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

308. Catching exceptions with ambiguous base classes

14.4 [except.handle] paragraph 3 contains the following text:

A handler is a match for a throw-expression with an 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

I propose to alter this text to allow to catch exceptions with ambiguous public base classes by some of the public subobjects. I'm really sure that if someone writes:

    try {
        // ...
    }
    catch (Matherr& m) {
        // ...
    }

    class SomeMatherr : public Matherr { /* */ };
    struct TrickyBase1 : public SomeMatherr { /* */ };
    struct TrickyBase2 : public SomeMatherr { /* */ };
    struct TrickyMatherr : public TrickyBase1, TrickyBase2 { /* */ };

According to the standard TrickyMatherr will leak through the catch (Matherr& m) clause. For example:

    #include <stdio.h>

    struct B {};
    struct B1 : B {};
    struct B2 : B {};
    struct D : B1, B2 {};  // D() has two B() subobjects

    void f() { throw D(); }

    int main()
    {
     try { f(); }
     catch (B& b) { puts("B&"); }  // passed
     catch (D& d) { puts("D&"); }  // really works _after_ B&!!!
    }

Also I see one more possible solution: to forbid objects with ambiguous base classes to be "exceptional objects" (for example Borland C++ goes this way) but it seems to be unnecessary restrictive.

Notes from the 10/01 meeting:

The Core Working Group did not feel this was a significant problem. Catching either of the ambiguous base classes would be surprising, and giving an error on throwing an object that has an ambiguous base class would break existing code.