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<title>Issue 3637: pmr::memory_resource::do_allocate needs clarification</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="3637"><a href="lwg-active.html#3637">3637</a>. <code>pmr::memory_resource::do_allocate</code> needs clarification</h3>
<p><b>Section:</b> 20.5.2 <a href="https://wg21.link/mem.res.class">[mem.res.class]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Jonathan Wakely <b>Opened:</b> 2021-11-12 <b>Last modified:</b> 2022-01-30</p>
<p><b>Priority: </b>3
</p>
<p><b>View all other</b> <a href="lwg-index.html#mem.res.class">issues</a> in [mem.res.class].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
20.5.2.3 <a href="https://wg21.link/mem.res.private">[mem.res.private]</a> says that <code>pmr::memory_resource::do_allocate</code> 
returns "a pointer to allocated storage" and references 6.8.6.5.2 <a href="https://wg21.link/basic.stc.dynamic.allocation">[basic.stc.dynamic.allocation]</a>. 
But it's not really clear which parts of 6.8.6.5.2 <a href="https://wg21.link/basic.stc.dynamic.allocation">[basic.stc.dynamic.allocation]</a> define 
"allocated storage". <code>pmr::memory_resource::allocate</code> is not "an allocation function" 
and not a "replaceable allocation function", so "the value returned by a replaceable allocation 
function is a non-null pointer value" doesn't apply here, and neither does "different from any 
previously returned value".
<p/>
Is <code>pmr::memory_resource::allocate</code> allowed to return a null pointer on success? Is it 
allowed to return the same address twice, without an intervening deallocation? What about if you call 
<code>pmr::monotonic_buffer_resource::release()</code>, is that a deallocation?
<p/>
When discussed on the reflector the consensus was that returning null should not be allowed, 
it should throw an exception or return a valid dereferenceable pointer. The reference to 
6.8.6.5.2 <a href="https://wg21.link/basic.stc.dynamic.allocation">[basic.stc.dynamic.allocation]</a> doesn't work to specify this though, so we should 
restate the requirements without directly using the core wording for <code>operator new</code>.
<p/>
It was also suggested that returning the same value should not be allowed without an intervening 
deallocation, but that "deallocation" should not only mean a call to deallocate on the resource, 
but include things like <code>pmr::monotonic_buffer_resource::release()</code>, and when a memory 
resource's destructor returns memory to an upstream resource.
</p>

<p><i>[2022-01-30; Reflector poll]</i></p>

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



<p id="res-3637"><b>Proposed resolution:</b></p>





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