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<title>Issue 2172: Does atomic_compare_exchange_* accept v == nullptr arguments?</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++14">C++14</a> status.</em></p>
<h3 id="2172"><a href="lwg-defects.html#2172">2172</a>. Does <code>atomic_compare_exchange_*</code> accept <code>v == nullptr</code> arguments?</h3>
<p><b>Section:</b> 99 [depr.util.smartptr.shared.atomic] <b>Status:</b> <a href="lwg-active.html#C++14">C++14</a>
 <b>Submitter:</b> Howard Hinnant <b>Opened:</b> 2012-07-28 <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#C++14">C++14</a> status.</p>
<p><b>Discussion:</b></p>

<p>
Looking at  [util.smartptr.shared.atomic]/p31
</p>
<blockquote><pre>
template&lt;class T&gt;
  bool
  atomic_compare_exchange_strong_explicit(shared_ptr&lt;T&gt;* p,
                                          shared_ptr&lt;T&gt;* v,
                                          shared_ptr&lt;T&gt; w,
                                          memory_order success, memory_order failure);
</pre><blockquote><p>
-31- <i>Requires</i>: <code>p</code> shall not be null.
</p></blockquote></blockquote>
<p>
What about <code>v</code>?  Can it be null?  And if so, what happens?
<p/>
This looks closely related to C++11 issue LWG <a href="lwg-defects.html#1030" title="Missing requirements for smart-pointer safety API (Status: C++11)">1030</a><sup><a href="https://cplusplus.github.io/LWG/issue1030" title="Latest snapshot">(i)</a></sup>, where we gave every signature in this section a:
</p>
<blockquote><p>
<i>Requires</i>: <code>p</code> shall not be null.
</p></blockquote>
<p>
It looks like a simple oversight to me that we did not add for the <code>atomic_compare_exchange_*</code>:
</p>
<blockquote><p>
<i>Requires</i>: <code>p</code> shall not be null <ins>and <code>v</code> shall not be null</ins>.
</p></blockquote>

<p><i>[2012-10 Portland: Move to Ready]</i></p>

<p>
This is clearly the right thing to do, and Lawrence concurs. 
</p>

<p><i>[2013-04-20 Bristol]</i></p>




<p id="res-2172"><b>Proposed resolution:</b></p>
<p>This wording is relative to N3376.</p>

<p>Change  [util.smartptr.shared.atomic] as indicated:</p>

<blockquote><pre>
template&lt;class T&gt;
  bool atomic_compare_exchange_weak(
    shared_ptr&lt;T&gt;* p, shared_ptr&lt;T&gt;* v, shared_ptr&lt;T&gt; w);
</pre><blockquote><p>
-27- <i>Requires</i>: <code>p</code> shall not be null <ins>and <code>v</code> shall not be null</ins>.
<p/>
[&hellip;]
</p>
</blockquote>
<pre>
template&lt;class T&gt;
  bool atomic_compare_exchange_weak_explicit(
    shared_ptr&lt;T&gt;* p, shared_ptr&lt;T&gt;* v, shared_ptr&lt;T&gt; w,
    memory_order success, memory_order failure);
template&lt;class T&gt;
  bool atomic_compare_exchange_strong_explicit(
    shared_ptr&lt;T&gt;* p, shared_ptr&lt;T&gt;* v, shared_ptr&lt;T&gt; w,
    memory_order success, memory_order failure);
</pre><blockquote><p>
-31- <i>Requires</i>: <code>p</code> shall not be null <ins>and <code>v</code> shall not be null</ins>.
<p/>
[&hellip;]
</p>
</blockquote></blockquote>






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