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<title>Issue 2357: Remaining "Assignable" requirement</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#C++14">C++14</a> status.</em></p>
<h3 id="2357"><a href="lwg-defects.html#2357">2357</a>. Remaining "<code>Assignable</code>" requirement</h3>
<p><b>Section:</b> 26.8.5 <a href="https://wg21.link/alg.partitions">[alg.partitions]</a> <b>Status:</b> <a href="lwg-active.html#C++14">C++14</a>
 <b>Submitter:</b> Daniel Kr&uuml;gler <b>Opened:</b> 2014-02-01 <b>Last modified:</b> 2017-07-05</p>
<p><b>Priority: </b>0
</p>
<p><b>View all other</b> <a href="lwg-index.html#alg.partitions">issues</a> in [alg.partitions].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++14">C++14</a> status.</p>
<p><b>Discussion:</b></p>
<p>
The <i>Requires</i> element of <code>partition_copy</code> says (emphasis mine):
</p>
<blockquote><p>
<i>Requires</i>: <code>InputIterator</code>'s value type shall be <b><code>Assignable</code></b>, and &hellip;
</p></blockquote>
<p>
The C++03 term <code>Assignable</code> was replaced by <code>CopyAssignable</code>, remaining cleanups happened via LWG issue
<a href="lwg-closed.html#972" title="The term &quot;Assignable&quot; undefined but still in use (Status: NAD Editorial)">972</a><sup><a href="https://cplusplus.github.io/LWG/issue972" title="Latest snapshot">(i)</a></sup>, but algorithm <code>partition_copy</code> was not affected at that time (during that time the requirements
of <code>partition_copy</code> didn't mention writable nor assignable, but I cannot track down at the moment where these requirements
had been added). Presumably this requirement should be corrected similarly to the approach used in <a href="lwg-closed.html#972" title="The term &quot;Assignable&quot; undefined but still in use (Status: NAD Editorial)">972</a><sup><a href="https://cplusplus.github.io/LWG/issue972" title="Latest snapshot">(i)</a></sup>.
<p/>
Another question is whether a <code>CopyAssignable</code> is needed here, given the fact that we already require "writable to" an 
<code>OutputIterator</code> which is defined in 24.3.1 <a href="https://wg21.link/iterator.requirements.general">[iterator.requirements.general]</a> and does already impose the necessary
statement
</p>
<blockquote><pre>
*out = *in;
</pre></blockquote>
<p>
Given the fact that <code>partition_copy</code> never touches any input value twice, there is no reason why anything more than
<i>writable to</i> should be necessary.
<p/>
The below suggested primary resolution does not respond to the second part of this question.
</p>

<p><i>[Issaquah 2014-02-11: Move to Immediate]</i></p>




<p id="res-2357"><b>Proposed resolution:</b></p>
<p>This wording is relative to N3797.</p>

<ol>
<li><p>Modify 26.8.5 <a href="https://wg21.link/alg.partitions">[alg.partitions]</a>, p12 as indicated:</p>

<blockquote><p>
-12- <i>Requires</i>: <code>InputIterator</code>'s value type shall be <code><ins>Copy</ins>Assignable</code>, and shall be writable 
to the <code>out_true</code> and <code>out_false</code> <code>OutputIterator</code>s, and shall be convertible to <code>Predicate</code>'s 
argument type. The input range shall not overlap with either of the output ranges.
</p></blockquote>
</li>
</ol>





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