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<title>Issue 2921: packaged_task and type-erased allocators</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++17">C++17</a> status.</em></p>
<h3 id="2921"><a href="lwg-defects.html#2921">2921</a>. <code>packaged_task</code> and type-erased allocators</h3>
<p><b>Section:</b> 32.10.10 <a href="https://wg21.link/futures.task">[futures.task]</a> <b>Status:</b> <a href="lwg-active.html#C++17">C++17</a>
 <b>Submitter:</b> United States <b>Opened:</b> 2017-02-03 <b>Last modified:</b> 2025-06-23</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all other</b> <a href="lwg-index.html#futures.task">issues</a> in [futures.task].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++17">C++17</a> status.</p>
<p><b>Discussion:</b></p>
<b>Addresses US 165</b>

<p>The constructor that type-erases an allocator has all of the problems of the similar function constructor that was removed 
for this CD. This constructor from <code>packaged_task</code> should similarly be removed as well. If we prefer to keep this 
constructor, the current wording is underspecified, as the <code>Allocator</code> argument is not required to be type satisfying 
the Allocator requirements, nor is <code>allocator_traits</code> used.</p>

<p>Proposed change: </p>
<p>Strike</p>
<blockquote><pre>
<del>template &lt;class F, class Allocator&gt;
packaged_task(allocator_arg_t, const Allocator&amp; a, F&amp;&amp; f);</del>
</pre></blockquote>
<p>
from the class definition in p2, and from <b>30.6.9.1 [futures.task.members]</b> p2.
Strike the last sentence of 30.6.9.1p4. In p3, revise "These constructors" to "This constructor"
</p>

<p><i>[Kona 2017-03-02]</i></p>

<p>Accepted as Immediate to resolve NB comment.</p>

<p><i>[Sofia 2025-06-21; this was reverted by <a href="https://wg21.link/P3503R3" title=" Make type-erased allocator use in promise and packaged_task consistent">P3503R3</a>]</i></p>




<p id="res-2921"><b>Proposed resolution:</b></p>
<p>This wording is relative to <a href="https://wg21.link/n4618">N4618</a>.</p>
<ol>
<li><p>Modify 32.10.10 <a href="https://wg21.link/futures.task">[futures.task]</a> as follows:</p>
<p>Strike</p>
<blockquote><pre><del>
template &lt;class F, class Allocator&gt;
packaged_task(allocator_arg_t, const Allocator&amp; a, F&amp;&amp; f);
</del></pre></blockquote>
<p>from the class definition in p2, and from [futures.task.members] p2.</p>
</li>

<li>
<p>Modify 32.10.10.2 <a href="https://wg21.link/futures.task.members">[futures.task.members]</a>/3:</p>
<blockquote>
<i>Remarks:</i> <del>These constructors</del><ins>This constructor</ins> shall not participate in overload resolution if 
<code>decay_t&lt;F&gt;</code> is the same type as <code>packaged_task&lt;R(ArgTypes...)&gt;</code>.
</blockquote>
</li>

<li>
<p>Strike the last sentence of 32.10.10.2 <a href="https://wg21.link/futures.task.members">[futures.task.members]</a>/4:</p>
<blockquote><del>
The constructors that take an <code>Allocator</code> argument use it to allocate memory needed to store the internal data structures.
</del></blockquote>
</li>


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