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<title>Issue 4065: Requirements for fancy pointers might be insufficient for self-referential implementation of containers</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#New">New</a> status.</em></p>
<h3 id="4065"><a href="lwg-active.html#4065">4065</a>. Requirements for fancy pointers might be insufficient for self-referential implementation of containers</h3>
<p><b>Section:</b> 16.4.4.6.1 <a href="https://wg21.link/allocator.requirements.general">[allocator.requirements.general]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Jiang An <b>Opened:</b> 2024-04-07 <b>Last modified:</b> 2024-05-01</p>
<p><b>Priority: </b>3
</p>
<p><b>View other</b> <a href="lwg-index-open.html#allocator.requirements.general">active issues</a> in [allocator.requirements.general].</p>
<p><b>View all other</b> <a href="lwg-index.html#allocator.requirements.general">issues</a> in [allocator.requirements.general].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
Currently, 16.4.4.6.1 <a href="https://wg21.link/allocator.requirements.general">[allocator.requirements.general]</a>/2.12 imposes a precondition for 
<code>pointer_traits::pointer_to</code> that <code>r</code> denotes an object whose storage is allocated 
by the allocator.
<p/>
However, as mentioned in <a href="https://wg21.link/P0773R0" title=" Towards meaningful fancy pointers">P0773R0</a>, some containers are self-referential in current 
implementations, and thus a fancy pointer stored in the last node (as required by LWG <a href="lwg-defects.html#2261" title="Are containers required to use their 'pointer' type internally? (Status: C++17)">2261</a><sup><a href="https://cplusplus.github.io/LWG/issue2261" title="Latest snapshot">(i)</a></sup>) 
needs to be able to point to a sentinel node in the container object. Given the container object can 
be constructed at any location, perhaps we should specify that <code>pointer_traits::pointer_to</code> 
works on any lvalue.
</p>

<p><i>[2024-05-01; Reflector poll]</i></p>

<p>
Set priority to 3 after reflector poll.
</p>
<p>"This applies to small-object optimizations too."</p>
<p>"An alternative way to support P0773's "near pointers" would be to require
that a sentinel-node container using a "near allocator" must itself be
constructed in the near segment. (This is P0773 (B) (2).)"</p>



<p id="res-4065"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4971" title=" Working Draft, Programming Languages — C++">N4971</a>.
</p>

<ol>
<li><p>Modify 16.4.4.6.1 <a href="https://wg21.link/allocator.requirements.general">[allocator.requirements.general]</a> as indicated:</p>

<blockquote>
<p>
-2- In subclause 16.4.4.6 <a href="https://wg21.link/allocator.requirements">[allocator.requirements]</a>,
</p>
<ol style="list-style-type: none">
<li><p>[&hellip;]</p></li>
<li><p>(2.10) &mdash; <code>p</code> denotes a value of type <code>XX::pointer</code> obtained by calling 
<code>a1.allocate</code>, where <code>a1 == a</code>,</p></li>
<li><p>(2.11) &mdash; [&hellip;],</p></li>
<li><p>(2.12) &mdash; <code>r</code> denotes a<ins>n l</ins>value of type <code>T<del>&amp;</del></code> 
<del>obtained by the expression <code>*p</code></del>,</p></li>
<li><p>[&hellip;]</p></li>
</ol>
<p>
[&hellip;]
</p>
<pre>
pointer_traits&lt;XX::pointer&gt;::pointer_to(r)
</pre>
<blockquote>
<p>
-33- <i>Result:</i> <code>XX::pointer</code>
<p/>
-34- <i>Postconditions:</i> <del>Same as <code>p</code></del><ins>Let <code>p1</code> denote the return value. 
<code>*p1</code> refers to the same object as <code>r</code></ins>.
</p>
</blockquote>
</blockquote>
</li>
</ol>





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