This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 116a. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2024-12-19

729. Qualification conversions and handlers of reference-to-pointer type

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

Given the following example:

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

can f() return 2? That is, does an int* exception object match a const int*& handler?

According to 14.4 [except.handle] paragraph 3, it does not:

A handler is a match for an exception object of type E if

• The handler is of type cv T or cv T& and E and T are the same type (ignoring the top-level cv-qualifiers), or

• the handler is of type cv T or cv T& and T is an unambiguous public base class of E, or

• the handler is of type cv1 T* cv2 and E is a pointer type that can be converted to the type of the handler by either or both of

  • a standard pointer conversion (7.3.12 [conv.ptr]) not involving conversions to pointers to private or protected or ambiguous classes

  • a qualification conversion

• the handler is a pointer or pointer to member type and E is std::nullptr_t.

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

    int* p;
    const int*& r = p;

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

However, implementation practice seems to be in the other direction; both EDG and g++ do match an int* with a const int*&, 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?

(See also issue 388.)

Notes from the October, 2009 meeting:

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

Proposed resolution (February, 2010):

This issue is resolved by the resolution of issue 388.