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    CWG Issue 1865</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="1865"></A><H4>1865.
  
Pointer arithmetic and multi-level qualification conversions
</H4>
<B>Section: </B>7.6.6&#160; [<A href="https://wg21.link/expr.add">expr.add</A>]
 &#160;&#160;&#160;

 <B>Status: </B>CD4
 &#160;&#160;&#160;

 <B>Submitter: </B>Geoffrey Romer
 &#160;&#160;&#160;

 <B>Date: </B>2014-02-15<BR>


<P>[Moved to DR at the November, 2014 meeting.]</P>

<P>The resolution of <A HREF="1504.html">issue 1504</A> added
7.6.6 [<A href="https://wg21.link/expr.add#7">expr.add</A>] paragraph 7:</P>

<BLOCKQUOTE>

For addition or subtraction, if the expressions <TT>P</TT> or <TT>Q</TT>
have type &#8220;pointer to <I>cv</I> <TT>T</TT>&#8221;, where <TT>T</TT>
is different from the cv-unqualified array element type, the behavior is
undefined.

</BLOCKQUOTE>

<P>This wording was intended to address derived-base conversion in pointer
arithmetic, but it inadvertently categorized as undefined behavior
previously well-defined pointer arithmetic on pointers that are the result
of multi-level qualification conversions.  For example:</P>

<PRE>
  void f() {
    int i = 0;
    int* arr[3] = {&amp;i, &amp;i, &amp;i};
    int const * const * aptr = arr;
    assert(aptr[2] == &amp;i);
  }
</PRE>

<P>This now has undefined behavior because the type of <TT>*aptr</TT> is
&#8220;pointer to <TT>const int</TT>,&#8221; which is different from the
cv-unqualified array element type, &#8220;pointer
to <TT>int</TT>.&#8221;</P>

<P>See also 
<A HREF="330.html">issue 330</A>.</P>

<P><B>Proposed Resolution (July, 2014):</B></P>

<P>Change 7.6.6 [<A href="https://wg21.link/expr.add#7">expr.add</A>] paragraph 7 as follows:</P>

<BLOCKQUOTE>

For addition or subtraction, if the expressions <TT>P</TT>
or <TT>Q</TT> have type &#8220;pointer
to <I>cv</I> <TT>T</TT>&#8221;, where <TT>T</TT> <DEL>is
different from the cv-unqualified</DEL> <INS>and the</INS>
array element type <INS>are not similar
(7.3.6 [<A href="https://wg21.link/conv.qual">conv.qual</A>])</INS>, the behavior is
undefined. [<I>Note:</I> In particular, a pointer to a base
class cannot be used for pointer arithmetic when the array
contains objects of a derived class type. &#8212;<I>end
note</I>]

</BLOCKQUOTE>

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