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    CWG Issue 771</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="771"></A><H4>771.
  
Move-construction of reference members of closure objects
</H4>
<B>Section: </B>7.5.6.2&#160; [<A href="https://wg21.link/expr.prim.lambda.closure">expr.prim.lambda.closure</A>]
 &#160;&#160;&#160;

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

 <B>Submitter: </B>Jonathan Caves
 &#160;&#160;&#160;

 <B>Date: </B>9 February, 2009<BR>


<P>[Voted into the WP at the July, 2009 meeting as part of N2927.]</P>

<P>According to 7.5.6 [<A href="https://wg21.link/expr.prim.lambda#11">expr.prim.lambda</A>] paragraph 11, the closure
class &#8220;has a public move constructor that performs a
member-wise move.&#8221; Although the terms &#8220;move
constructor&#8221; and &#8220;member-wise move&#8221; are not currently
defined (see <A HREF="680.html">issue 680</A>), this presumably
means that a lambda like <TT>[&amp;i]{}</TT> results in a closure class
similar to:</P>

<PRE>
    class F {
        int&amp; i;
    public:
        F(&amp;&amp; other):
            i(std::move(other.i)) { }
        // etc.
    };
</PRE>

<P>This constructor is ill-formed because it attempts to initialize
an lvalue reference to non-const int with the rvalue returned by
<TT>std::move</TT>.</P>

<P>It is not clear whether this should be handled by:</P>

<OL>

<LI><P>Not generating the move constructor.</P></LI>

<LI><P>Generating the declaration of the move constructor but only
defining it (and giving the corresponding error) if the move
constructor would be used, similar to the handling of other
implicitly-defined special member functions.</P></LI>

<P><LI>Generating the move constructor but copy-constructing any
reference members.</LI></P>

</OL>

<P><B>Proposed resolution (July, 2009)</B></P>

<P>See document PL22.16/09-0117 = WG21 N2927.</P>

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