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<title>Issue 2445: "Stronger" memory ordering</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="2445"><a href="lwg-defects.html#2445">2445</a>. "Stronger" memory ordering</h3>
<p><b>Section:</b> 99 [depr.util.smartptr.shared.atomic], 99 [atomics.types.operations.req] <b>Status:</b> <a href="lwg-active.html#Resolved">Resolved</a>
 <b>Submitter:</b> JF Bastien <b>Opened:</b> 2014-10-08 <b>Last modified:</b> 2017-11-29</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all other</b> <a href="lwg-index.html#depr.util.smartptr.shared.atomic">issues</a> in [depr.util.smartptr.shared.atomic].</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 definitions of compare and exchange in  [util.smartptr.shared.atomic] p32 and 
99 [atomics.types.operations.req] p20 state:
</p>
<blockquote><p>
<i>Requires</i>: The failure argument shall not be <code>memory_order_release</code> nor <code>memory_order_acq_rel</code>. 
The <code>failure</code> argument shall be no stronger than the <code>success</code> argument.
</p></blockquote>
<p>
The term "stronger" isn't defined by the standard.
<p/>
It is hinted at by 99 [atomics.types.operations.req] p21:
</p>
<blockquote><p>
When only one <code>memory_order</code> argument is supplied, the value of <code>success</code> is <code>order</code>, and the 
value of <code>failure</code> is <code>order</code> except that a value of <code>memory_order_acq_rel</code> shall be replaced 
by the value <code>memory_order_acquire</code> and a value of <code>memory_order_release</code> shall be replaced by the 
value <code>memory_order_relaxed</code>.
</p></blockquote>
<p>
Should the standard define a partial ordering for memory orders, where consume and acquire are incomparable with release?
</p>

<p><i>[2014-11 Urbana]</i></p>

<p>
Move to SG1.
</p>
<p><i>[2016-11-12, Issaquah]</i></p>

<p>Resolved by P0418R2</p>


<p id="res-2445"><b>Proposed resolution:</b></p>





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