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    CWG Issue 320</TITLE>
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<P><EM>This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21
  Core Issues List revision 118b.
  See http://www.open-std.org/jtc1/sc22/wg21/ for the official
  list.</EM></P>
<P>2025-09-28</P>
<HR>
<A NAME="320"></A><H4>320.
  
Question on copy constructor elision example
</H4>
<B>Section: </B>6.8.7&#160; [<A href="https://wg21.link/class.temporary">class.temporary</A>]
 &#160;&#160;&#160;

 <B>Status: </B>CD1
 &#160;&#160;&#160;

 <B>Submitter: </B>Steve Clamage
 &#160;&#160;&#160;

 <B>Date: </B>2 Nov 2001<BR>


<P>[Voted into WP at April 2005 meeting.]</P>

<P>Section 6.8.7 [<A href="https://wg21.link/class.temporary#2">class.temporary</A>] paragraph 2, abridged:</P>
<BLOCKQUOTE>
<PRE>
  X f(X);
  void g()
  {
	X a;
	a = f(a);
  }
</PRE>
<P>
<TT>a=f(a)</TT> requires a temporary for either the argument <TT>a</TT>
or the result of <TT>f(a)</TT> to avoid undesired aliasing of <TT>a</TT>.
</P>
</BLOCKQUOTE>

<P>The note seems to imply that an implementation is allowed to omit
copying "a" to f's formal argument, or to omit using a temporary for
the return value of f.  I don't find that license in normative text.</P>

<P>Function f returns an X by value, and in the expression the value is
assigned (not copy-constructed) to "a".  I don't see how that
temporary can be omitted.  (See also 11.4.5.3 [<A href="https://wg21.link/class.copy.ctor">class.copy.ctor</A>] p 15)</P>

<P>Since "a" is an lvalue and not a temporary, I don't see how copying
"a" to f's formal parameter can be avoided.</P>

<P>Am I missing something, or is 6.8.7 [<A href="https://wg21.link/class.temporary">class.temporary</A>] p 2
misleading?</P>

<P><B>Proposed resolution (October, 2004):</B></P>

<P>In 6.8.7 [<A href="https://wg21.link/class.temporary#2">class.temporary</A>] paragraph 2, change the last
sentence as indicated:</P>

<BLOCKQUOTE>

On the other hand, the expression <TT>a=f(a)</TT> requires a
temporary for <DEL>either the argument <TT>a</TT> or the result of
<TT>f(a)</TT> to avoid undesired aliasing of <TT>a</TT></DEL>
<INS>the result of <TT>f(a)</TT>, which is then assigned to
<TT>a</TT></INS>.

</BLOCKQUOTE>

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