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added comment about abstract base classes as value_type

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[SVN r9584]
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Jeremy Siek 2001-03-19 17:31:27 +00:00
parent feb370b201
commit 2cd1422514
1 changed files with 16 additions and 1 deletions
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17
iterator_adaptors.htm
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@ -170,7 +170,8 @@ struct iterator_adaptor;
<td>The <tt>value_type</tt> of the resulting iterator, unless const. If
Value is <tt>const X</tt> the
<tt>value_type</tt> will be (<i>non-</i><tt>const</tt>) <tt>X</tt><a href=
"#1">[1]</a>.<br>
"#1">[1]</a>. If the <tt>value_type</tt> you wish to use is an abstract
base class see note <a href="#5">[5]</a>.<br>
<b>Default:</b>
<tt>std::iterator_traits&lt;BaseType&gt;::value_type</tt> <a href=
"#2">[2]</a>
@ -829,6 +830,20 @@ bool operator==(const iterator_adaptor&lt;B1,P,V1,R1,P1,C,D&gt;&amp;,
returning a reference could cause serious memory problems due to the
reference being bound to a temporary object whose lifetime ends inside of
the <tt>operator[]</tt>.
<p><a name="5">[5]</a>
The <tt>value_type</tt> of an iterator may not be
an abstract base class, however many common uses of iterators
never need the <tt>value_type</tt>, only the <tt>reference</tt> type.
If you wish to create such an iterator adaptor, use a dummy
type such as <tt>char</tt> for the <tt>Value</tt> parameter,
and use a reference to your abstract base class for
the <tt>Reference</tt> parameter. Note that such an iterator
does not fulfill the C++ standards requirements for a
<a href= "http://www.sgi.com/tech/stl/ForwardIterator.html">
Forward Iterator</a>, so you will need to use a less restrictive
iterator category such as <tt>std::input_iterator_tag</tt>.
<hr>
<p>Revised