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From: shopiro@alice.UUCP (Jonathan Shopiro)
Newsgroups: comp.lang.c++
Subject: Re: pointer to virtual function; how to use it
Summary: Use pointers to members
Keywords: virtual function pointer
Message-ID: <9228@alice.UUCP>
Date: 22 Apr 89 22:53:41 GMT
References: <856@garya.Solbourne.COM>
Organization: AT&T Bell Laboratories, Murray Hill NJ
Lines: 62

In article <856@garya.Solbourne.COM>, garya@Solbourne.COM (Gary Aitken) writes:
(with condensation and a few syntax errors fixed)
> 
> Consider the following:
> 
> class A {
>  public:
>   virtual void f() ;
	void g();	// not virtual
> } ;
> 
> class B : public A {
>  public:
>           void f() ;
	void g();	// not virtual
> } ;
> 
> 	A*	ap = new A;
> 	B*	bp = new B;
> 	A*	objp  = bp;   // no cast necessary
> 
> It is not unusual to need to take the address of a function, and pass it
> as an argument.  The following possibilities arise:
> 
> 	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

For virtual functions, only dynamic (time of call) binding is supported.
Do it this way:

	typedef void  A::MEMF();  // MEMF is the type of a member function of A
	MEMF*	fp = &A::f;

(There is a way to do it without a typedef, but this is easier).
Then the calls go like this:

	(ap->*fp)()	// A::f()
	(bp->*fp)()	// B::f()
	(objp->*fp)()	// B::f()
	fp = &A::g;	// note the variable can refer
			// to a virtual or non-virtual function
	(ap->*fp)()	// A::g()
	(bp->*fp)()	// A::g()
	(objp->*fp)()	// A::g()

There is no way to apply A::f() to a B object (but you're not
supposed to want to :-)).

> 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?

void dispatch(A *ap, MEMF* fp)	// this is how
{
	(ap->*fp)();  // This is what I really want...
			// you got it
}
-- 
		Jonathan Shopiro
		AT&T Bell Laboratories, Warren, NJ  07060-0908
		research!shopiro   (201) 580-4229