From:	"A.Appleyard" <A DOT APPLEYARD AT fs2 DOT mt DOT umist DOT ac DOT uk>
To:	DJGPP AT SUN DOT SOE DOT CLARKSON DOT EDU
Date:	Wed, 26 Jul 1995 11:16:45 BST
Subject:	class and subclass funny with operator=

  This test program:-
class ground{public: double x,y;
    operator= (ground&g){x =g.x; y =g.y;};
    operator+=(ground&g){x+=g.x; y+=g.y;};};
class air : public ground {public: double z;};
main(){ground G; air A; double n;
    n=A.x; n=A.y; n=A.z;
    A=A;
    A=G;
    G=A;
    G=G;
    A+=A;
    A+=G;
    G+=A;
    G+=G;}
  caused this compile error:-
t$.cc: In function `int main()':
t$.cc:8: bad argument 1 for function
  `class air & air::operator =(class air &)' (type was class ground)

  What's the matter? What's special about `='? I thought that the idea of
subclasses was that e.g. an `air' value could be used in every way that a
`ground' value could. What then is special about `=', so it faulted line 8
(`A=G;') but not line 12 (`A+=G;')? OK, in that case, `A.z' is not given a
value; but perhaps I don't want to give it a value.

  If I declare a
    `class zxcvbnm{........};' or a
    `class zxcvbnm: public qwerty{.......};',
  when and when not is a
    `zxcvbnm::operator=(zxcvbnm&x){.......... /* binary copy */};'
  thereby automatically invisibly declared?

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