dolfin team mailing list archive

[Jed Brown <jed@xxxxxxxx>]

Anders Logg wrote:
>>>     I've narrowed one of the problems down to the following simple
>>>     example. Say that a class C inherits from both A and B (which both
>>>     have member variables - this is important). Then I would expect an
>>>     object c of class C to have the same address/pointer as seen from all
>>>     three classes.
>> It could not possibly work this way because B must be able to function
>> correctly with no knowledge of A and C.
> 
> I don't understand why that would not work. It looks to me like it's
> just some trick to simplify the cast from a C to a B, and that cast is
> handled by an offset in the struct.

Static casts are eliminated at compile time, they can't allocate memory
to make copies of the data structure.  The definition of a struct
completely determines it's layout in memory.  The definition

class B
{
public:
 B() { cout << "B pointer: " << this << endl; }
 int bvar;
};

requires a particular layout, specifically sizeof B == sizeof bvar and
offsetof(B,bvar) == 0.  If you give me a pointer to one of these things,
I have to be able to use it this way.  The same applies to A, therefore
it is not possible for (void*)(A*)&c == (void*)(B*)&c.

Jed

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