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<title>Issue 1185: Iterator categories and output iterators</title>
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<p><em>This page is a snapshot from the LWG issues list, see the <a href="lwg-active.html">Library Active Issues List</a> for more information and the meaning of <a href="lwg-active.html#Resolved">Resolved</a> status.</em></p>
<h3 id="1185"><a href="lwg-defects.html#1185">1185</a>. Iterator categories and output iterators</h3>
<p><b>Section:</b> 24.3 <a href="https://wg21.link/iterator.requirements">[iterator.requirements]</a> <b>Status:</b> <a href="lwg-active.html#Resolved">Resolved</a>
 <b>Submitter:</b> Alisdair Meredith <b>Opened:</b> 2009-07-31 <b>Last modified:</b> 2016-01-28</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all other</b> <a href="lwg-index.html#iterator.requirements">issues</a> in [iterator.requirements].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#Resolved">Resolved</a> status.</p>
<p><b>Discussion:</b></p>
<p>
(wording relative to
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2723.pdf">N2723</a>
pending new working paper)
</p>

<p>
According to p3 24.3 <a href="https://wg21.link/iterator.requirements">[iterator.requirements]</a>, Forward iterators,
Bidirectional iterators and Random Access iterators all satisfy the
requirements for an Output iterator:
</p>

<blockquote><p>
XXX iterators satisfy all the requirements of the input and output iterators
and can be used whenever either kind is specified ...
</p></blockquote>

<p>
Meanwhile, p4 goes on to contradict this:
</p>

<blockquote><p>
Besides its category, a forward, bidirectional, or random access
iterator can also be mutable or constant...
</p></blockquote>

<blockquote><p>
... Constant iterators do not satisfy the requirements for output iterators
</p></blockquote>

<p>
The latter seems to be the overriding concern, as the iterator tag
hierarchy does not define <code>forward_iterator_tag</code> as multiply derived from
both <code>input_iterator_tag</code> and <code>output_iterator_tag</code>.
</p>

<p>
The work on concepts for iterators showed us that output iterator really
is fundamentally a second dimension to the iterator categories, rather
than part of the linear input -&gt; forward -&gt; bidirectional -&gt;
random-access sequence.  It would be good to clear up these words to
reflect that, and separately list output iterator requirements in the
requires clauses for the appropriate algorithms and operations.
</p>

<p><i>[
2010 Pittsburgh:  Moved to <del>NAD Editorial</del><ins>Resolved</ins>.  Rationale added below.
]</i></p>




<p><b>Rationale:</b></p>
<p>
Solved by
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2010/n3066.html">N3066</a>.
</p>


<p id="res-1185"><b>Proposed resolution:</b></p>





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