Path: utzoo!attcan!uunet!cs.utexas.edu!tut.cis.ohio-state.edu!ucbvax!ulysses!hector!jss
From: jss@hector.UUCP (Jerry Schwarz)
Newsgroups: comp.lang.c++
Subject: Re: "Inheritance" of operator=()
Keywords: user error or cfront error?
Message-ID: <11543@ulysses.homer.nj.att.com>
Date: 18 May 89 17:28:27 GMT
References: <863@ethz.UUCP> <11519@ulysses.homer.nj.att.com> <933@ethz.UUCP>
Sender: netnews@ulysses.homer.nj.att.com
Reply-To: jss@hector.UUCP (Jerry Schwarz)
Organization: AT&T Bell Laboratories
Lines: 51

In article <933@ethz.UUCP> marti@ethz.UUCP (Robert Marti) writes:
>In article <11519@ulysses.homer.nj.att.com>, jss@hector.UUCP (Jerry Schwarz)
>writes:
>
>I am glad to hear that the semantics of (user-defined) assignment has
>changed in 2.0, although I am not quite sure what you mean when you say
>that it is defined to be "recursive".  Could you elaborate on that?
>

The semantics of user-defined assignment has not changed.  It remains
that you call the user-defined function.  Its the semantics of
builtin assignment on classes that has changed.

Assignment for a class K means to assign each "part" of K according
(recursively) to the appropriate definition of assignment for that
part.  A part is either a member declared as a member of K or
the parts of K that result from inheritance. 

For example.

        class K {
        public:
                K& operator=(const K&) ;
        } ;

        class B {
        private:
                int b ;
        protected:
                virtual void f() ;
                B& operator=(const B&) ;
        } ;

        class D : public B {
                K k ;
                int x ;
                } 

        D d1, d2 ;

	// The "parts" of d1 are d1.k, d1.x (B&)d1

        d1 = d2 ; 
                // copy d2.x to d1.x call K::operator= to "assign"
                // d2.k to d1.k.  And call B::operator= to "assign"
                // the base part of d2 to the base part of d1.
		// The vtable pointer is not copied.


Jerry Schwarz
AT&T Bell Labs, Murray Hill