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<title>Issue 538: 241 again: Does unique_copy() require CopyConstructible and Assignable?</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="538"><a href="lwg-defects.html#538">538</a>. 241 again: Does unique_copy() require CopyConstructible and Assignable?</h3>
<p><b>Section:</b> 26.7.9 <a href="https://wg21.link/alg.unique">[alg.unique]</a> <b>Status:</b> <a href="lwg-active.html#CD1">CD1</a>
 <b>Submitter:</b> Howard Hinnant <b>Opened:</b> 2006-02-09 <b>Last modified:</b> 2016-01-28</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View other</b> <a href="lwg-index-open.html#alg.unique">active issues</a> in [alg.unique].</p>
<p><b>View all other</b> <a href="lwg-index.html#alg.unique">issues</a> in [alg.unique].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#CD1">CD1</a> status.</p>
<p><b>Discussion:</b></p>
<p>
I believe I botched the resolution of
<a href="lwg-defects.html#241" title="Does unique_copy() require CopyConstructible and Assignable? (Status: CD1)">241</a><sup><a href="https://cplusplus.github.io/LWG/issue241" title="Latest snapshot">(i)</a></sup>
"Does unique_copy() require CopyConstructible and Assignable?" which now
has WP status.
</p>

<p>
This talks about <code>unique_copy</code> requirements and currently reads:
</p>

<blockquote><p>
-5- <i>Requires:</i> The ranges <code>[<i>first</i>, <i>last</i>)</code> and
<code>[<i>result</i>, <i>result</i>+(<i>last</i>-<i>first</i>))</code>
shall not overlap. The expression <code>*<i>result</i> = *<i>first</i></code> shall
be valid. If neither <code>InputIterator</code> nor <code>OutputIterator</code> meets the
requirements of forward iterator then the value type of <code>InputIterator</code>
must be CopyConstructible (20.1.3). Otherwise CopyConstructible is not required.
</p></blockquote>

<p>
The problem (which Paolo discovered) is that when the iterators are at their
most restrictive (<code>InputIterator</code>, <code>OutputIterator</code>), then we want
<code>InputIterator::value_type</code> to be both <code>CopyConstructible</code> and
<code>CopyAssignable</code> (for the most efficient implementation).  However this
proposed resolution only makes it clear that it is <code>CopyConstructible</code>,
and that one can assign from <code>*<i>first</i></code> to <code>*<i>result</i></code>.
This latter requirement does not necessarily imply that you can:
</p>

<blockquote><pre>
*<i>first</i> = *<i>first</i>;
</pre></blockquote>


<p id="res-538"><b>Proposed resolution:</b></p>
<blockquote><p>
-5- <i>Requires:</i> The ranges <code>[<i>first</i>, <i>last</i>)</code> and
<code>[<i>result</i>, <i>result</i>+(<i>last</i>-<i>first</i>))</code>
shall not overlap. The expression <code>*<i>result</i> = *<i>first</i></code>
shall
be valid. If neither <code>InputIterator</code> nor <code>OutputIterator</code> meets the
requirements of forward iterator then the <del>value type</del> 
<ins><code>value_type</code></ins> of <code>InputIterator</code>
must be CopyConstructible (20.1.3) <ins>and Assignable</ins>.
Otherwise CopyConstructible is not required.
</p></blockquote>





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