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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="../../c++boost.gif"
ALT="C++ Boost">
<BR Clear>
<H2>
<A NAME="concept:MultiPassInputIterator"></A>
MultiPassInputIterator
</H2>
This concept is a refinement of <a
href="http://www.sgi.com/Technology/STL/InputIterator.html">InputIterator</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
MultiPassInputIterator is very similar to the <a
href="http://www.sgi.com/Technology/STL/ForwardIterator.hmtl">ForwardIterator</a>. The
only difference is that a <a
href="http://www.sgi.com/Technology/STL/ForwardIterator.hmtl">ForwardIterator</a>
requires the <TT>reference</TT> type to be <TT>value_type&amp;</TT>, whereas
MultiPassInputIterator is like <a
href="http://www.sgi.com/Technology/STL/InputIterator.html">InputIterator</a>
in that the <TT>reference</TT> type merely has to be convertible to
<TT>value_type</TT>.
<h3>Design Notes</h3>
comments by Valentin Bonnard:
<p> I think that introducing MultiPassInputIterator isn't the right
solution. Do you also want to define MultiPassBidirectionnalIterator
and MultiPassRandomAccessIterator ? 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> The terms Forward, Bidirectionnal and RandomAccess 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 MultiPassInputIterator
is just called a ForwardIterator.
<p>
Other translations are:<br>
std::ForwardIterator -> ForwardIterator & LvalueIterator<br>
std::BidirectionnalIterator -> BidirectionnalIterator & LvalueIterator<br>
std::RandomAccessIterator -> RandomAccessIterator & LvalueIterator<br>
<p>
Note that in practice the only operation not allowed on my
ForwardIterator which is allowed on std::ForwardIterator is
<tt>&*it</tt>. I think that <tt>&*</tt> is rarely needed in generic code.
<p>
reply by Jeremy Siek:
<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 ForwardIterator. The right course
of action is to get ForwardIterator, etc. changed in the C++ standard.
Once that is done we can drop MultiPassInputIterator.
<br>
<HR>
<TABLE>
<TR valign=top>
<TD nowrap>Copyright &copy 2000</TD><TD>
<A HREF=http://www.boost.org/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>
</BODY>
</HTML>