Re: vtbl inheritance
On Jan 30, 4:29 pm, Leigh Johnston <le...@i42.co.uk> wrote:
On 30/01/2011 16:14, Serve Laurijssen wrote:
Consider this code:
class RefCounted {
private: long m_nRefCount;
public: virtual ~RefCounted();
};
struct Header {
short us;
int i1;
int i2;
};
struct UnitHeader: public Header {
BYTE filler[sizeof(ULONG) - (sizeof(UnitHeader) & (sizeof(ULONG) - 1))];
};
class CHeader : public UnitHeader, public RefCounted {
};
RefCounted has a virtual destructor, UnitHeader and Header are POD structs.
CHeader inherits from UnitHeader and RefCounted.
Now consider this:
void CHeader::MakeDummy() {
memset((UnitHeader*)this, 0, sizeof(UnitHeader));
}
The 'this' pointer in CHeader is casted to UnitHeader struct and that
memory area set to zero.
But since the class inherits from a class with a virtual destructor Im
not sure this works. How does MSVC(2005) handle the Vtbl when inheriting
from a class with a vtbl?
That should work yes as you are calling memset on a POD base sub-object
(of type UnitHeader).
I don't think it will fail in this particular case, because of
the data types involved and their respective alignment
requirements. But in general, the actual number of bytes
occupied by UnitHeader may be less than sizeof(UnitHeader) when
UnitHeader is a base class, so brutally writing
sizeof(UnitHeader) bytes is not a safe operation. (The "empty
base class optimization" is somewhat miss-named, because it
doesn't apply to only empty base classes.)
In VC++ the vtable pointer will be in the
RefCounted base sub-object so will not be affected by the memset.
Really? I would have expected that it be in front of the
UnitHeader element, with another vptr in the RefCounted
subclass. (The vptr for CHeader has to point to a different
vtable than that of RefCounted.)
--
James Kanze