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<title>Issue 3471: polymorphic_allocator::allocate does not satisfy Cpp17Allocator requirements</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++23">C++23</a> status.</em></p>
<h3 id="3471"><a href="lwg-defects.html#3471">3471</a>. <code>polymorphic_allocator::allocate</code> does not satisfy <i>Cpp17Allocator</i> requirements</h3>
<p><b>Section:</b> 20.5 <a href="https://wg21.link/mem.res">[mem.res]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Alisdair Meredith <b>Opened:</b> 2020-07-27 <b>Last modified:</b> 2023-11-22</p>
<p><b>Priority: </b>3
</p>
<p><b>View all other</b> <a href="lwg-index.html#mem.res">issues</a> in [mem.res].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++23">C++23</a> status.</p>
<p><b>Discussion:</b></p>
<p>
With the adoption of <a href="https://wg21.link/p0593r6">P0593R6</a> in Prague,
<code>std::ptr::polymorphic_allocator</code> no longer satisfies the of <i>Cpp17Allocator</i>
requirements. Specifically, all calls to <code>allocate(n)</code> need to create an object
for an array of <code>n T</code>s (but not initialize any of those elements).
<code>std::pmr::polymorphic_allocator</code> calls its underlying memory resource to allocate
sufficient bytes of storage, but it does not create (and start the lifetime of) the array
object within that storage.
</p>

<p><i>[2020-08-03; Billy comments]</i></p>

<p>
It's worth noting that the resolution of <a href="https://wg21.link/cwg2382">CWG 2382</a>
has impact on implementors for this issue.
</p>

<p><i>[2020-08-21; Reflector prioritization]</i></p>

<p>
Set priority to 3 after reflector discussions.
</p>

<p><strong>Previous resolution [SUPERSEDED]:</strong></p>
<blockquote class="note">
<p>
This wording is relative to <a href="https://wg21.link/n4861">N4861</a>.
</p>

<blockquote class="note">
<p>
[<i>Drafting note:</i> The proposed wording is inspired by the example given in the "Assertion/note" column of the expression
<code>a.allocate(n)</code> in table [tab:cpp17.allocator].]
</p>
</blockquote>

<ol>
<li><p>Modify 20.5.2.2 <a href="https://wg21.link/mem.res.public">[mem.res.public]</a> as indicated:</p>

<blockquote>
<pre>
[[nodiscard]] void* allocate(size_t bytes, size_t alignment = max_align);
</pre>
<blockquote>
<p>
-2- <i>Effects:</i> Equivalent to: <del><code>return do_allocate(bytes, alignment);</code></del>
</p>
<blockquote><pre>
<ins>void* p = do_allocate(bytes, alignment);
return launder(new (p) byte[bytes]);</ins>
</pre></blockquote>
</blockquote>
</blockquote>
</li>
</ol>
</blockquote>

<p><i>[2021-05-20 Tim comments and updates wording]</i></p>

<p>
<code>memory_resource::allocate</code> is the PMR equivalent of <code>malloc</code>
and <code>operator new</code>. It therefore needs the "suitable created object"
wording (see 6.8.2 <a href="https://wg21.link/intro.object">[intro.object]</a>, 20.2.12 <a href="https://wg21.link/c.malloc">[c.malloc]</a>).
This ensures that it creates the requisite array object automatically for all
the allocation functions of <code>polymorphic_allocator</code>,
and returns the correct pointer value in all cases.
</p>

<p><i>[2022-01-31; Reflector poll]</i></p>

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

<p><i>[2022-02-10 Approved at February 2022 virtual plenary. Status changed: Tentatively Ready &rarr; WP.]</i></p>



<p id="res-3471"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/n4885">N4885</a>.
</p>

<ol>
<li><p>Modify 20.5.2.2 <a href="https://wg21.link/mem.res.public">[mem.res.public]</a> as indicated:</p>

<blockquote>
<pre>
[[nodiscard]] void* allocate(size_t bytes, size_t alignment = max_align);
</pre>
<blockquote>
<p>
-2- <i>Effects:</i> <del>Equivalent to: <code>return</code></del>
<ins>Allocates storage by calling</ins> <code>do_allocate(bytes, alignment)</code><del><code>;</code></del>
<ins>and implicitly creates objects within the allocated region of storage.</ins>
<p/>
<ins>-?- <i>Returns:</i> A pointer to a suitable created object
(6.8.2 <a href="https://wg21.link/intro.object">[intro.object]</a>) in the allocated region of storage.</ins>
<p/>
<ins>-?- <i>Throws:</i> What and when the call to <code>do_allocate</code> throws.</ins>
</p>
</blockquote>
</blockquote>
</li>
</ol>





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