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<title>Issue 2776: shared_ptr unique() and use_count()</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#Resolved">Resolved</a> status.</em></p>
<h3 id="2776"><a href="lwg-defects.html#2776">2776</a>. <code>shared_ptr unique()</code> and <code>use_count()</code></h3>
<p><b>Section:</b> 20.3.2.2.6 <a href="https://wg21.link/util.smartptr.shared.obs">[util.smartptr.shared.obs]</a> <b>Status:</b> <a href="lwg-active.html#Resolved">Resolved</a>
 <b>Submitter:</b> Hans Boehm <b>Opened:</b> 2016-09-22 <b>Last modified:</b> 2018-06-12</p>
<p><b>Priority: </b>2
</p>
<p><b>View all other</b> <a href="lwg-index.html#util.smartptr.shared.obs">issues</a> in [util.smartptr.shared.obs].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#Resolved">Resolved</a> status.</p>
<p><b>Discussion:</b></p>
<p>
The removal of the "debug only" restriction for <code>use_count()</code> and <code>unique()</code> in <code>shared_ptr</code>  
by LWG <a href="lwg-defects.html#2434" title="shared_ptr::use_count() is efficient (Status: C++17)">2434</a><sup><a href="https://cplusplus.github.io/LWG/issue2434" title="Latest snapshot">(i)</a></sup> introduced a bug. In order for <code>unique()</code> to produce a useful and reliable value, 
it needs a synchronize clause to ensure that prior accesses through another reference are visible to the successful 
caller of <code>unique()</code>. Many current implementations use a relaxed load, and do not provide this guarantee, 
since it's not stated in the standard. For debug/hint usage that was OK. Without it the specification is unclear 
and probably misleading.
<p/>
I would vote for making <code>unique()</code> use <code>memory_order_acquire</code>, and specifying that reference count 
decrement operations synchronize with <code>unique()</code>. That still doesn't give us sequential consistency by default, 
like we're supposed to have. But the violations seem sufficiently obscure that I think it's OK. All uses that 
anybody should care about will work correctly, and the bad uses are clearly bad. I agree with Peter that this 
version of <code>unique()</code> may be quite useful.
<p/>
I would prefer to specify <code>use_count()</code> as only providing an unreliable hint of the actual count (another way 
of saying debug only). Or deprecate it, as JF suggested. We can't make <code>use_count()</code> reliable without adding 
substantially more fencing. We really don't want someone waiting for <code>use_count() == 2</code> to determine that 
another thread got that far. And unfortunately, I don't think we currently say anything to make it clear that's a 
mistake.
<p/>
This would imply that <code>use_count()</code> normally uses <code>memory_order_relaxed</code>, and <code>unique</code> is 
neither specified nor implemented in terms of <code>use_count()</code>.
</p>

<p><i>[2016-10-27 Telecon]</i></p>

<p>Priority set to 2</p>

<p><i>[2018-06 Rapperswil Thursday issues processing]</i></p>

<p>This was resolved by <a href="https://wg21.link/P0521">P0521</a>, which was adopted in Jacksonville.</p>


<p id="res-2776"><b>Proposed resolution:</b></p>






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