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-- Copyright (c) Jeremy Siek 2000
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--
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-- Permission to use, copy, modify, distribute and sell this software
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-- and its documentation for any purpose is hereby granted without fee,
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-- provided that the above copyright notice appears in all copies and
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-- that both that copyright notice and this permission notice appear
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-- in supporting documentation. Silicon Graphics makes no
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-- representations about the suitability of this software for any
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-- purpose. It is provided "as is" without express or implied warranty.
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-->
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<Head>
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<Title>MultiPassInputIterator</Title>
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<BODY BGCOLOR="#ffffff" LINK="#0000ee" TEXT="#000000" VLINK="#551a8b"
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ALINK="#ff0000">
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<IMG SRC="../../c++boost.gif"
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ALT="C++ Boost" width="277" height="86">
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<BR Clear>
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<H2>
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<A NAME="concept:MultiPassInputIterator"></A>
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Multi-Pass Input Iterator
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</H2>
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This concept is a refinement of <a
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href="http://www.sgi.com/tech/stl/InputIterator.html">Input Iterator</a>,
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adding the requirements that the iterator can be used to make multiple
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passes through a range, and that if <TT>it1 == it2</TT> and
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<TT>it1</TT> is dereferenceable then <TT>++it1 == ++it2</TT>. The
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Multi-Pass Input Iterator is very similar to the <a
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href="http://www.sgi.com/tech/stl/ForwardIterator.hmtl">Forward Iterator</a>. The
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only difference is that a <a
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href="http://www.sgi.com/tech/stl/ForwardIterator.hmtl">Forward Iterator</a>
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requires the <TT>reference</TT> type to be <TT>value_type&</TT>, whereas
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MultiPassInputIterator is like <a
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href="http://www.sgi.com/tech/stl/InputIterator.html">Input Iterator</a>
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in that the <TT>reference</TT> type merely has to be convertible to
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<TT>value_type</TT>.
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<h3>Design Notes</h3>
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comments by Valentin Bonnard:
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2001-03-22 16:05:48 +00:00
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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
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and Multi-Pass Random Access Iterator ? I don't, definitly. It only
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confuses the issue. The problem lies into the existing hierarchy of
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iterators, which mixes movabillity, modifiabillity and lvalue-ness,
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and these are clearly independant.
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2001-03-22 16:05:48 +00:00
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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
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completly orthogonal way, iterators can be immutable, mutable, or
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neither. Lvalueness of iterators is also orthogonal with
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immutabillity. With these clean concepts, your Multi-Pass Input Iterator
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is just called a Forward Iterator.
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<p>
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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 Iterator<br>
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std::Random Access Iterator -> Random Access Iterator & Lvalue Iterator<br>
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<p>
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Note that in practice the only operation not allowed on my
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Forward Iterator which is allowed on std::Forward Iterator is
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<tt>&*it</tt>. I think that <tt>&*</tt> is rarely needed in generic code.
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<p>
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reply by Jeremy Siek:
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<p>
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The above analysis by Valentin is right on. Of course, there is
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the problem with backward compatibility. The current STL implementations
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are based on the old definition of Forward Iterator. The right course
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of action is to get Forward Iterator, etc. changed in the C++ standard.
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Once that is done we can drop Multi-Pass Input Iterator.
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<br>
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<HR>
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<TABLE>
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<TR valign=top>
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<TD nowrap>Copyright © 2000</TD><TD>
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2001-08-20 13:04:43 +00:00
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<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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2000-09-18 16:03:04 +00:00
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</TD></TR></TABLE>
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</BODY>
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</HTML>
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