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<title>Issue 200: Forward iterator requirements don't allow constant 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#CD1">CD1</a> status.</em></p>
<h3 id="200"><a href="lwg-defects.html#200">200</a>. Forward iterator requirements don't allow constant iterators</h3>
<p><b>Section:</b> 24.3.5.5 <a href="https://wg21.link/forward.iterators">[forward.iterators]</a> <b>Status:</b> <a href="lwg-active.html#CD1">CD1</a>
 <b>Submitter:</b> Matt Austern <b>Opened:</b> 1999-11-19 <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#forward.iterators">issues</a> in [forward.iterators].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#CD1">CD1</a> status.</p>
<p><b>Discussion:</b></p>
<p>
In table 74, the return type of the expression <code>*a</code> is given
as <code>T&amp;</code>, where <code>T</code> is the iterator's value type.
For constant iterators, however, this is wrong.  ("Value type"
is never defined very precisely, but it is clear that the value type
of, say, <code>std::list&lt;int&gt;::const_iterator</code> is supposed to be
<code>int</code>, not <code>const int</code>.)
</p>


<p id="res-200"><b>Proposed resolution:</b></p>
<p>
In table 74, in the <code>*a</code> and <code>*r++</code> rows, change the
return type from "<code>T&amp;</code>" to "<code>T&amp;</code>
if <code>X</code> is mutable, otherwise <code>const T&amp;</code>".
In the <code>a-&gt;m</code> row, change the return type from
"<code>U&amp;</code>" to "<code>U&amp;</code> if <code>X</code> is mutable,
otherwise <code>const U&amp;</code>".
</p>

<p><i>[Tokyo: The LWG believes this is the tip of a larger iceberg;
there are multiple const problems with the STL portion of the library
and that these should be addressed as a single package.&nbsp; Note
that issue <a href="lwg-defects.html#180" title="Container member iterator arguments constness has unintended consequences (Status: CD1)">180</a><sup><a href="https://cplusplus.github.io/LWG/issue180" title="Latest snapshot">(i)</a></sup> has already been declared NAD Future for
that very reason.]</i></p>


<p><i>[Redmond: the LWG thinks this is separable from other constness
issues.  This issue is just cleanup; it clarifies language that was
written before we had iterator_traits.  Proposed resolution was
modified: the original version only discussed *a.  It was pointed out
that we also need to worry about *r++ and a-&gt;m.]</i></p>







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