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From: ark@alice.UUCP (Andrew Koenig)
Newsgroups: comp.lang.c++
Subject: Re: VIRTUAL-NESS LOST  - A C++ mystery involving funcs returning objects not ptrs
Keywords: virtual C++ overloaded operator interpreter
Message-ID: <10588@alice.UUCP>
Date: 17 Mar 90 14:20:32 GMT
References: <33629@genrad.UUCP> <33897@genrad.UUCP> <2887@ohm.sw.mcc.com>
Organization: AT&T Bell Laboratories, Liberty Corner NJ
Lines: 60

In article <2887@ohm.sw.mcc.com>, creemer@ohm.sw.mcc.com (David Zachary Creemer) writes:

> Yes, but, the AT&T 2.0 language reference manual (the one I get with
> my Sun CC compiler) reads,
> "The interpretation of the call of a virtual function depends on the
> type of an object for which it was called,..."

Did my earlier posting on this subject get lost?

This example is a bug in cfront 2.0.
It's fixed in cfront 2.1.

The statement in the manual is correct.  HOWEVER, if you have a function

	void f(Base b) { /* stuff that deals with b */ }

then calling f with a derived object:

	Derived d;
	f(d);

is a request to use the Base(const Base&) constructor to form a copy
of the Base part of d and use that as the argument to f.

Once inside f, the formal parameter is a Base, not a Derived.
If you had said

	void f(Base& b) { /* stuff that deals with b */ }

the situation would be quite different.

> It makes no mention of the pointer-to-object vs. object argument going
> on here. So, as I read it, cfront is behaving exactly as the Language
> Ref. manual says it should., Furthermore, given the following:

> class Base {
>   public:
>     virtual void print() { cout << "base\n"; }
> };

> class Der : public Base {
>   public:
>     virtual void print() { cout << "der\n"; }
> };

> Base& func() { Der *x = new Der; return *x; }

> main() {   func().print(); }

> both g++ (1.37.something) and CC call the derived class' print method.
> I argue that this approach is best.  References are supposed to give
> "pointer like" behavior (call by reference) without having to be
> bothered by pointer dereferencing problems. In any event, it's a
> curious fact which will undoubtedly cause someone a headache at some
> time.

Both g++ and CC are correct this time.
-- 
				--Andrew Koenig
				  ark@europa.att.com