TITLE: Casting pointer to member functions


PROBLEM: Randy Chapman <chapman@u.washington.edu>

I am trying to get some old (1991) code working, and it appears that
current versions of C++ break it :(

The basic juist of the problem is that I have a pointer type that is
a pointer to a member function of class A.  I want to assign to
that variable (a member of class A, via a SetCallback()),
a member function pointer of class B (same paramters, etc).
Compiler (g++) gives me a casting error:

> cat foo.C

class A;

typedef void (A::*Aptr)();

class A {
  Aptr foo;
public:
        void SetFoo(char *event, Aptr fooP) { foo = fooP; }
};

class B: protected A {
        void junk();
        B() { SetFoo("SomeOddThing",junk); }
};

> gcc foo.C
foo.C: In method `B::B()':
foo.C:14: type `B' is not a base type for type `A'
foo.C:14:    in pointer to member function conversion


RESPONSE: clamage@Eng.Sun.COM (Steve Clamage), 7 Jun 95

First, you have a syntax error. The call to SetFoo should look like
        B() { SetFoo("SomeOddThing", &B::junk); }

Second, casting among pointer-to-member-function works backwards from
ordinary casts. Normally, you can cast a derived to a base, since a
derived is a base (loosely speaking).

But with pointer-to-member-function, the reverse is the case. A
pointer to a Derived member function cannot be used where a pointer
to Base member function is expected. The Base::*ptr will be used
in conjunction with Base object, and a member of class Derived
will expect to operate on a Derived object. In general, this will
lead to disaster. (E.g., the Derived member function stores into data
which is part of a Derived object, but isn't part of a Base object.)
That's why it isn't allowed.