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<title>Issue 1469: atomic&lt;T*&gt; inheritance from atomic_address breaks type safety</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="1469"><a href="lwg-defects.html#1469">1469</a>. <code>atomic&lt;T*&gt;</code> inheritance from <code>atomic_address</code> breaks type safety</h3>
<p><b>Section:</b> 32.5.8 <a href="https://wg21.link/atomics.types.generic">[atomics.types.generic]</a> <b>Status:</b> <a href="lwg-active.html#Resolved">Resolved</a>
 <b>Submitter:</b> BSI <b>Opened:</b> 2010-08-25 <b>Last modified:</b> 2016-01-28</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all other</b> <a href="lwg-index.html#atomics.types.generic">issues</a> in [atomics.types.generic].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#Resolved">Resolved</a> status.</p>
<p><b>Discussion:</b></p>
<p><b>Addresses GB-133</b></p>

<p>
The free functions that operate on <code>atomic_address</code> can be
used to store a pointer to an unrelated type in an <code>atomic&lt;T*&gt;</code> 
without a cast. e.g.
</p>
<blockquote><pre>
int i;
atomic&lt;int*&gt; ai(&amp;i);
string s;
atomic_store(&amp;ai,&amp;s);
</pre></blockquote>
<p>
Overload the <code>atomic_store</code>, <code>atomic_exchange</code> and
<code>atomic_compare_exchange_[weak/strong]</code> operations for 
<code>atomic&lt;T*&gt;</code> to allow storing only pointers to <code>T</code>.
</p>

<p><i>[
2010-10-27 Daniel adds:
]</i></p>


<blockquote>
<p>
Resolving this issue will also solve <a href="lwg-defects.html#1467" title="Deriving atomic&lt;T*&gt; from atomic_address breaks type safety (Status: Resolved)">1467</a><sup><a href="https://cplusplus.github.io/LWG/issue1467" title="Latest snapshot">(i)</a></sup>
<p/>
Accepting <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2010/n3164.html">n3164</a> would solve this issue by
removing <code>atomic_address</code>.
</p>
</blockquote>

<p><i>[Resolved in Batavia by accepting
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2010/n3193.htm">n3193</a>.
]</i></p>



<p id="res-1469"><b>Proposed resolution:</b></p>
<p>Add the following overloads to 32.5.8 <a href="https://wg21.link/atomics.types.generic">[atomics.types.generic]</a>, the synopsis around the specialization
<code>atomic&lt;T*&gt;</code>, as indicated:</p>
<blockquote><pre>
namespace std {
  [..]
  template &lt;class T&gt; struct atomic&lt;T*&gt; : atomic_address {
    [..]
  };

  <ins>template&lt;typename T&gt;</ins>
  <ins>void atomic_store(atomic&lt;T*&gt;&amp;,T*);</ins>
  <ins>template&lt;typename T&gt;</ins>
  <ins>void atomic_store(atomic&lt;T*&gt;&amp;,void*) = delete;</ins>
  <ins>template&lt;typename T&gt;</ins>
  <ins>void atomic_store_explicit(atomic&lt;T*&gt;&amp;,T*,memory_order);</ins>
  <ins>template&lt;typename T&gt;</ins>
  <ins>void atomic_store_explicit(atomic&lt;T*&gt;&amp;,void*,memory_order) = delete;</ins>
  <ins>template&lt;typename T&gt;</ins>
  <ins>T* atomic_exchange(atomic&lt;T*&gt;&amp;,T*);</ins>
  <ins>template&lt;typename T&gt;</ins>
  <ins>T* atomic_exchange(atomic&lt;T*&gt;&amp;,void*) = delete;</ins>
  <ins>template&lt;typename T&gt;</ins>
  <ins>T* atomic_exchange_explicit(atomic&lt;T*&gt;&amp;,T*,memory_order);</ins>
  <ins>template&lt;typename T&gt;</ins>
  <ins>T* atomic_exchange_explicit(atomic&lt;T*&gt;&amp;,void*,memory_order) = delete;</ins>
  <ins>template&lt;typename T&gt;</ins>
  <ins>T* atomic_compare_exchange_weak(atomic&lt;T*&gt;&amp;,T**,T*);</ins>
  <ins>template&lt;typename T&gt;</ins>
  <ins>T* atomic_compare_exchange_weak(atomic&lt;T*&gt;&amp;,void**,void*) = delete;</ins>
  <ins>template&lt;typename T&gt;</ins>
  <ins>T* atomic_compare_exchange_weak_explicit(atomic&lt;T*&gt;&amp;,T**,T*,memory_order);</ins>
  <ins>template&lt;typename T&gt;</ins>
  <ins>T* atomic_compare_exchange_weak_explicit(atomic&lt;T*&gt;&amp;,void**,void*,memory_order) = delete;</ins>
  <ins>template&lt;typename T&gt;</ins>
  <ins>T* atomic_compare_exchange_strong(atomic&lt;T*&gt;&amp;,T**,T*);</ins>
  <ins>template&lt;typename T&gt;</ins>
  <ins>T* atomic_compare_exchange_strong(atomic&lt;T*&gt;&amp;,void**,void*) = delete;</ins>
  <ins>template&lt;typename T&gt;</ins>
  <ins>T* atomic_compare_exchange_strong_explicit(atomic&lt;T*&gt;&amp;,T**,T*,memory_order);</ins>
  <ins>template&lt;typename T&gt;</ins>
  <ins>T* atomic_compare_exchange_strong_explicit(atomic&lt;T*&gt;&amp;,void**,void*,memory_order) = delete;</ins>

}
</pre></blockquote>





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