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<title>Issue 3012: atomic&lt;T&gt; is unimplementable for non-is_trivially_copy_constructible T</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++20">C++20</a> status.</em></p>
<h3 id="3012"><a href="lwg-defects.html#3012">3012</a>. <code>atomic&lt;T&gt;</code> is unimplementable for non-<code>is_trivially_copy_constructible T</code></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#C++20">C++20</a>
 <b>Submitter:</b> Billy O'Neal III <b>Opened:</b> 2017-08-16 <b>Last modified:</b> 2021-02-25</p>
<p><b>Priority: </b>2
</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#C++20">C++20</a> status.</p>
<p><b>Discussion:</b></p>
<p>
32.5.8 <a href="https://wg21.link/atomics.types.generic">[atomics.types.generic]</a> requires that <code>T</code> for <code>std::atomic</code> is trivially copyable. 
Unfortunately, that's not sufficient to implement <code>atomic</code>. Consider <code>atomic&lt;T&gt;::load</code>, which 
wants to look something like this:
</p>
<blockquote><pre>
template&lt;class T&gt;
struct atomic {
  __compiler_magic_storage_for_t storage;

  T load(memory_order = memory_order_seq_cst) const {
    return __magic_intrinsic(storage);
  }

};
</pre></blockquote>
<p>
Forming this return statement, though, requires that <code>T</code> is copy constructible &mdash; trivially copyable things 
aren't necessarily copyable! For example, the following is trivially copyable but breaks libc++, libstdc++, and msvc++:
</p>
<blockquote><pre>
struct NonAssignable {
  int i;
  NonAssignable() = delete;
  NonAssignable(int) : i(0) {}
  NonAssignable(const NonAssignable&amp;) = delete;
  NonAssignable(NonAssignable&amp;&amp;) = default;
  NonAssignable&amp; operator=(const NonAssignable&amp;) = delete;
  NonAssignable&amp; operator=(NonAssignable&amp;&amp;) = delete;
  ~NonAssignable() = default;
};
</pre></blockquote>
<p>
All three standard libraries are happy as long as <code>T</code> is trivially copy constructible, assignability is not required. 
Casey Carter says that we might want to still require trivially copy assignable though, since what happens when you do an 
<code>atomic&lt;T&gt;::store</code> is morally an "assignment" even if it doesn't use the user's assignment operator.
</p>

<p><i>[2017-11 Albuquerque Wednesday issue processing]</i></p>

<p>Status to Open; Casey and STL to work with Billy for better wording.</p>
<p>Should this include trivially copyable as well as trivially copy assignable?</p>

<p>2017-11-09, Billy O'Neal provides updated wording.</p>
<p><strong>Previous resolution [SUPERSEDED]:</strong></p>
<blockquote class="note">
<p>This resolution is relative to <a href="https://wg21.link/n4687">N4687</a>.</p>

<ol>
<li><p>Edit 32.5.8 <a href="https://wg21.link/atomics.types.generic">[atomics.types.generic]</a> as indicated:</p>

<blockquote>
<p>
-1- <ins>If <code>is_trivially_copy_constructible_v&lt;T&gt;</code> is <code>false</code>, the program is ill-formed</ins><del>The
template argument for <code>T</code> shall be trivially copyable (6.9 <a href="https://wg21.link/basic.types">[basic.types]</a>)</del>. [<i>Note:</i> Type
arguments that are not also statically initializable may be difficult to use. &mdash; <i>end note</i>]
</p>
</blockquote>
</li>
</ol>
</blockquote>

<p><strong>Previous resolution [SUPERSEDED]:</strong></p>
<blockquote class="note">
<p>This resolution is relative to <a href="https://wg21.link/n4687">N4687</a>.</p>

<ol>
<li><p>Edit 32.5.8 <a href="https://wg21.link/atomics.types.generic">[atomics.types.generic]</a> as indicated:</p>

<blockquote>
<p>
-1- <ins>If <code>is_copy_constructible_v&lt;T&gt;</code> is <code>false</code> or if
<code>is_trivially_copyable_v&lt;T&gt;</code> is <code>false</code>, the program is ill-formed</ins><del>The
template argument for <code>T</code> shall be trivially copyable (6.9 <a href="https://wg21.link/basic.types">[basic.types]</a>)</del>. [<i>Note:</i> Type
arguments that are not also statically initializable may be difficult to use. &mdash; <i>end note</i>]
</p>
</blockquote>
</li>
</ol>
</blockquote>

<p><i>[2017-11-12, Tomasz comments and suggests alternative wording]</i></p>

<p>
According to my understanding during Albuquerque Saturday issue processing we agreed that we want the type used 
with the atomics to have non-deleted and trivial copy/move construction and assignment.
<p/>
Wording note: <code>CopyConstructible</code> and <code>CopyAssignable</code> include semantic requirements that are not 
checkable at compile time, so these are requirements imposed on the user and cannot be validated by an
implementation without heroic efforts.
</p>

<p><i>[2018-11 San Diego Thursday night issue processing]</i></p>

<p>Status to Ready.</p>


<p id="res-3012"><b>Proposed resolution:</b></p>
<p>This resolution is relative to <a href="https://wg21.link/n4700">N4700</a>.</p>

<ol>
<li><p>Edit 32.5.8 <a href="https://wg21.link/atomics.types.generic">[atomics.types.generic]</a> as indicated:</p>

<blockquote>
<p>
-1- The template argument for <code>T</code> shall <ins>meet the <code>CopyConstructible</code> and <code>CopyAssignable</code>
requirements. If <code>is_trivially_copyable_v&lt;T&gt; &amp;&amp; is_copy_constructible_v&lt;T&gt; &amp;&amp; 
is_move_constructible_v&lt;T&gt; &amp;&amp; is_copy_assignable_v&lt;T&gt; &amp;&amp; is_move_assignable_v&lt;T&gt;</code> 
is <code>false</code>, the program is ill-formed</ins><del>be trivially copyable (6.9 <a href="https://wg21.link/basic.types">[basic.types]</a>)</del>. 
[<i>Note:</i> Type arguments that are not also statically initializable may be difficult to use. &mdash; <i>end note</i>]
</p>
</blockquote>
</li>
</ol>




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