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From: mjv@objects.mv.com (Michael J. Vilot)
Newsgroups: comp.lang.c++
Subject: Re: Invoking Member Functions Via Pointers
Message-ID: <1001@zinn.MV.COM>
Date: 10 Nov 90 04:11:03 GMT
Sender: nuucp@zinn.MV.COM
Lines: 49

Anne Wilson asks for help:
> 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.

class Base_class { ... };

class Derived_class : public Base_class {
	void new_function();
};

typedef void (*v_fn_ptr) (void *);
!! This is declaring a type named `v_fn_ptr' which points to a _non_ member
!! function.

class Container_class {
	...
	void apply (v_fn_ptr);
!! This declares a function taking a non-member function pointer
};

c.apply (&Derived_class::new_function);		// compiles, but crashes
!! This should fail at compile time: it is placing a pointer of type
!! `void (Derived_class::*)(void)' into a pointer of type `void (*)(void*)'

!! It would make more sense to declare the type of `v_fn_ptr' as:
	typedef void (Base_class::* v_fn_ptr)();

> 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 ...)
>
>		typedef void (*PROC) (void*, int);
>
It's a non-portable approach, at best.  The example you cite, from ``The C++
Programming Language'' Section 5.4.5, has a footnote in the version of the book
I have (second printing, March 1986):
	``Later versions of C++ support a concept of pointer to member; cl::*
	means "pointer to member of cl".  For example,
		typedef void (cl::*PROC)(int)''

Note the difference in type between this declaration, and the one above.

Hope this helps,

--
Mike Vilot,  ObjectWare Inc, Nashua NH
mjv@objects.mv.com  (UUCP:  ...!decvax!zinn!objects!mjv)