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| 7 | <title>Problem with reference and old/new iterator category correspondance</title> |
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| 9 | </head> |
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| 10 | <body> |
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| 11 | <h1 class="title">Problem with <tt class="literal"><span class="pre">reference</span></tt> and old/new iterator category correspondance</h1> |
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| 12 | <div class="document" id="problem-with-reference-and-old-new-iterator-category-correspondance"> |
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| 13 | <table class="field-list" frame="void" rules="none"> |
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| 14 | <col class="field-name" /> |
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| 15 | <col class="field-body" /> |
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| 16 | <tbody valign="top"> |
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| 17 | <tr class="field"><th class="field-name">Author:</th><td class="field-body">David Abrahams and Jeremy Siek</td> |
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| 18 | </tr> |
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| 19 | <tr class="field"><th class="field-name">Contact:</th><td class="field-body"><a class="reference" href="mailto:dave@boost-consulting.com">dave@boost-consulting.com</a>, <a class="reference" href="mailto:jsiek@osl.iu.edu">jsiek@osl.iu.edu</a></td> |
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| 20 | </tr> |
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| 21 | <tr class="field"><th class="field-name">Organization:</th><td class="field-body"><a class="reference" href="http://www.boost-consulting.com">Boost Consulting</a>, Indiana University Bloomington</td> |
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| 22 | </tr> |
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| 23 | <tr class="field"><th class="field-name">date:</th><td class="field-body">$Date: 2004/11/02 14:31:13 $</td> |
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| 24 | </tr> |
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| 25 | <tr class="field"><th class="field-name">Copyright:</th><td class="field-body">Copyright David Abrahams, Jeremy Siek 2003. Use, modification and |
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| 26 | distribution is subject to the Boost Software License, |
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| 27 | Version 1.0. (See accompanying file LICENSE_1_0.txt or copy |
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| 28 | at <a class="reference" href="http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt</a>)</td> |
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| 29 | </tr> |
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| 30 | </tbody> |
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| 31 | </table> |
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| 32 | <div class="section" id="introduction"> |
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| 33 | <h1><a name="introduction">Introduction</a></h1> |
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| 34 | <p>The new iterator categories are intended to correspond to the old |
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| 35 | 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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| 36 | an iterator categorized as a mutable Forward Iterator under the old |
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| 37 | scheme is now a Writable, Lvalue, and Foward Traversal iterator. |
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| 38 | However, there is a problem with this correspondance, the new iterator |
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| 39 | categories place requirements on the <tt class="literal"><span class="pre">iterator_traits<X>::reference</span></tt> |
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| 40 | type whereas the standard iterator requirements say nothing about the |
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| 41 | <tt class="literal"><span class="pre">reference</span></tt> type . In particular, the new Readable Iterator |
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| 42 | requirements say that the return type of <tt class="literal"><span class="pre">*a</span></tt> must be |
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| 43 | <tt class="literal"><span class="pre">iterator_traits<X>::reference</span></tt> and the Lvalue Iterator requirements |
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| 44 | says that <tt class="literal"><span class="pre">iterator_traits<X>::reference</span></tt> must be <tt class="literal"><span class="pre">T&</span></tt> or <tt class="literal"><span class="pre">const</span> |
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| 45 | <span class="pre">T&</span></tt>.</p> |
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| 46 | </div> |
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| 47 | <div class="section" id="proposed-resolution"> |
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| 48 | <h1><a name="proposed-resolution">Proposed Resolution</a></h1> |
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| 49 | <p>Change the standard requirements to match the requirements of the new |
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| 50 | iterators. (more details to come)</p> |
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| 51 | </div> |
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| 52 | <div class="section" id="rationale"> |
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| 53 | <h1><a name="rationale">Rationale</a></h1> |
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| 54 | <p>The lack of specification in the standard of the <tt class="literal"><span class="pre">reference</span></tt> type is |
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| 55 | certainly a defect. Without specification, it is entirely useless in a |
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| 56 | generic function. The current practice in the community is generally |
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| 57 | to assume there are requirements on the <tt class="literal"><span class="pre">reference</span></tt> type, such as |
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| 58 | those proposed in the new iterator categories.</p> |
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| 59 | <p>There is some danger in <em>adding</em> requirements to existing concepts. |
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| 60 | This will mean that some existing iterator types will no longer meet |
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| 61 | the iterator requirements. However, we feel that the impact of this is |
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| 62 | small enough to warrant going ahead with this change.</p> |
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| 63 | <p>An alternative solution would be to leave the standard requirements as |
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| 64 | is, and to remove the requirements for the <tt class="literal"><span class="pre">reference</span></tt> type in the |
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| 65 | new iterator concepts. We are not in favor of this approach because it |
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| 66 | extends what we see as a defect further into the future.</p> |
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| 67 | </div> |
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| 68 | </div> |
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| 69 | <hr class="footer" /> |
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| 70 | <div class="footer"> |
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| 71 | <a class="reference" href="ref_problem.rst">View document source</a>. |
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| 73 | </div> |
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