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From: leo@atcmp.nl (Leo  Willems)
Newsgroups: comp.lang.c++
Subject: Virtual table problem
Keywords: virtual pointer table
Message-ID: <527@atcmpe.atcmp.nl>
Date: 22 Jun 89 12:27:22 GMT
Organization: AT Computing, Nijmegen, The Netherlands
Lines: 80


I recently put a question on the net, which, i think, did not
reach many places, or the right readers.

(When re-reading this example i discovered a bug of which i'm not sure
i corrected it in my initial posting, to be sure i marked the line in this
posting. Sorry for that)

Since then, i found that my problem which (still) exists for our C++ compiler 
(a version of cfront) does compile correct under G++.
So here it is again, if it's a stupid question, please e-flame anyway so 
i now this message got out.

I recently made a dispatch function which calls a memberfunction from
some class B.  This code is shown below

	class B{
	public:
		void put() { puts("BBBBBB"); }
	};
	
	typedef void (B::*PF)();
	
	void disp(B* bp, PF f)
	{
		(bp->*f)();
	}
	main()
	{
		B ab;
	
		disp(&ab, &B::put);
	}

That worked quit well.

After that i extended the B class with a virtual member put().

	class B{
	public:
		virtual void put() { puts("BBBBBB"); }	// BUG: the previous
							// version did not
							// have the word
							// virtual
	};

Our C++ compiler (1.2E) did not complain, but our C compiler did.
The  C code generated was somewhat complex, using a ternary operator.

Sorry, to put this in, but explains the error.
void disp (_au0_bp , _au0_f )
struct B *_au0_bp ;
PF _au0_f ;
{ 
(*(((void (*)(/* struct B*  */))(((((unsigned int )_au0_f ))& (~ 127))? _au0_f : (_au0_bp -> 
_B__vptr [(((unsigned int )_au0_f ))- 1])))))( _au0_bp ) ;
}

Our C compiler is complaining that the =false= part of the ternary operator
is of the wrong pointer type, which is true i think:

	true part: _au0_f  is of type PF.
	false part: this gives  a pointer of type  int (*)();
			(it is one of the elements of the vtable from B:
			implemented as:  int (**_B__vptr)();  )

If i patch the generated code witch a cast (PF) in front of the false part
everything works fine :-(

Is there any chance that my solution (without a cast!) should work?
Or is this kind of solution not supposed to work for classes with virtuals? (it
works fine in G++)

Thanks, any reaction is welcome.

Leo Willems			Internet: leo@atcmp.nl
AT Computing			UUCP:     mcvax!hp4nl!kunivv1!atcmpe!leo
P. O. Box 1428				
6501 BK  Nijmegen
The Netherlands