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    CWG Issue 887</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="887"></A><H4>887.
  
Move construction of thrown object
</H4>
<B>Section: </B>11.4.5.3&#160; [<A href="https://wg21.link/class.copy.ctor">class.copy.ctor</A>]
 &#160;&#160;&#160;

 <B>Status: </B>CD2
 &#160;&#160;&#160;

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

 <B>Date: </B>6 May, 2009<BR>


<P>[Voted into WP at March, 2010 meeting.]</P>

<P>11.4.5.3 [<A href="https://wg21.link/class.copy.ctor#16">class.copy.ctor</A>] paragraph 16 details the conditions
under which a thrown object can be moved instead of copied.  However,
the optimization as currently described is unsafe.  Consider the
following example:</P>

<PRE>
    void f() {
        X x;
        try {
            throw x;
        } catch (...) {
        }
        // x may have been moved from but can still be accessed here
    }
</PRE>

<P>When the operation is a throw, as opposed to a return, there
must be a restriction that the object potentially being moved
be defined within the innermost enclosing try block.</P>

<P><B>Notes from the July, 2009 meeting:</B></P>

<P>It is not clear how important this optimization is in the context
of <TT>throw</TT>: how often is a large object with substantial copying
overhead thrown?  Also, throwing an exception is already a heavyweight
operation, so presumably moving instead of copying an object would not
make much difference.</P>

<P><B>Proposed resolution (October, 2009):</B></P>

<P>Change 11.4.5.3 [<A href="https://wg21.link/class.copy.ctor#17">class.copy.ctor</A>] paragraph 17 second bullet as
follows:</P>

<UL><LI>in a <I>throw-expression</I>, when the operand is the name of
a non-volatile automatic object <INS>whose scope does not extend
beyond the end of the innermost enclosing <I>try-block</I> (if there
is one)</INS>, the copy operation from the operand to the exception
object (14.2 [<A href="https://wg21.link/except.throw">except.throw</A>]) can be omitted by constructing the
automatic object directly into the exception object</LI></UL>

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