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<title>Issue 4147: Precondition on inplace_vector::emplace</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#WP">WP</a> status.</em></p>
<h3 id="4147"><a href="lwg-defects.html#4147">4147</a>. Precondition on <code>inplace_vector::emplace</code></h3>
<p><b>Section:</b> 23.2.4 <a href="https://wg21.link/sequence.reqmts">[sequence.reqmts]</a> <b>Status:</b> <a href="lwg-active.html#WP">WP</a>
 <b>Submitter:</b> Arthur O'Dwyer <b>Opened:</b> 2024-08-26 <b>Last modified:</b> 2024-11-28</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View other</b> <a href="lwg-index-open.html#sequence.reqmts">active issues</a> in [sequence.reqmts].</p>
<p><b>View all other</b> <a href="lwg-index.html#sequence.reqmts">issues</a> in [sequence.reqmts].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#WP">WP</a> status.</p>
<p><b>Discussion:</b></p>
<p>
Inserting into the middle of an <code>inplace_vector</code>, just like inserting into the middle of a 
<code>vector</code> or <code>deque</code>, requires that we construct the new element out-of-line, shift 
down the trailing elements (<i>Cpp17MoveAssignable</i>), and then move-construct the new element 
into place (<i>Cpp17MoveInsertable</i>). <a href="https://wg21.link/P0843R14" title=" inplace_vector">P0843R14</a> failed to make this change, but 
it should have.
</p>

<p><i>[2024-09-18; Reflector poll]</i></p>

<p>
Set status to Tentatively Ready after nine votes in favour during reflector poll.
</p>

<p><i>[Wrocław 2024-11-23; Status changed: Voting &rarr; WP.]</i></p>



<p id="res-4147"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4988" title=" Working Draft, Programming Languages — C++">N4988</a>.
</p>

<ol>
<li><p>Modify 23.2.4 <a href="https://wg21.link/sequence.reqmts">[sequence.reqmts]</a> as indicated:</p>

<blockquote><pre>
a.emplace(p, args)
</pre>
<blockquote>
<p>
-19- <i>Result</i>: <code>iterator</code>.
<p/>
-20- <i>Preconditions</i>: <code>T</code> is <i>Cpp17EmplaceConstructible</i> into <code>X</code> 
from <code>args</code>. For <code>vector</code><ins>, inplace_vector,</ins> and <code>deque</code>, 
<code>T</code> is also <i>Cpp17MoveInsertable</i> into <code>X</code> and <i>Cpp17MoveAssignable</i>.
<p/>
-21- <i>Effects</i>: Inserts an object of type <code>T</code> constructed with 
<code>std::forward&lt;Args&gt;(args)...</code> before <code>p</code>.
<p/>
[<i>Note 1</i>: <code>args</code> can directly or indirectly refer to a value in <code>a</code>. 
&mdash; <i>end note</i>]
<p/>
-22- <i>Returns</i>: An iterator that points to the new element constructed from <code>args</code> 
into <code>a</code>.
</p>
</blockquote>
</blockquote>
</li>

</ol>






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