finding offset of subclasses

[Petar#M]

Dear all,

Is there any way to derive the offset of a subclass without instantiating
the class (in other words none of the *_cast operators can be used).

So, e.g.:

class A {};
class B {};
class C : public A, public B {};

I want to know the offset of B within C.

Thanks,
Petar

[Jack Klein]

Re: finding offset of subclasses

On 9 Feb 2004 17:50:48 -0800, petar@scs.cs.ny u.edu (Petar#M) wrote in
comp.lang.c++:
[color=blue]
> Dear all,
>
> Is there any way to derive the offset of a subclass without instantiating
> the class (in other words none of the *_cast operators can be used).
>
> So, e.g.:
>
> class A {};
> class B {};
> class C : public A, public B {};
>
> I want to know the offset of B within C.
>
> Thanks,
> Petar[/color]

Why do you want to know this? Perhaps you need to rethink your
design.

In any case, if the class is a POD class, which means respectively
that all its base classes are POD classes, the standard offsetof()
macro from <stddef.h> or <cstddef> can be used.

If it's not a POD class, there is no standard C++ way to do this.

--
Jack Klein
Home: http://JK-Technology.Com
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[Pete Becker]

Re: finding offset of subclasses

Jack Klein wrote:[color=blue]
>
> If it's not a POD class, there is no standard C++ way to do this.
>[/color]

And there may not be a single value: virtual bases can be at different
offsets in the same derived type, depending on the most derived type.

--

Pete Becker
Dinkumware, Ltd. (http://www.dinkumware.com)

[Victor Bazarov]

Re: finding offset of subclasses

"Pete Becker" <petebecker@acm .org> wrote...[color=blue]
> Jack Klein wrote:[color=green]
> >
> > If it's not a POD class, there is no standard C++ way to do this.
> >[/color]
>
> And there may not be a single value: virtual bases can be at different
> offsets in the same derived type, depending on the most derived type.[/color]

I am not sure I understand that. So, if 'B' is a stand-alone object,
it can have a different layout than if it's a sub-object of another
object, when virtual inheritance is involved?

Thanks.

Victor

[Rob Williscroft]

Re: finding offset of subclasses

Victor Bazarov wrote in news:f26Wb.2655 95$I06.2850867@ attbi_s01:
[color=blue]
> "Pete Becker" <petebecker@acm .org> wrote...[color=green]
>> Jack Klein wrote:[color=darkred]
>> >
>> > If it's not a POD class, there is no standard C++ way to do this.
>> >[/color]
>>
>> And there may not be a single value: virtual bases can be at different
>> offsets in the same derived type, depending on the most derived type.[/color]
>
> I am not sure I understand that. So, if 'B' is a stand-alone object,
> it can have a different layout than if it's a sub-object of another
> object, when virtual inheritance is involved?
>[/color]

yes,

struct B_base { int bb; };

struct B : virtual B_base
{
int data;
};

actual layout of b is say:

struct __like_B
{
__implementatio n_defined __ptr_to_B_base ;
int data;
B_base __actual_B_base ; /* actual subobject */
};

struct C: B_base, B
{
};

actual layout of C:

struct __like_C
{
B_base __actual_B_base ; /* C & B subobject */
__implementatio n_defined __ptr_to_B_base ; /* B */
int data; /* B */
};

__ptr_to_B_base need to be initialized by B or C's ctor.

int get_bb( B *bp )
{
return bp->bb;

gets translated to something like:

return bp->__ptr_to_B_bas e->bb;
}

Rob.
--

http://www.victim-prime.dsl.pipex.com/

[Pete Becker]

Re: finding offset of subclasses

Victor Bazarov wrote:[color=blue]
>
> "Pete Becker" <petebecker@acm .org> wrote...[color=green]
> > Jack Klein wrote:[color=darkred]
> > >
> > > If it's not a POD class, there is no standard C++ way to do this.
> > >[/color]
> >
> > And there may not be a single value: virtual bases can be at different
> > offsets in the same derived type, depending on the most derived type.[/color]
>
> I am not sure I understand that. So, if 'B' is a stand-alone object,
> it can have a different layout than if it's a sub-object of another
> object, when virtual inheritance is involved?
>[/color]

Yup. Let's look at two classes, B and C, each with a virtual base A:

+--------+--------+
| B data | A data |
+--------+--------+

+--------+--------+
| C data | A data |
+--------+--------+

Now derived a class D from both B and C. One possible layout is:

+--------+--------+--------+--------+
| D data | B data | A data | C data |
+--------+--------+--------+--------+

Another is:

+--------+--------+--------+--------+
| D data | B data | C data | A data |
+--------+--------+--------+--------+

In the first, the offset of the A subobject from the C subobject is
different from its offset in a standalone C object. In the second it's
the offset from the B subobject that's different.

--

Pete Becker
Dinkumware, Ltd. (http://www.dinkumware.com)