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    CWG Issue 645</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="645"></A><H4>645.
  
Are bit-field and non-bit-field members layout compatible?
</H4>
<B>Section: </B>11.4&#160; [<A href="https://wg21.link/class.mem">class.mem</A>]
 &#160;&#160;&#160;

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

 <B>Submitter: </B>Alan Stokes
 &#160;&#160;&#160;

 <B>Date: </B>9 Aug 2007<BR>


<P>[Voted into the WP at the March, 2009 meeting.]</P>



<P>The current wording defining a &#8220;common initial sequence&#8221;
in 11.4 [<A href="https://wg21.link/class.mem#17">class.mem</A>] paragraph 17 does not address the case
in which one member is a bit-field and the corresponding member is
not:</P>

<BLOCKQUOTE>

Two standard-layout structs share a common initial sequence if
corresponding members have layout-compatible types (and, for
bit-fields, the same widths) for a sequence of one or more initial
members.

</BLOCKQUOTE>

<P>Presumably the intent was something like, &#8220;(and, if one
of the pair is a bit-field, the other is also a bit-field of the
same width).&#8221;</P>

<P><B>Proposed Resolution (September, 2008):</B></P>

<P>Change 11.4 [<A href="https://wg21.link/class.mem#18">class.mem</A>] paragraph 18 as follows:</P>
<BLOCKQUOTE>

... Two standard-layout structs share a common initial sequence
if corresponding members have layout-compatible types <DEL>(and,
for bit-fields, the same widths)</DEL> <INS>and either neither member
is a bit-field or both are bit-fields with the same widths</INS>
for a sequence of one or more initial members.

</BLOCKQUOTE>

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