Re: New vtable ABI (was Re: Strange behaviour in C++...)

["Martin v. Loewis" <martin@mira.isdn.cs.tu-berlin.de>]

> I confess to not being able to follow more than the very basic idea;
> I guess I'm rather rusty on g++ vtable management.  However, it does
> not seem like this will do much for Java, since the big deal is
> how to handle the offsets, while for Java object the offsets are
> always zero.

Is that mandatory? The naïve way of implementing interfaces is to take
the usual C++ approach: interfaces are virtual classes with pure
virtual methods. So

interface FooBar{
  public void x();
}
class Foo extends Bar implements FooBar{public void x(){...}}

becomes

class FooBar{
  virtual void x()=0;
};

class Foo:Bar, virtual FooBar{
  void x(){...}
};

Does that meet all requirements? If so, you will have adjustments when
you have a FooBar pointer and invoke x. The reason is that Foo has two
embedded vtbl pointers: the one of Foo, and the one of FooBar. A
FooBar pointer is represented as a pointer to the location of the
FooBar vtbl pointer inside the Foo object.

In the current ABI proposal (by Jason), the FooBar vtbl is represented
as

-1: RTTI (at negative offset for COM compatibility)
0:  x

Inside Foo, Foo::x has two entry points:

x_for_FooBar: this = this + this->_vtbl[1] ;fallthrough
x__3Foo:      ...

So, the FooBar-in-Foo vtbl has a different layout:

-1: RTTI
0:  x_for_FooBar
1:  Adjust FooBar to Foo (say, -16)

An interface call would then be a normal virtual call: Using the
pointer you have, retrieve the vtbl (at offset 0), retrieve the method
pointer, and call it. The adjustment to the full object is made at the
target.

Would that work for Java?

Martin