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<title>Issue 760: The emplace issue</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="760"><a href="lwg-closed.html#760">760</a>. The emplace issue</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> Paolo Carlini <b>Opened:</b> 2007-11-11 <b>Last modified:</b> 2017-07-17</p>
<p><b>Priority: </b>2
</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>
In an <code>emplace</code> member function the function parameter pack may be bound
to a priori unlimited number of objects: some or all of them can be
elements of the container itself. Apparently, in order to conform to the
blanket statement 23.2 <a href="https://wg21.link/container.requirements">[container.requirements]</a>/11, the
implementation must check all of them for that possibility. A possible
solution can involve extending the exception in 23.2 <a href="https://wg21.link/container.requirements">[container.requirements]</a>/12 also to the emplace member. As a
side note, the <code>push_back</code> and <code>push_front</code> member
functions are luckily not affected by this problem, can be efficiently
implemented anyway.
</p>

<p><i>[
Related to <a href="lwg-defects.html#767" title="Forwarding and backward compatibility (Status: Resolved)">767</a><sup><a href="https://cplusplus.github.io/LWG/issue767" title="Latest snapshot">(i)</a></sup> and to <a href="lwg-defects.html#2164" title="What are the semantics of vector.emplace(vector.begin(), vector.back())? (Status: C++20)">2164</a><sup><a href="https://cplusplus.github.io/LWG/issue2164" title="Latest snapshot">(i)</a></sup>
]</i></p>


<p><i>[
Bellevue:
]</i></p>


<blockquote>
<p>
The proposed addition (13) is partially redundant with the existing
paragraph 12. Why was the qualifier "rvalues" added to paragraph 12? Why
does it not cover subelements and pointers?
</p>
<p>
Resolution: Alan Talbot to rework language, then set state to Review.
</p>
</blockquote>

<p><i>[
2009-07 Frankfurt
]</i></p>


<blockquote>
<p>
The problem is broader than <code>emplace</code>. The LWG doesn't
feel that it knows how to write wording that prohibits all of the
problematic use cases at this time.
</p>
<p>
NAD Future.
</p>
</blockquote>

<p><i>[2015-02 Cologne]</i></p>

<p>
LWG believes that <a href="lwg-defects.html#2164" title="What are the semantics of vector.emplace(vector.begin(), vector.back())? (Status: C++20)">2164</a><sup><a href="https://cplusplus.github.io/LWG/issue2164" title="Latest snapshot">(i)</a></sup> addresses this issue and therefore considers <a href="lwg-closed.html#760" title="The emplace issue (Status: NAD)">760</a><sup><a href="https://cplusplus.github.io/LWG/issue760" title="Latest snapshot">(i)</a></sup> as NAD. 
</p>


<p id="res-760"><b>Proposed resolution:</b></p>
<p>
Add after 23.2 <a href="https://wg21.link/container.requirements">[container.requirements]</a>/12:
</p>

<blockquote>
<p>
-12- Objects passed to member functions of a container as rvalue
references shall not be elements of that container. No diagnostic
required.
</p>
<p>
<ins>
-13- Objects bound to the function parameter pack of the
<code>emplace</code> member function shall not be elements or sub-objects of
elements of the container. No diagnostic required.
</ins>
</p>

</blockquote>






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