003-11-17" />
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<meta name="copyright" content="Copyright David Abrahams, Jeremy Siek 2003. Use, modification and distribution is subject to the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)" />
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<body>
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<div class="document" id="problem-with-reference-and-old-new-iterator-category-correspondance">
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<h1 class="title">Problem with <tt class="docutils literal"><span class="pre">reference</span></tt> and old/new iterator category correspondance</h1>
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<table class="docinfo" frame="void" rules="none">
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<col class="docinfo-name" />
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<col class="docinfo-content" />
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<tbody valign="top">
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<tr><th class="docinfo-name">Author:</th>
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<td>David Abrahams and Jeremy Siek</td></tr>
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<tr><th class="docinfo-name">Contact:</th>
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<td><a class="first reference" href="mailto:dave@boost-consulting.com">dave@boost-consulting.com</a>, <a class="last reference" href="mailto:jsiek@osl.iu.edu">jsiek@osl.iu.edu</a></td></tr>
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<tr><th class="docinfo-name">Organization:</th>
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<td><a class="first reference" href="http://www.boost-consulting.com">Boost Consulting</a>, Indiana University Bloomington</td></tr>
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<tr><th class="docinfo-name">Date:</th>
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<td>2003-11-17</td></tr>
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<tr><th class="docinfo-name">Copyright:</th>
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<td>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 <a class="reference" href="http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt</a>)</td></tr>
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</tbody>
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</table>
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<div class="section">
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<h1><a id="introduction" name="introduction">Introduction</a></h1>
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<p>The new iterator categories are intended to correspond to the old
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iterator categories, as specified in a diagram in <a class="reference" href="http://www.boost-consulting.com/writing/n1550.html">N1550</a>. 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 <tt class="docutils literal"><span class="pre">iterator_traits<X>::reference</span></tt>
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type whereas the standard iterator requirements say nothing about the
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<tt class="docutils literal"><span class="pre">reference</span></tt> type . In particular, the new Readable Iterator
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requirements say that the return type of <tt class="docutils literal"><span class="pre">*a</span></tt> must be
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<tt class="docutils literal"><span class="pre">iterator_traits<X>::reference</span></tt> and the Lvalue Iterator requirements
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says that <tt class="docutils literal"><span class="pre">iterator_traits<X>::reference</span></tt> must be <tt class="docutils literal"><span class="pre">T&</span></tt> or <tt class="docutils literal"><span class="pre">const</span>
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<span class="pre">T&</span></tt>.</p>
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</div>
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<div class="section">
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<h1><a id="proposed-resolution" name="proposed-resolution">Proposed Resolution</a></h1>
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<p>Change the standard requirements to match the requirements of the new
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iterators. (more details to come)</p>
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</div>
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<div class="section">
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<h1><a id="rationale" name="rationale">Rationale</a></h1>
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<p>The lack of specification in the standard of the <tt class="docutils literal"><span class="pre">reference</span></tt> 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 <tt class="docutils literal"><span class="pre">reference</span></tt> type, such as
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those proposed in the new iterator categories.</p>
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<p>There is some danger in <em>adding</em> 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.</p>
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<p>An alternative solution would be to leave the standard requirements as
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is, and to remove the requirements for the <tt class="docutils literal"><span class="pre">reference</span></tt> 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.</p>
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</div>
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