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<title>Issue 279: const and non-const iterators should have equivalent typedefs</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#NAD">NAD</a> status.</em></p>
<h3 id="279"><a href="lwg-closed.html#279">279</a>. const and non-const iterators should have equivalent typedefs</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#NAD">NAD</a>
 <b>Submitter:</b> Steve Cleary <b>Opened:</b> 2000-11-27 <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#NAD">NAD</a> status.</p>
<p><b>Discussion:</b></p>

<p>
This came from an email from Steve Cleary to Fergus in reference to
issue <a href="lwg-defects.html#179" title="Comparison of const_iterators to iterators doesn't work (Status: CD1)">179</a><sup><a href="https://cplusplus.github.io/LWG/issue179" title="Latest snapshot">(i)</a></sup>. The library working group briefly discussed
this in Toronto and believes it should be a separate issue.
</p>

<p>
Steve said: &quot;We may want to state that the const/non-const iterators must have
the same difference type, size_type, and category.&quot;
</p>

<p>
(Comment from Judy)
I'm not sure if the above sentence should be true for all
const and non-const iterators in a particular container, or if it means 
the container's iterator can't be compared with the container's
const_iterator unless the above it true. I suspect the former.
</p>


<p id="res-279"><b>Proposed resolution:</b></p>
<p>
In <b>Section:</b> 23.2 <a href="https://wg21.link/container.requirements">[container.requirements]</a>,
table 65, in the assertion/note pre/post condition for X::const_iterator,
add the following:
</p>

<blockquote>
<p>
typeid(X::const_iterator::difference_type) == typeid(X::iterator::difference_type)
</p>

<p>
typeid(X::const_iterator::size_type) == typeid(X::iterator::size_type)
</p>

<p>
typeid(X::const_iterator::category) == typeid(X::iterator::category)
</p>
</blockquote>


<p><b>Rationale:</b></p>
<p>Going through the types one by one: Iterators don't have a
<code>size_type</code>.  We already know that the difference types are
identical, because the container requirements already say that the
difference types of both X::iterator and X::const_iterator are both
X::difference_type.  The standard does not require that X::iterator
and X::const_iterator have the same iterator category, but the LWG
does not see this as a defect: it's possible to imagine cases in which
it would be useful for the categories to be different.</p>

<p>It may be desirable to require X::iterator and X::const_iterator to
have the same value type, but that is a new issue. (Issue <a href="lwg-defects.html#322" title="iterator and const_iterator should have the same value type (Status: CD1)">322</a><sup><a href="https://cplusplus.github.io/LWG/issue322" title="Latest snapshot">(i)</a></sup>.)</p>






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