| Brian Silverman | 60e3e2a | 2018-08-04 23:57:12 -0700 | [diff] [blame^] | 1 | <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> |
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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= |
| 14 | "86"><br clear="none"> |
| 15 | |
| 16 | <h2><a name="concept:MultiPassInputIterator" id= |
| 17 | "concept:MultiPassInputIterator"></a> Multi-Pass Input Iterator</h2> |
| 18 | |
| 19 | <p>This concept is a refinement of <a href= |
| 20 | "http://www.sgi.com/tech/stl/InputIterator.html">Input Iterator</a>, adding |
| 21 | the requirements that the iterator can be used to make multiple passes |
| 22 | through a range, and that if <tt>it1 == it2</tt> and <tt>it1</tt> is |
| 23 | dereferenceable then <tt>++it1 == ++it2</tt>. The Multi-Pass Input Iterator |
| 24 | is very similar to the <a href= |
| 25 | "http://www.sgi.com/tech/stl/ForwardIterator.html">Forward Iterator</a>. |
| 26 | The only difference is that a <a href= |
| 27 | "http://www.sgi.com/tech/stl/ForwardIterator.html">Forward Iterator</a> |
| 28 | requires the <tt>reference</tt> type to be <tt>value_type&</tt>, |
| 29 | whereas MultiPassInputIterator is like <a href= |
| 30 | "http://www.sgi.com/tech/stl/InputIterator.html">Input Iterator</a> in that |
| 31 | the <tt>reference</tt> type merely has to be convertible to |
| 32 | <tt>value_type</tt>.</p> |
| 33 | |
| 34 | <h3>Design Notes</h3> |
| 35 | |
| 36 | <p>comments by Valentin Bonnard:</p> |
| 37 | |
| 38 | <p>I think that introducing Multi-Pass Input Iterator isn't the right |
| 39 | solution. Do you also want to define Multi-Pass Bidirectionnal Iterator and |
| 40 | Multi-Pass Random Access Iterator ? I don't, definitly. It only confuses |
| 41 | the issue. The problem lies into the existing hierarchy of iterators, which |
| 42 | mixes movabillity, modifiabillity and lvalue-ness, and these are clearly |
| 43 | independant.</p> |
| 44 | |
| 45 | <p>The terms Forward, Bidirectionnal and Random Access are about |
| 46 | movabillity and shouldn't be used to mean anything else. In a completly |
| 47 | orthogonal way, iterators can be immutable, mutable, or neither. Lvalueness |
| 48 | of iterators is also orthogonal with immutabillity. With these clean |
| 49 | concepts, your Multi-Pass Input Iterator is just called a Forward |
| 50 | Iterator.</p> |
| 51 | |
| 52 | <p>Other translations are:<br> |
| 53 | std::Forward Iterator -> ForwardIterator & Lvalue Iterator<br> |
| 54 | std::Bidirectionnal Iterator -> Bidirectionnal Iterator & Lvalue |
| 55 | Iterator<br> |
| 56 | std::Random Access Iterator -> Random Access Iterator & Lvalue |
| 57 | Iterator<br></p> |
| 58 | |
| 59 | <p>Note that in practice the only operation not allowed on my Forward |
| 60 | Iterator which is allowed on std::Forward Iterator is <tt>&*it</tt>. I |
| 61 | think that <tt>&*</tt> is rarely needed in generic code.</p> |
| 62 | |
| 63 | <p>reply by Jeremy Siek:</p> |
| 64 | |
| 65 | <p>The above analysis by Valentin is right on. Of course, there is the |
| 66 | problem with backward compatibility. The current STL implementations are |
| 67 | based on the old definition of Forward Iterator. The right course of action |
| 68 | is to get Forward Iterator, etc. changed in the C++ standard. Once that is |
| 69 | done we can drop Multi-Pass Input Iterator.<br></p> |
| 70 | <hr> |
| 71 | |
| 72 | <p><a href="http://validator.w3.org/check?uri=referer"><img border="0" src= |
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| 74 | height="31" width="88"></a></p> |
| 75 | |
| 76 | <p>Revised |
| 77 | <!--webbot bot="Timestamp" s-type="EDITED" s-format="%d %B, %Y" startspan -->05 |
| 78 | December, 2006<!--webbot bot="Timestamp" endspan i-checksum="38516" --></p> |
| 79 | |
| 80 | <table summary=""> |
| 81 | <tr valign="top"> |
| 82 | <td nowrap><i>Copyright © 2000</i></td> |
| 83 | |
| 84 | <td><i><a href="http://www.lsc.nd.edu/~jsiek">Jeremy Siek</a>, Univ.of |
| 85 | Notre Dame (<a href= |
| 86 | "mailto:jsiek@lsc.nd.edu">jsiek@lsc.nd.edu</a>)</i></td> |
| 87 | </tr> |
| 88 | </table> |
| 89 | |
| 90 | <p><i>Distributed under the Boost Software License, Version 1.0. (See |
| 91 | accompanying file <a href="../../LICENSE_1_0.txt">LICENSE_1_0.txt</a> or |
| 92 | copy at <a href= |
| 93 | "http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt</a>)</i></p> |
| 94 | </body> |
| 95 | </html> |