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    CWG Issue 729</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="729"></A><H4>729.
  
Qualification conversions and handlers of reference-to-pointer type
</H4>
<B>Section: </B>14.4&#160; [<A href="https://wg21.link/except.handle">except.handle</A>]
 &#160;&#160;&#160;

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

 <B>Submitter: </B>John Spicer
 &#160;&#160;&#160;

 <B>Date: </B>6 October, 2008<BR>


<P>[Voted into the WP at the February, 2012 meeting;
moved to DR at the October, 2012 meeting.]</P>

<P>Given the following example:</P>

<PRE>
    int f() {
        try { /* ... */ }
        catch(const int*&amp;) {
            return 1;
        }
        catch(int*&amp;) {
            return 2;
        }
        return 3;
    }
</PRE>

<P>can <TT>f()</TT> return <TT>2</TT>?  That is, does an
<TT>int*</TT> exception object match a <TT>const int*&amp;</TT>
handler?</P>

<P>According to 14.4 [<A href="https://wg21.link/except.handle#3">except.handle</A>] paragraph 3, it does not:</P>

<BLOCKQUOTE>

<P>A <I>handler</I> is a match for an exception object of type
<TT>E</TT> if</P>

<UL>
<LI><P>The <I>handler</I> is of type <I>cv</I> <TT>T</TT> or <I>cv</I>
<TT>T&amp;</TT> and <TT>E</TT> and <TT>T</TT> are the same type
(ignoring the top-level <I>cv-qualifier</I>s), or</P></LI>

<LI><P>the <I>handler</I> is of type <I>cv</I> <TT>T</TT> or <I>cv</I>
<TT>T&amp;</TT> and <TT>T</TT> is an unambiguous public base class of
<TT>E</TT>, or</P></LI>

<LI>
<P>the handler is of type <I>cv1</I> <TT>T*</TT> <I>cv2</I> and
<TT>E</TT> is a pointer type that can be converted to the type of the
handler by either or both of</P>

<UL>
<LI><P>a standard pointer conversion (7.3.12 [<A href="https://wg21.link/conv.ptr">conv.ptr</A>]) not
involving conversions to pointers to private or protected or ambiguous
classes</P></LI>

<LI><P>a qualification conversion</P></LI>
</UL>
</LI>

<LI><P>the <I>handler</I> is a pointer or pointer to member type and
<TT>E</TT> is <TT>std::nullptr_t</TT>.</P></LI>

</UL>

</BLOCKQUOTE>

<P>Only the third bullet allows qualification conversions, but
only the first bullet applies to a <I>handler</I> of
reference-to-pointer type.  This is consistent with how other
reference bindings work; for example, the following is ill-formed:</P>

<PRE>
    int* p;
    const int*&amp; r = p;
</PRE>

<P>(The consistency is not complete; the reference binding would be
permitted if <TT>r</TT> had type <TT>const int* const &amp;</TT>, but
a handler of that type would still not match an <TT>int*</TT>
exception object.)</P>

<P>However, implementation practice seems to be in the other
direction; both EDG and g++ do match an <TT>int*</TT> with a <TT>const
int*&amp;</TT>, and the Microsoft compiler issues an error for the
presumed hidden handler in the code above. Should the Standard be
changed to reflect existing practice?</P>

<P>(See also <A HREF="388.html">issue 388</A>.)</P>



<P><B>Notes from the October, 2009 meeting:</B></P>

<P>The CWG agreed that matching the exception object with a handler
should, to the extent possible, mimic ordinary reference binding in
cases like this.</P>

<P><B>Proposed resolution (February, 2010):</B></P>

<P>This issue is resolved by the resolution of <A HREF="388.html">issue 388</A>.</P>

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