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Andreas Huber
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<HTML>
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<Head>
<Title>MultiPassInputIterator</Title>
<BODY BGCOLOR="#ffffff" LINK="#0000ee" TEXT="#000000" VLINK="#551a8b"
ALINK="#ff0000">
<IMG SRC="../../boost.png"
ALT="C++ Boost" width="277" height="86">
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<BR Clear>
<html>
<head>
<meta http-equiv="Content-Language" content="en-us">
<meta http-equiv="Content-Type" content="text/html; charset=us-ascii">
<H2>
<A NAME="concept:MultiPassInputIterator"></A>
Multi-Pass Input Iterator
</H2>
<title>MultiPassInputIterator</title>
</head>
This concept is a refinement of <a
href="http://www.sgi.com/tech/stl/InputIterator.html">Input Iterator</a>,
adding the requirements that the iterator can be used to make multiple
passes through a range, and that if <TT>it1 == it2</TT> and
<TT>it1</TT> is dereferenceable then <TT>++it1 == ++it2</TT>. The
Multi-Pass Input Iterator is very similar to the <a
href="http://www.sgi.com/tech/stl/ForwardIterator.html">Forward Iterator</a>. The
only difference is that a <a
href="http://www.sgi.com/tech/stl/ForwardIterator.html">Forward Iterator</a>
requires the <TT>reference</TT> type to be <TT>value_type&amp;</TT>, whereas
MultiPassInputIterator is like <a
href="http://www.sgi.com/tech/stl/InputIterator.html">Input Iterator</a>
in that the <TT>reference</TT> type merely has to be convertible to
<TT>value_type</TT>.
<body bgcolor="#FFFFFF" link="#0000EE" text="#000000" vlink="#551A8B" alink=
"#FF0000">
<img src="../../boost.png" alt="C++ Boost" width="277" height=
"86"><br clear="none">
<h2><a name="concept:MultiPassInputIterator" id=
"concept:MultiPassInputIterator"></a> Multi-Pass Input Iterator</h2>
<h3>Design Notes</h3>
<p>This concept is a refinement of <a href=
"http://www.sgi.com/tech/stl/InputIterator.html">Input Iterator</a>, adding
the requirements that the iterator can be used to make multiple passes
through a range, and that if <tt>it1 == it2</tt> and <tt>it1</tt> is
dereferenceable then <tt>++it1 == ++it2</tt>. The Multi-Pass Input Iterator
is very similar to the <a href=
"http://www.sgi.com/tech/stl/ForwardIterator.html">Forward Iterator</a>.
The only difference is that a <a href=
"http://www.sgi.com/tech/stl/ForwardIterator.html">Forward Iterator</a>
requires the <tt>reference</tt> type to be <tt>value_type&amp;</tt>,
whereas MultiPassInputIterator is like <a href=
"http://www.sgi.com/tech/stl/InputIterator.html">Input Iterator</a> in that
the <tt>reference</tt> type merely has to be convertible to
<tt>value_type</tt>.</p>
comments by Valentin Bonnard:
<h3>Design Notes</h3>
<p> I think that introducing Multi-Pass Input Iterator isn't the right
solution. Do you also want to define Multi-Pass Bidirectionnal Iterator
and Multi-Pass Random Access Iterator ? I don't, definitly. It only
confuses the issue. The problem lies into the existing hierarchy of
iterators, which mixes movabillity, modifiabillity and lvalue-ness,
and these are clearly independant.
<p>comments by Valentin Bonnard:</p>
<p> The terms Forward, Bidirectionnal and Random Access are about
movabillity and shouldn't be used to mean anything else. In a
completly orthogonal way, iterators can be immutable, mutable, or
neither. Lvalueness of iterators is also orthogonal with
immutabillity. With these clean concepts, your Multi-Pass Input Iterator
is just called a Forward Iterator.
<p>I think that introducing Multi-Pass Input Iterator isn't the right
solution. Do you also want to define Multi-Pass Bidirectionnal Iterator and
Multi-Pass Random Access Iterator ? I don't, definitly. It only confuses
the issue. The problem lies into the existing hierarchy of iterators, which
mixes movabillity, modifiabillity and lvalue-ness, and these are clearly
independant.</p>
<p>
Other translations are:<br>
std::Forward Iterator -> ForwardIterator & Lvalue Iterator<br>
std::Bidirectionnal Iterator -> Bidirectionnal Iterator & Lvalue Iterator<br>
std::Random Access Iterator -> Random Access Iterator & Lvalue Iterator<br>
<p>The terms Forward, Bidirectionnal and Random Access are about
movabillity and shouldn't be used to mean anything else. In a completly
orthogonal way, iterators can be immutable, mutable, or neither. Lvalueness
of iterators is also orthogonal with immutabillity. With these clean
concepts, your Multi-Pass Input Iterator is just called a Forward
Iterator.</p>
<p>
Note that in practice the only operation not allowed on my
Forward Iterator which is allowed on std::Forward Iterator is
<tt>&*it</tt>. I think that <tt>&*</tt> is rarely needed in generic code.
<p>Other translations are:<br>
std::Forward Iterator -&gt; ForwardIterator &amp; Lvalue Iterator<br>
std::Bidirectionnal Iterator -&gt; Bidirectionnal Iterator &amp; Lvalue
Iterator<br>
std::Random Access Iterator -&gt; Random Access Iterator &amp; Lvalue
Iterator<br></p>
<p>
reply by Jeremy Siek:
<p>Note that in practice the only operation not allowed on my Forward
Iterator which is allowed on std::Forward Iterator is <tt>&amp;*it</tt>. I
think that <tt>&amp;*</tt> is rarely needed in generic code.</p>
<p>
The above analysis by Valentin is right on. Of course, there is
the problem with backward compatibility. The current STL implementations
are based on the old definition of Forward Iterator. The right course
of action is to get Forward Iterator, etc. changed in the C++ standard.
Once that is done we can drop Multi-Pass Input Iterator.
<p>reply by Jeremy Siek:</p>
<p>The above analysis by Valentin is right on. Of course, there is the
problem with backward compatibility. The current STL implementations are
based on the old definition of Forward Iterator. The right course of action
is to get Forward Iterator, etc. changed in the C++ standard. Once that is
done we can drop Multi-Pass Input Iterator.<br></p>
<hr>
<br>
<HR>
<TABLE>
<TR valign=top>
<TD nowrap>Copyright &copy 2000</TD><TD>
<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>)
</TD></TR></TABLE>
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<p>Revised
<!--webbot bot="Timestamp" s-type="EDITED" s-format="%d %B, %Y" startspan -->05
December, 2006<!--webbot bot="Timestamp" endspan i-checksum="38516" --></p>
<table summary="">
<tr valign="top">
<td nowrap><i>Copyright &copy; 2000</i></td>
<td><i><a href="http://www.lsc.nd.edu/~jsiek">Jeremy Siek</a>, Univ.of
Notre Dame (<a href=
"mailto:jsiek@lsc.nd.edu">jsiek@lsc.nd.edu</a>)</i></td>
</tr>
</table>
<p><i>Distributed under the Boost Software License, Version 1.0. (See
accompanying file <a href="../../LICENSE_1_0.txt">LICENSE_1_0.txt</a> or
copy at <a href=
"http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt</a>)</i></p>
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