Double abstract class Inheritance concern.

[Tiago Stein <tiago at lisha dot ufsc dot br>]

Hy! I have  question concerning inheritance of empty classes.

Supose that I have:

Class A{

};

Class B: public A{
  unsigned int foo1;
};

because of C++ constraints on object sizes.. sizeof(A) is  4 (I am in a
32bit PC).


In class B, due to the attribute.. this extra size form A is supressed. So
sizeof B= 4 too..


Until here everything is OK, but... given:


class A{
};

class B :public A{
};


class C: public A{
 int g;
};

class D: public B, public C
{
  unsigned int foo;
};


Why sizeof class D is 12? Should not be 8? Why the extra size from the
empty classes is not eliminated this time? Is this due to any rules in the
standard? I passed hours looking for it but found nothing that could
obligate so.  This behavior is causing some real nasty side-effects in our
operating system due to memory restrictions in our embedded targets.


So, can someone explain me why of this? Maybe I am leaving something wrong
pass at my design if there is any rule that dictates this behavior.

Thanks for any help.