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| 3 | -- Copyright (c) Jeremy Siek 2000 |
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| 13 | <Head> |
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| 14 | <Title>MultiPassInputIterator</Title> |
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| 15 | <BODY BGCOLOR="#ffffff" LINK="#0000ee" TEXT="#000000" VLINK="#551a8b" |
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| 16 | ALINK="#ff0000"> |
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| 17 | <IMG SRC="../../boost.png" |
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| 18 | ALT="C++ Boost" width="277" height="86"> |
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| 19 | |
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| 20 | <BR Clear> |
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| 21 | |
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| 22 | <H2> |
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| 23 | <A NAME="concept:MultiPassInputIterator"></A> |
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| 24 | Multi-Pass Input Iterator |
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| 25 | </H2> |
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| 26 | |
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| 27 | This concept is a refinement of <a |
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| 28 | href="http://www.sgi.com/tech/stl/InputIterator.html">Input Iterator</a>, |
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| 29 | adding the requirements that the iterator can be used to make multiple |
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| 30 | passes through a range, and that if <TT>it1 == it2</TT> and |
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| 31 | <TT>it1</TT> is dereferenceable then <TT>++it1 == ++it2</TT>. The |
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| 32 | Multi-Pass Input Iterator is very similar to the <a |
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| 33 | href="http://www.sgi.com/tech/stl/ForwardIterator.html">Forward Iterator</a>. The |
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| 34 | only difference is that a <a |
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| 35 | href="http://www.sgi.com/tech/stl/ForwardIterator.html">Forward Iterator</a> |
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| 36 | requires the <TT>reference</TT> type to be <TT>value_type&</TT>, whereas |
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| 37 | MultiPassInputIterator is like <a |
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| 38 | href="http://www.sgi.com/tech/stl/InputIterator.html">Input Iterator</a> |
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| 39 | in that the <TT>reference</TT> type merely has to be convertible to |
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| 40 | <TT>value_type</TT>. |
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| 41 | |
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| 42 | |
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| 43 | <h3>Design Notes</h3> |
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| 44 | |
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| 45 | comments by Valentin Bonnard: |
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| 46 | |
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| 47 | <p> I think that introducing Multi-Pass Input Iterator isn't the right |
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| 48 | solution. Do you also want to define Multi-Pass Bidirectionnal Iterator |
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| 49 | and Multi-Pass Random Access Iterator ? I don't, definitly. It only |
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| 50 | confuses the issue. The problem lies into the existing hierarchy of |
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| 51 | iterators, which mixes movabillity, modifiabillity and lvalue-ness, |
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| 52 | and these are clearly independant. |
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| 53 | |
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| 54 | <p> The terms Forward, Bidirectionnal and Random Access are about |
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| 55 | movabillity and shouldn't be used to mean anything else. In a |
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| 56 | completly orthogonal way, iterators can be immutable, mutable, or |
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| 57 | neither. Lvalueness of iterators is also orthogonal with |
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| 58 | immutabillity. With these clean concepts, your Multi-Pass Input Iterator |
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| 59 | is just called a Forward Iterator. |
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| 60 | |
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| 61 | <p> |
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| 62 | Other translations are:<br> |
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| 63 | std::Forward Iterator -> ForwardIterator & Lvalue Iterator<br> |
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| 64 | std::Bidirectionnal Iterator -> Bidirectionnal Iterator & Lvalue Iterator<br> |
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| 65 | std::Random Access Iterator -> Random Access Iterator & Lvalue Iterator<br> |
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| 66 | |
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| 67 | <p> |
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| 68 | Note that in practice the only operation not allowed on my |
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| 69 | Forward Iterator which is allowed on std::Forward Iterator is |
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| 70 | <tt>&*it</tt>. I think that <tt>&*</tt> is rarely needed in generic code. |
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| 71 | |
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| 72 | <p> |
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| 73 | reply by Jeremy Siek: |
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| 74 | |
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| 75 | <p> |
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| 76 | The above analysis by Valentin is right on. Of course, there is |
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| 77 | the problem with backward compatibility. The current STL implementations |
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| 78 | are based on the old definition of Forward Iterator. The right course |
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| 79 | of action is to get Forward Iterator, etc. changed in the C++ standard. |
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| 80 | Once that is done we can drop Multi-Pass Input Iterator. |
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| 81 | |
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| 82 | |
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| 83 | <br> |
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| 84 | <HR> |
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| 85 | <TABLE> |
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| 86 | <TR valign=top> |
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| 87 | <TD nowrap>Copyright © 2000</TD><TD> |
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| 88 | <a HREF="../../people/jeremy_siek.htm">Jeremy Siek</a>, Univ.of Notre Dame (<A HREF="mailto:jsiek@lsc.nd.edu">jsiek@lsc.nd.edu</A>) |
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| 89 | </TD></TR></TABLE> |
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| 90 | |
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| 91 | </BODY> |
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| 92 | </HTML> |
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