96 lines
4.0 KiB
HTML
96 lines
4.0 KiB
HTML
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<meta http-equiv="Content-Language" content="en-us">
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<meta http-equiv="Content-Type" content="text/html; charset=us-ascii">
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<title>MultiPassInputIterator</title>
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<body bgcolor="#FFFFFF" link="#0000EE" text="#000000" vlink="#551A8B" alink=
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"#FF0000">
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<img src="../../boost.png" alt="C++ Boost" width="277" height=
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"86"><br clear="none">
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<h2><a name="concept:MultiPassInputIterator" id=
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"concept:MultiPassInputIterator"></a> Multi-Pass Input Iterator</h2>
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<p>This concept is a refinement of <a href=
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"http://www.sgi.com/tech/stl/InputIterator.html">Input Iterator</a>, adding
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the requirements that the iterator can be used to make multiple passes
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through a range, and that if <tt>it1 == it2</tt> and <tt>it1</tt> is
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dereferenceable then <tt>++it1 == ++it2</tt>. The Multi-Pass Input Iterator
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is very similar to the <a href=
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"http://www.sgi.com/tech/stl/ForwardIterator.html">Forward Iterator</a>.
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The only difference is that a <a href=
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"http://www.sgi.com/tech/stl/ForwardIterator.html">Forward Iterator</a>
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requires the <tt>reference</tt> type to be <tt>value_type&</tt>,
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whereas MultiPassInputIterator is like <a href=
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"http://www.sgi.com/tech/stl/InputIterator.html">Input Iterator</a> in that
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the <tt>reference</tt> type merely has to be convertible to
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<tt>value_type</tt>.</p>
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<h3>Design Notes</h3>
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<p>comments by Valentin Bonnard:</p>
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<p>I think that introducing Multi-Pass Input Iterator isn't the right
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solution. Do you also want to define Multi-Pass Bidirectionnal Iterator and
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Multi-Pass Random Access Iterator ? I don't, definitly. It only confuses
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the issue. The problem lies into the existing hierarchy of iterators, which
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mixes movabillity, modifiabillity and lvalue-ness, and these are clearly
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independant.</p>
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<p>The terms Forward, Bidirectionnal and Random Access are about
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movabillity and shouldn't be used to mean anything else. In a completly
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orthogonal way, iterators can be immutable, mutable, or neither. Lvalueness
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of iterators is also orthogonal with immutabillity. With these clean
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concepts, your Multi-Pass Input Iterator is just called a Forward
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Iterator.</p>
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<p>Other translations are:<br>
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std::Forward Iterator -> ForwardIterator & Lvalue Iterator<br>
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std::Bidirectionnal Iterator -> Bidirectionnal Iterator & Lvalue
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Iterator<br>
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std::Random Access Iterator -> Random Access Iterator & Lvalue
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Iterator<br></p>
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<p>Note that in practice the only operation not allowed on my Forward
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Iterator which is allowed on std::Forward Iterator is <tt>&*it</tt>. I
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think that <tt>&*</tt> is rarely needed in generic code.</p>
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<p>reply by Jeremy Siek:</p>
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<p>The above analysis by Valentin is right on. Of course, there is the
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problem with backward compatibility. The current STL implementations are
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based on the old definition of Forward Iterator. The right course of action
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is to get Forward Iterator, etc. changed in the C++ standard. Once that is
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done we can drop Multi-Pass Input Iterator.<br></p>
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<hr>
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<p><a href="http://validator.w3.org/check?uri=referer"><img border="0" src=
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"../../doc/images/valid-html401.png" alt="Valid HTML 4.01 Transitional"
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height="31" width="88"></a></p>
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<p>Revised
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<!--webbot bot="Timestamp" s-type="EDITED" s-format="%d %B, %Y" startspan -->05
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December, 2006<!--webbot bot="Timestamp" endspan i-checksum="38516" --></p>
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<table summary="">
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<tr valign="top">
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<td nowrap><i>Copyright © 2000</i></td>
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<td><i><a href="http://www.lsc.nd.edu/~jsiek">Jeremy Siek</a>, Univ.of
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Notre Dame (<a href=
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"mailto:jsiek@lsc.nd.edu">jsiek@lsc.nd.edu</a>)</i></td>
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</tr>
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</table>
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<p><i>Distributed under the Boost Software License, Version 1.0. (See
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accompanying file <a href="../../LICENSE_1_0.txt">LICENSE_1_0.txt</a> or
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copy at <a href=
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"http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt</a>)</i></p>
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</body>
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</html>
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