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    CWG Issue 2404</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="2404"></A><H4>2404.
  
<TT>[[no_unique_address]]</TT> and allocation order
</H4>
<B>Section: </B>11.4&#160; [<A href="https://wg21.link/class.mem">class.mem</A>]
 &#160;&#160;&#160;

 <B>Status: </B>CD5
 &#160;&#160;&#160;

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

 <B>Date: </B>2018-12-13<BR>


<P>[Accepted as a DR at the July, 2019 meeting.]</P>

<P>According to 11.4 [<A href="https://wg21.link/class.mem#19">class.mem</A>] paragraph 19,</P>

<BLOCKQUOTE>

Non-static data members of a (non-union) class with the same
access control (11.8 [<A href="https://wg21.link/class.access">class.access</A>]) are allocated so
that later members have higher addresses within a class
object.

</BLOCKQUOTE>

<P>With the advent of the <TT>[[no_unique_address]]</TT>
attribute, &#8220;higher addresses&#8221; is no longer
strictly accurate. According to the FAQ in
<A HREF="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0840r2.html">
P0840R2</A>, next-to-last question:</P>

<BLOCKQUOTE>

<P>Q: Suppose I have members <TT>a</TT>, <TT>b</TT>, <TT>c</TT>
(in that order, with the
same access). Today we guarantee that <TT>&amp;a
&lt; &amp;b
&lt; &amp;c</TT>. What happens if <TT>b</TT>
has the attribute? </P>

<P>Two cases:</P>

<OL>
<LI><P>If the type of <TT>b</TT> is empty, then there is
no guarantee about the address of <TT>b</TT> (other than that
it is somewhere within the containing object).</P></LI>

<LI><P>If the type of <TT>b</TT>
is nonempty, then we still guarantee that
<TT>&amp;a &lt; &amp;b &lt; &amp;c</TT>.</P></LI>

</OL>

</BLOCKQUOTE>

<P>Presumably the wording in 11.4 [<A href="https://wg21.link/class.mem#19">class.mem</A>] paragraph 19
needs to be changed to reflect that intent.</P>

<P><B>Proposed resolution, March, 2019:</B></P>

<P>Change 11.4 [<A href="https://wg21.link/class.mem#19">class.mem</A>] paragraph 19 as follows:</P>

<BLOCKQUOTE>

<INS>[<I>Note:</I></INS> Non-static data members of a
(non-union) class with the same access control
(11.8 [<A href="https://wg21.link/class.access">class.access</A>]) <INS>and non-zero size
(6.8.2 [<A href="https://wg21.link/intro.object">intro.object</A>])</INS> are allocated so that
later members have higher addresses within a class
object. The order of allocation of non-static data members
with different access control is unspecified
<DEL>(11.8 [<A href="https://wg21.link/class.access">class.access</A>])</DEL>. Implementation alignment
requirements might cause two adjacent members not to be
allocated immediately after each other; so might
requirements for space for managing virtual functions
(11.7.3 [<A href="https://wg21.link/class.virtual">class.virtual</A>]) and virtual base classes
(11.7.2 [<A href="https://wg21.link/class.mi">class.mi</A>]). <INS>&#8212;<I>end note</I>]</INS>

</BLOCKQUOTE>

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