Hi!
This is not a newbie asking what a pointer or object is. I''m just wondering about how the compiler solves a specific problem of memory layout in classes
This could be a followup of the thread
When does a class have a vtable ?
If I derive some classes from a common base class like this:
class Base {
public:
virtual void virtualMethod(void) = 0;
void normalMethod(void) { cout << "normal"; }
private:
int variableInBase;
};
class Derived1 : public Base {
public:
virtual void virtualMethod(void) { cout << "virtual1"; }
};
class Derived2 : public Base {
public:
virtual void virtualMethod(void) { cout << "virtual2"; }
private:
int variableInDerived;
}
So now I''ve got the following function (which could be sealed in a precompiled library):
void callVirtualMethod(Base *pClass) {
pClass->virtualMethod();
}
If I call this method with a pointer to either Derived1 or Derived2, the compiler would see that the function is expecting a Base * pointer (from the function prototype of course)
Now what exactly is passed to the function ?
- A pointer to the vtbl of the derived class wouldn''t allow the methods to access the data of the class instance.
- A pointer to the data of the class wouldn''t suffice to access the vtbl of the class.
So, how is the class instance data connected with the vtbl of the class ?
In the thread referenced above, the statement was made that there''s only one vtbl for all instances of a class. If the compiler doesn''t pass two pointers for each class pointer argument, one for the instance data and once for the vtable, then the instance data requires a pointer to the vtbl at least.
However, remember that it''s possible (not good practice) to do memset(this, 0, sizeof(*this)). This would overwrite the vtbl pointer with 0 if it wasn''t located before the actual class adress, maybe ?
-Markus-
P.S.
Of course, a lot of this is compiler specific. And such knowledge of the internal workings of a compiler implementation should under no circumstances be relied upon.
I''m just curious how a compiler would solve those things
