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| 8 | <title>MultiPassInputIterator</title> |
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| 11 | <body bgcolor="#FFFFFF" link="#0000EE" text="#000000" vlink="#551A8B" alink= |
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| 13 | <img src="../../boost.png" alt="C++ Boost" width="277" height= |
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| 14 | "86"><br clear="none"> |
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| 15 | |
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| 16 | <h2><a name="concept:MultiPassInputIterator" id= |
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| 17 | "concept:MultiPassInputIterator"></a> Multi-Pass Input Iterator</h2> |
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| 18 | |
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| 19 | <p>This concept is a refinement of <a href= |
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| 20 | "http://www.sgi.com/tech/stl/InputIterator.html">Input Iterator</a>, adding |
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| 21 | the requirements that the iterator can be used to make multiple passes |
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| 22 | through a range, and that if <tt>it1 == it2</tt> and <tt>it1</tt> is |
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| 23 | dereferenceable then <tt>++it1 == ++it2</tt>. The Multi-Pass Input Iterator |
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| 24 | is very similar to the <a href= |
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| 25 | "http://www.sgi.com/tech/stl/ForwardIterator.html">Forward Iterator</a>. |
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| 26 | The only difference is that a <a href= |
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| 27 | "http://www.sgi.com/tech/stl/ForwardIterator.html">Forward Iterator</a> |
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| 28 | requires the <tt>reference</tt> type to be <tt>value_type&</tt>, |
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| 29 | whereas MultiPassInputIterator is like <a href= |
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| 30 | "http://www.sgi.com/tech/stl/InputIterator.html">Input Iterator</a> in that |
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| 31 | the <tt>reference</tt> type merely has to be convertible to |
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| 32 | <tt>value_type</tt>.</p> |
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| 33 | |
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| 34 | <h3>Design Notes</h3> |
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| 35 | |
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| 36 | <p>comments by Valentin Bonnard:</p> |
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| 37 | |
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| 38 | <p>I think that introducing Multi-Pass Input Iterator isn't the right |
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| 39 | solution. Do you also want to define Multi-Pass Bidirectionnal Iterator and |
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| 40 | Multi-Pass Random Access Iterator ? I don't, definitly. It only confuses |
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| 41 | the issue. The problem lies into the existing hierarchy of iterators, which |
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| 42 | mixes movabillity, modifiabillity and lvalue-ness, and these are clearly |
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| 43 | independant.</p> |
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| 44 | |
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| 45 | <p>The terms Forward, Bidirectionnal and Random Access are about |
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| 46 | movabillity and shouldn't be used to mean anything else. In a completly |
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| 47 | orthogonal way, iterators can be immutable, mutable, or neither. Lvalueness |
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| 48 | of iterators is also orthogonal with immutabillity. With these clean |
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| 49 | concepts, your Multi-Pass Input Iterator is just called a Forward |
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| 50 | Iterator.</p> |
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| 51 | |
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| 52 | <p>Other translations are:<br> |
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| 53 | std::Forward Iterator -> ForwardIterator & Lvalue Iterator<br> |
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| 54 | std::Bidirectionnal Iterator -> Bidirectionnal Iterator & Lvalue |
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| 55 | Iterator<br> |
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| 56 | std::Random Access Iterator -> Random Access Iterator & Lvalue |
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| 57 | Iterator<br></p> |
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| 58 | |
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| 59 | <p>Note that in practice the only operation not allowed on my Forward |
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| 60 | Iterator which is allowed on std::Forward Iterator is <tt>&*it</tt>. I |
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| 61 | think that <tt>&*</tt> is rarely needed in generic code.</p> |
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| 62 | |
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| 63 | <p>reply by Jeremy Siek:</p> |
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| 64 | |
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| 65 | <p>The above analysis by Valentin is right on. Of course, there is the |
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| 66 | problem with backward compatibility. The current STL implementations are |
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| 67 | based on the old definition of Forward Iterator. The right course of action |
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| 68 | is to get Forward Iterator, etc. changed in the C++ standard. Once that is |
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| 69 | done we can drop Multi-Pass Input Iterator.<br></p> |
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| 70 | <hr> |
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| 71 | |
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| 72 | <p><a href="http://validator.w3.org/check?uri=referer"><img border="0" src= |
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| 73 | "http://www.w3.org/Icons/valid-html401" alt="Valid HTML 4.01 Transitional" |
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| 74 | height="31" width="88"></a></p> |
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| 75 | |
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| 76 | <p>Revised |
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| 77 | <!--webbot bot="Timestamp" s-type="EDITED" s-format="%d %B, %Y" startspan -->05 |
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| 78 | December, 2006<!--webbot bot="Timestamp" endspan i-checksum="38516" --></p> |
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| 79 | |
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| 80 | <table summary=""> |
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| 81 | <tr valign="top"> |
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| 82 | <td nowrap><i>Copyright © 2000</i></td> |
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| 83 | |
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| 84 | <td><i><a href="http://www.lsc.nd.edu/~jsiek">Jeremy Siek</a>, Univ.of |
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| 85 | Notre Dame (<a href= |
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| 86 | "mailto:jsiek@lsc.nd.edu">jsiek@lsc.nd.edu</a>)</i></td> |
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| 87 | </tr> |
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| 88 | </table> |
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| 89 | |
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| 90 | <p><i>Distributed under the Boost Software License, Version 1.0. (See |
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| 91 | accompanying file <a href="../../LICENSE_1_0.txt">LICENSE_1_0.txt</a> or |
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| 92 | copy at <a href= |
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| 93 | "http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt</a>)</i></p> |
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| 94 | </body> |
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| 95 | </html> |
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