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    CWG Issue 922</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="922"></A><H4>922.
  
Implicit default constructor definitions and <TT>const</TT> variant members
</H4>
<B>Section: </B>11.4.5&#160; [<A href="https://wg21.link/class.ctor">class.ctor</A>]
 &#160;&#160;&#160;

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

 <B>Submitter: </B>Daveed Vandevoorde
 &#160;&#160;&#160;

 <B>Date: </B>19 June, 2009<BR>


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

<P>According to 11.4.5 [<A href="https://wg21.link/class.ctor#5">class.ctor</A>] paragraph 5,</P>

<BLOCKQUOTE>

An implicitly-declared default constructor for class <TT>X</TT> is
defined as deleted if: ... any non-static data member of
const-qualified type (or array thereof) does not have a user-provided
default constructor, or...

</BLOCKQUOTE>

<P>It is not clear if this adequately covers the case in which some
variant members are const-qualified but others are not.  The intent
of the restriction is to prevent creation of an object with
uninitialized members that would require a <TT>const_cast</TT> to
set their value later, but const-qualified members of an anonymous
union in which other members are not const do not seem to present
that problem.</P>

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

<P>Change 11.4.5 [<A href="https://wg21.link/class.ctor#5.3">class.ctor</A>] bullet 5.3
of the second list and add a fourth bullet as follows:</P>

<UL>
<LI><P>...</P></LI>

<LI><P>any <INS>non-variant</INS> non-static data member of
const-qualified type (or array thereof) does not have a user-provided
default constructor, <DEL>or</DEL>
</P></LI>

<LI><P><INS>all variant members are of const-qualified type (or array
thereof), or</INS></P></LI>

<LI><P>...</P></LI>

</UL>

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

<P>Change 11.4.5 [<A href="https://wg21.link/class.ctor#5.3">class.ctor</A>] bullet 5.3
of the second list and add two bullets as follows:</P>

<UL>
<LI><P>...</P></LI>

<LI><P>any <INS>non-variant</INS> non-static data member of
const-qualified type (or array thereof) does not have a user-provided
default constructor, <DEL>or</DEL>
</P></LI>

<LI><P><INS><TT>X</TT> is a union and all its variant members are of
const-qualified type (or array thereof),</INS></P></LI>

<LI><P><INS><TT>X</TT> is a non-union class and all members of any
anonymous union member are of const-qualified type (or array
thereof), or</INS></P></LI>

<LI><P>...</P></LI>

</UL>

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