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From: wilson@brillig.cs.umd.edu (Anne Wilson)
Newsgroups: comp.lang.c++
Subject: Invoking Member Functions Via Pointers
Message-ID: <27515@mimsy.umd.edu>
Date: 7 Nov 90 17:13:48 GMT
Sender: news@mimsy.umd.edu
Reply-To: wilson@brillig.cs.umd.edu (Anne Wilson)
Organization: U of Maryland, Dept. of Computer Science, Coll. Pk., MD 20742
Lines: 71

(I tried to post this yesterday, and it has not yet appeared.  My
apologies if this appears twice. -aw)

AUGHH!

I am having a devil of a time trying to invoke a pointer to member function
through an object which is a base class object for the derived class to
which the member function belongs.  The whole thing is so compliciated,
I have tried to "paraphrase" an example:

class Base_class {
}

class Derived_class : public class {
	void new_function;
}

typedef void (*v_fn_ptr) (void *);

class Container_class {
	Base_class* *list;	// array of pointers to Base_class elements
	Container_class (int obj_count, ... );
	void apply (v_fn_ptr);	// apply function to each element in list
}

Derived_class D1, D2;

Container_class c (2, &D1, &D2);		// create c

// Idea is to apply new_function to both D1 and D2
c.apply (&Derived_class::new_function);		// compiles, but crashes
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

I have running code which does something very similar and works correctly,
the only difference being that the apply function is called on Base_class
objects rather than Derived_class objects.  So I suspect the binding between
the Derived_class member function and a list item, which is also of type 
Derived_class, is not happening properly.

I have been trying the trick of letting the function appear like any old function
and passing it the address of the Derived_class object as its first argument,
but to no avail.  I typed in the following example from Straustrup, and (guess 
what ...) it ran fine.

#include <stream.h>

class cl
{
public:
	char* val;
	void print (int x) { cout << val << " " << x << nl;};
	cl (char *v) {val = v;}
};

typedef void (*PROC) (void*, int);

main () {
	cl z1 ("z1");
	cl z2 ("z2");
	PROC pf1 = PROC (&z1.print);
	PROC pf2 = PROC (&z2.print);
	z1.print(1);
	(*pf1) (&z1,2);
	z2.print(3);
	(*pf2) (&z2,4);
}

Can anyone help with this??  I am running g++ V 1.37.1 on a VAX.
I would be **very** grateful for any suggestions!

Anne Wilson