Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!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: <11519@ulysses.homer.nj.att.com>
Date: 11 May 89 15:59:03 GMT
References: <863@ethz.UUCP>
Sender: netnews@ulysses.homer.nj.att.com
Reply-To: jss@hector.UUCP (Jerry Schwarz)
Organization: AT&T Bell Laboratories
Lines: 40

In article <863@ethz.UUCP> marti@ethz.UUCP (Robert Marti) asks
some questions about inheritance of assignment.

Assignment is special because it is defined for all classes, (thus it
never needs to be inherited).

But the meaning of assignment has changed between 1.2 and 2.0. In 1.2
the synthesized assignment operator was always "bitwise copy".  This
was seen to be wrong and in 2.0 assignment is defined to be
"recursive".

The example is something like

struct B {
	// some data members
	B& operator=(B&) ;
} ; 

struct D : public B {
	// some more data members
} ;

void f()
{
	D d1, d2 ;
	d1 = d2 ;	// what should happen here?
	}

Note that it doesn't make much sense to just "inherit" assignment
because that would mean that the added members of D wouldn't
participate in assignment of D.  Since 1.2 didn't implement a
"recursive" assignment, and a bitwise copy would clearly be wrong. It
insisted that if assignment was defined in the base class that it
also be defined in the derived class.

In 2.0 the synthesized assignment operation usually "does the right
thing" so this restriction is not imposed.

Jerry Schwarz
AT&T Bell Labs, Murray Hill