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From: jss@hector.UUCP (Jerry Schwarz)
Newsgroups: comp.lang.c++
Subject: Re: More Questions on operator=
Keywords: inheritance and assignment, default behaviour of assignment
Message-ID: <11549@ulysses.homer.nj.att.com>
Date: 18 May 89 19:12:01 GMT
References: <925@ethz.UUCP>
Sender: netnews@ulysses.homer.nj.att.com
Reply-To: jss@hector.UUCP (Jerry Schwarz)
Organization: AT&T Bell Laboratories
Lines: 57

In article <925@ethz.UUCP> marti@ethz.UUCP (Robert Marti) writes:
>
>
>On a related issue, what is the approved way to get the default
>behaviour of assignment (ie., bitwise copying) INSIDE the definition
>of a user-defined operator=?  

No. And in 2.0 the default behavior never copies the _vptr. The _vptr
is associated with an object when the object is allocated and cannot
be changed. Copying the _vptr "breaks" the C++ type system.

Consider something like

	class B {
		virtual void f() { }
		} ;

	class D : class B 
		int x ;
		void  f() { x = 99 ; }
		} ;

	void g() {
		B b ;
		D d ;
		b = d ;  // legal because D is derived from B.
		b.f() ;
			// If the vptr has been copied then this
			// call will be nonense, b has no x member
			// for f to store 99 in.
		} 

>(I need to do this to make sure that
>an invisible _vptr field is alway copied along with all data members.)
>::operator=(*this, another) does not work (why?), so I currently use
>bcopy((char*)&another, (char*)this, sizeof(Object)) instead, e.g.
>

bcopy (or the ANSI equivalent memcpy) is the appropriate way to do
this operation.  Just be sure you know what you're doing when you
circumvent the type system this way.


>Object& operator=(Object& another)
>{
>  // do some stuff (e.g., manipulating reference counts)
>  bcopy((char*)&another, (char*)this, sizeof(Object));  // copy EVERYTHING
>  // do some more stuff
>  return *this;
>}

This example doesn't look like an appropriate use. It is subject to
the same "bug" as my earlier example. What if the "another" was
allocated as a D (where D is a class derived from Object)?

Jerry Schwarz
AT&T Bell Labs, Murray Hill