Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!ncar!boulder!stan!garya
From: garya@Solbourne.COM (Gary Aitken)
Newsgroups: comp.lang.c++
Subject: pointer to virtual function; how to use it
Keywords: virtual function pointer
Message-ID: <856@garya.Solbourne.COM>
Date: 21 Apr 89 22:29:33 GMT
Organization: Solbourne Computer Inc., Longmont, Co.
Lines: 98


Consider the following:

class A : public {
 public:
  virtual void f() ;
} ;

class B::A : public {
 public:
          void f() ;
} ;

It is not unusual to need to take the address of a function, and pass it
as an argument.  The following possibilities arise:

	A	*ap ;
	B	*bp ;
	A	*objp ;
	void (*fp)() ;

	ap = new A ;
	bp = new B ;
	objp = (A*) bp ;

	fp = &A::f ;     // static reference to a known function
	fp = &B::f ;     // static reference to a known function
	fp = &objp->f ;  // dynamic ref to unknown (at compile time) virtual funct

The intent is to get the pointer to the proper function via the virtual
table.  This does, in fact, work with cfront, although you get a warning
telling you to use :: (which won't compile, since it expects a class name
in front of it instead of a pointer to a class object).

Now consider a function which has been passed two arguments, one a pointer
to an object, and the other a pointer to a member function for that object.
How do you invoke the member function?  The only way I know to do it is
by using straight C code, relying on the fact that cfront passes "this" as
the first argument.  But what about other implementations?  Example of
what works in cfront:

//	void dispatch(A *ap, void (*A::fp)())  How would one write this???
	void dispatch(A *ap, void (*fp)(A*))
	{
//		ap->(*fp)() ;  // This is what I really want...
		(*fp)(ap) ;    // This is what works
		return ;
	}

Can anyone help me out on this (ooooooo is that a pun?) ?

Here's a complete program to play with:

=============================  Cut Here  ================================

void printf(...) ;
class A {
 public:
  virtual void f() ;
} ;
void A::f() {printf("A.f\n");}

class B : public A {
 public:
          void f() ;
} ;
void B::f() {printf("B.f\n");}

//void dispatch(A *ap, void (*A::fp)())
void dispatch(A *ap, void (*fp)(A*))
{
//	ap->(*fp)() ;	/* how do we do this???? */
	(*fp)(ap) ;	/* this works for cfront, but what about others?? */
	return ;
}

main()
{
	A	*ap ;
	B	*bp ;
	A	*objp ;
	auto void (*fp)() ;

	ap = new A ;
	bp = new B ;
	objp = (A*) bp ;

	fp = &A::f ;     // static reference to a known function
	fp = &B::f ;     // static reference to a known function
	fp = &objp->f ;  // dynamic ref to unknown (at compile time) virtual function
//	dispatch(objp, (void (*)()) fp) ;
	dispatch(objp, (void (*)(A*)) fp) ;
}
-- 
Gary Aitken

Solbourne Computer Inc.    ARPA: garya@Solbourne.COM
Longmont, CO               UUCP: ...!{boulder,sun}!stan!garya