Re: About the address offset when assign address of a derived class to a Base class pointer.

junyangzou <>
Thu, 29 Aug 2013 03:02:55 -0700 (PDT)
On Thursday, August 29, 2013 2:30:21 PM UTC+8, Paavo Helde wrote:

junyangzou <> wrote in

On Thursday, August 29, 2013 11:27:34 AM UTC+8, Tobias M=FCller wrote:


junyangzou <> wrote:




Say we have two base class Base1, Base2 and a Derived : public


Base1, p


ublic Base2.




When we assign the address of a Derived Object to a Base2 Pointer.






address need to be adjust to the beginning of the Base2:








Base2 *pBase2;




Derived d;




pBase2 = &d;








In my machine, the value of pBase2 and &d is 0079FAB0 0079FAB8




respectively, indicating that sizeof( Base1 ) is 8 bytes.








But when and where is the offset added? This can not be done in








time. Because though the example above is quite straightforward.






times we may write code like:




pBase2 = Factory.get();








We known the real type of the returned object only in rumtime. So








can explain when and where is the offset added? Thanks!








The "real" type of the object actually doesn't matter. What matters


is th






(statically known) type of the return value of Factory.get(). This


type h






to be a common base class of all possible "real" types and the






adjustment from the "real" type to that common base type happens






the Factory.get() method, where the "real" type is statically known.












This make sense. So if the Factory method returns an pointer to


Derived( Derived* ) in which the real object is an instance of a


derived class from Derived, say Derived2. The pointer may be( if


needed ) offset twice?

Yes, if there is again multiple derivation involved. E.g.
class Base1 {...};
class Base2 {...};
class Base3 {...};
class Derived1: public Base1, public Base2 {...};
class Derived2: public Base3, public Derived1 {...};
if we now have:
Derived2* x = new Derived2();
Derived1* y = x;
Base2* z = y;
then a typical compiler would translate this into something approximately=


like this (space optimizations might change this a bit):
// pseudocode:
x = ...
y = x + sizeof(Base3);
z = y + sizeof(Base1);
Note that after the adjustment has been done, the rest of the code
working with the base class pointer does not have any a priori knowledge=


that the complete object is Derived2. If this becomes important, one must=


add relevant virtual functions to classes so that all the subobjects
would have vtable pointers where the code can dig up this information at=


run-time, for example when processing things like
Derived2* w = dynamic_cast<Derived2*>(z);
delete z;

Yes, now I get it. I am reading the implementation details of vtables, and =
see in some case the virtual function need to adjust `this` pointer to the =
original address. That is why am I wandering when is it offset from the ori=
ginal, hah. Thank you!

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