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<title>Issue 322: iterator and const_iterator should have the same value type</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#CD1">CD1</a> status.</em></p>
<h3 id="322"><a href="lwg-defects.html#322">322</a>. iterator and const_iterator should have the same value type</h3>
<p><b>Section:</b> 23.2 <a href="https://wg21.link/container.requirements">[container.requirements]</a> <b>Status:</b> <a href="lwg-active.html#CD1">CD1</a>
 <b>Submitter:</b> Matt Austern <b>Opened:</b> 2001-05-17 <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#container.requirements">issues</a> in [container.requirements].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#CD1">CD1</a> status.</p>
<p><b>Discussion:</b></p>
<p>
It's widely assumed that, if X is a container,
iterator_traits&lt;X::iterator&gt;::value_type and
iterator_traits&lt;X::const_iterator&gt;::value_type should both be
X::value_type.  However, this is nowhere stated.  The language in
Table 65 is not precise about the iterators' value types (it predates
iterator_traits), and could even be interpreted as saying that
iterator_traits&lt;X::const_iterator&gt;::value_type should be "const
X::value_type".
</p>

<p>Related issue: <a href="lwg-closed.html#279" title="const and non-const iterators should have equivalent typedefs (Status: NAD)">279</a><sup><a href="https://cplusplus.github.io/LWG/issue279" title="Latest snapshot">(i)</a></sup>.</p>


<p id="res-322"><b>Proposed resolution:</b></p>
<p>In Table 65 ("Container Requirements"), change the return type for
X::iterator to "iterator type whose value type is T".  Change the
return type for X::const_iterator to "constant iterator type whose
value type is T".</p>


<p><b>Rationale:</b></p>
<p>
This belongs as a container requirement, rather than an iterator
requirement, because the whole notion of iterator/const_iterator
pairs is specific to containers' iterator.
</p>
<p>
It is existing practice that (for example) 
iterator_traits&lt;list&lt;int&gt;::const_iterator&gt;::value_type
is "int", rather than "const int".  This is consistent with
the way that const pointers are handled: the standard already 
requires that iterator_traits&lt;const int*&gt;::value_type is int.
</p>





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