Path: utzoo!utgpu!news-server.csri.toronto.edu!bonnie.concordia.ca!uunet!microsoft!jimad
From: jimad@microsoft.UUCP (Jim ADCOCK)
Newsgroups: comp.std.c++
Subject: Re: backward virtual function call
Message-ID: <72185@microsoft.UUCP>
Date: 6 May 91 19:12:04 GMT
References: <1991Apr30.024010.4331@csi.uottawa.ca>
Reply-To: jimad@microsoft.UUCP (Jim ADCOCK)
Organization: Microsoft Corp., Redmond WA
Lines: 61

In article <1991Apr30.024010.4331@csi.uottawa.ca> hitz@sim5.csi.uottawa.ca (Martin Hitz) writes:
|I wonder if the following is guaranteed to print 1:
|
|#include <stream.h>
|struct X {
|	virtual f() { return 1; }
|};
|struct Y : X {
|	f() { return 2; }
|};
|
|main()
|{
|	X x;
|	Y * y = (Y *) &x;
|	cout << y->f();
|}
|
|The ARM explains the usual case, that a call of f() for an object of
|class Y invokes Y::f(), even if it is called via a pointer to X,
|but I couldn't find anything about the above case. Zortech and g++
|both yield 1 for this example. Is this considered to be the standard
|behaviour?

What would a C++ compiler then do with the following?
[All that I'm aware of act in an ill-defined manner, which is expected
since the ARM prohibits such erroneous down-casts.  Your example just
happens to work on most C++ compilers]

extern "C" int printf(const char*, ...);

class A
{
	private: virtual void illegalToAccess() 
	{ printf("erroneous access of A private method\n"); }
	public: virtual void print() { printf("A\n"); }
};

class B
{
	public: virtual void print() { printf("B\n"); }
	private: virtual void illegalToAccess() 
	{ printf("erroneous access of B private method\n"); }
};

class AB: public A, public B
{
	public: virtual void print() { printf("AB\n"); }
};

main()
{
	A aA;
	B aB;
	A* a = &aA;
	((AB*)a)->print();
	B* b = &aB;
	((AB*)b)->print();

	return 0;
}