Path: utzoo!attcan!uunet!mcvax!cernvax!ethz!marti
From: marti@ethz.UUCP (Robert Marti)
Newsgroups: comp.lang.c++
Subject: More Questions on operator=
Keywords: inheritance and assignment, default behaviour of assignment
Message-ID: <925@ethz.UUCP>
Date: 17 May 89 07:44:10 GMT
Organization: ETH Zuerich
Lines: 59

In article <863@ethz.UUCP>, I posed a question concerning the
inheritance of operator=.  Essentially, cfront 1.2.1 produced the
error message indicated by <<<<< :

class Base { public:
  int data;
  Base& operator=(Base& another) { data = another.data; return *this; }
  void foo() { ++data; }
};

class Derived : public Base { public:
  Derived() { data = 0; }
};

main() {
  Derived d, d1, d2;
  d.foo();
  d = d1;  	// <<<<< error: assignment not defined for class Derived
  d.operator=(d2);
}

Since the C++ bible states on p.265 that for a binary operator @,
x @ y can be interpreted as x.operator@(y), and since member functions
are inherited by a derived class, the error message seemed like a bug
to me.

However, as Clifford Beshers of Columbia kindly pointed out to me,
the bible also says that assignment is not implicitly defined for
objects of a class derived from a class for which operator= has been
defined p.277).  Now, if you accept this restriction (which I have
a very hard time to do, but I guess I don't have much of a choice!),
the above example should actually produce TWO errors instead of one,
because d.operator=(d2) seems "just as wrong" as d = d1, since
d = d1 is interpreted as d.operator=(d1) anyway.

Clarifications on the topic, anyone?  (I do hope the silly restriction
is lifted in 2.0!)


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=?  (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.

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;
}

-- 
Robert Marti                      Phone:      +41 1 256 52 36
Institut fur Informationssysteme
ETH-Zentrum                       CSNET/ARPA: marti%inf.ethz.ch@relay.cs.net
CH-8092 Zurich, Switzerland       UUCP:       ...uunet!mcvax!ethz!marti