<HTML>
<HEAD>
<meta http-equiv="Content-Type" content="text/html; charset=us-ascii">
<TITLE>
    CWG Issue 1533</TITLE>
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<STYLE TYPE="text/css">
  INS { text-decoration:none; font-weight:bold; background-color:#A0FFA0 }
  .INS { text-decoration:none; background-color:#D0FFD0 }
  DEL { text-decoration:line-through; background-color:#FFA0A0 }
  .DEL { text-decoration:line-through; background-color: #FFD0D0 }
  @media (prefers-color-scheme: dark) {
    HTML { background-color:#202020; color:#f0f0f0; }
    A { color:#5bc0ff; }
    A:visited { color:#c6a8ff; }
    A:hover, a:focus { color:#afd7ff; }
    INS { background-color:#033a16; color:#aff5b4; }
    .INS { background-color: #033a16; }
    DEL { background-color:#67060c; color:#ffdcd7; }
    .DEL { background-color:#67060c; }
  }
  SPAN.cmnt { font-family:Times; font-style:italic }
</STYLE>
</HEAD>
<BODY>
<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="1533"></A><H4>1533.
  
Function pack expansion for member initialization
</H4>
<B>Section: </B>13.7.4&#160; [<A href="https://wg21.link/temp.variadic">temp.variadic</A>]
 &#160;&#160;&#160;

 <B>Status: </B>CD3
 &#160;&#160;&#160;

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

 <B>Date: </B>2012-08-06<BR>


<P>[Moved to DR at the April, 2013 meeting.]</P>

<P>The list of pack expansion contexts in 13.7.4 [<A href="https://wg21.link/temp.variadic#4">temp.variadic</A>] paragraph 4
includes a <I>mem-initializer-list</I>, with no restriction
on whether the <I>mem-initializer</I> corresponds to a base class or
a member. However, it appears from 11.9.3 [<A href="https://wg21.link/class.base.init#15">class.base.init</A>] paragraph 15
that such a pack expansion is intended for bases:

<BLOCKQUOTE>

A <I>mem-initializer</I> followed by an ellipsis is a pack expansion
(13.7.4 [<A href="https://wg21.link/temp.variadic">temp.variadic</A>]) that initializes the base classes
specified by a pack expansion in the <I>base-specifier-list</I> for
the class.

</BLOCKQUOTE>
</P>

<P>This is not conclusive, however, and use of a pack expansion with
a <I>mem-initializer</I> for a member could be used with packs containing
zero or one element:</P>

<PRE>
  class S { };

  template&lt;typename... T&gt; class X {
  public:
    X(T ...args) : data_(args)... { }
  private:
    S data_;
  };

  int main() {
    S s;
    X&lt;&gt; x1;
    X&lt;S&gt; x2(s);
  }
</PRE>

<P>The Standard should be clarified as to whether such a usage is
permitted or not.</P>

<P><B>Proposed resolution (October, 2012):</B></P>

<P>Change 13.7.4 [<A href="https://wg21.link/temp.variadic#4">temp.variadic</A>] paragraph 4 as follows:</P>

<BLOCKQUOTE>

<P>...Pack expansions can occur in the following contexts:</P>

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

<LI><P>In a <I>mem-initializer-list</I> (11.9.3 [<A href="https://wg21.link/class.base.init">class.base.init</A>])
<INS>for a <I>mem-initializer</I> whose <I>mem-initializer-id</I>
denotes a base class</INS>; the pattern is <DEL>a</DEL> <INS>the</INS>
<I>mem-initializer</I>.</P></LI>

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

</UL>

</BLOCKQUOTE>

<BR><BR>
</BODY>
</HTML>
