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<title>Issue 749: Currently has_nothrow_copy_constructor&lt;T&gt;::value is true if T has 'a' nothrow copy constructor.</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#CD1">CD1</a> status.</em></p>
<h3 id="749"><a href="lwg-defects.html#749">749</a>. Currently <code>has_nothrow_copy_constructor&lt;T&gt;::value</code> is true if <code>T</code> has 'a' nothrow copy constructor.</h3>
<p><b>Section:</b> 21.3.6.4 <a href="https://wg21.link/meta.unary.prop">[meta.unary.prop]</a> <b>Status:</b> <a href="lwg-active.html#CD1">CD1</a>
 <b>Submitter:</b> Alisdair Meredith <b>Opened:</b> 2007-10-10 <b>Last modified:</b> 2016-01-28</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View other</b> <a href="lwg-index-open.html#meta.unary.prop">active issues</a> in [meta.unary.prop].</p>
<p><b>View all other</b> <a href="lwg-index.html#meta.unary.prop">issues</a> in [meta.unary.prop].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#CD1">CD1</a> status.</p>
<p><b>Discussion:</b></p>
<p>
Unfortunately a class can have multiple copy constructors, and I believe to
be useful this trait should only return true is ALL copy constructors are
no-throw.
</p>
<p>
For instance:
</p>
<blockquote>
<pre>
struct awkward {
 awkward( const awkward &amp; ) throw() {}
 awkward( awkward &amp; ) { throw "oops"; } };
</pre>
</blockquote>


<p id="res-749"><b>Proposed resolution:</b></p>
<p>
Change 21.3.6.4 <a href="https://wg21.link/meta.unary.prop">[meta.unary.prop]</a>:
</p>

<blockquote>
<pre>has_trivial_copy_constructor</pre>
<blockquote><p>
<code>T</code> is a trivial type (3.9) or a reference type or a class type <del>with a trivial copy constructor</del>
<ins>where all copy constructors are trivial</ins> (12.8).
</p></blockquote>
</blockquote>

<blockquote>
<pre>has_trivial_assign</pre>
<blockquote><p>
<code>T</code> is neither <code>const</code> nor a reference type, and <code>T</code> is a trivial type (3.9)
or a class type <del>with a trivial copy assignment operator</del> <ins>where all copy assignment operators are trivial</ins> (12.8).
</p></blockquote>
</blockquote>

<blockquote>
<pre>has_nothrow_copy_constructor</pre>
<blockquote><p>
<code>has_trivial_copy_constructor&lt;T&gt;::value</code> is <code>true</code> or <code>T</code> is a class type <del>with
a</del> <ins>where all</ins> copy constructor<ins>s</ins> <del>that is</del> <ins>are</ins> 
known not to throw any exceptions or <code>T</code> is an
array of such a class type
</p></blockquote>
</blockquote>

<blockquote>
<pre>has_nothrow_assign</pre>
<blockquote><p>
<code>T</code> is neither <code>const</code> nor a reference type, and
<code>has_trivial_assign&lt;T&gt;::value</code> is <code>true</code> or <code>T</code> is a class type <del>with a</del>
<ins>where all</ins> copy
assignment operator<ins>s</ins> tak<ins>e</ins><del>ing</del> an lvalue of type <code>T</code> that is known not to
throw any exceptions or <code>T</code> is an array of such a class type.
</p></blockquote>
</blockquote>






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