TITLE: What goes on in casting pointers?

PROBLEM: alan@saturn.cs.swin.oz.au (Alan Christiansen)

[ ... ]

class A *a;
class F *f;
f = a;   if ( (void *)f != (void *)a ) sometimes these are not equal!


RESPONSE: cs931003@ariel.yorku.ca (ANETA COSTIN), 27 Aug 93
	  cs931003, aka Jonathan Shekter, from sunny Toronto, Canada

	Yes, this is true. I have seen this. It occurs when you are
doing multiple inheritance, I have not seen it otherwise. Let's say you
have the following class structure:

	A
	|  
	B   C
	 \ /
   	  E

	Now, say you have an object of type E. What you will actually
have in memory is the data of type B followed by the data of type C (or
reversed, depending on the order inwhich you declared them on the first
line of you class declaration for class E). Say you have a pointer to an
E. This pointer will actually point to the first data member of B, since
B is first _in_memory_. Now, say you have a function that expects an
argument of type C *. When you cast an E * to a C *, i.e. (C*)&myE, the
compiler will actually add sizeof(B) bytes to the pointer, so that it is
now pointing at the first member of the C data contained in E.

	This can make some pretty interesting casts. For example, say
you want to convert a C pointer, which you know is actually an E ptr,
into an A ptr. (Yes, stuff like this actually comes up!). You must go:

	(A *)(E *)Cptr, 

	and not

	(A *)Cptr,

	as this will not do the correct pointer subtraction required to
convert a C* into an E*. Hence, some wierd things, which you only really
see in multiple inheritance.

	Now, as far as your question, if you don't have multiple
inheritance, casting should work fine. If you do have multiple
inheritance, make sure you cast through all the steps needed so the
compiler can keep up with you and do the neccessary pointer adjustments.

	If you are really worried, or you have problems, then a "safe"
way is to use virtual functions which access the data. Or, if each
object is supposed to perform some specific action, you can have one
vertual Do() or Execute() function, which is the same for each type of
object, and you don't even need to know what type of object it actually
is.