typecasting with virtual functions
c++, casting, virtual
Solution
You're seeing a different value for your pointer because the new virtual function is causing the injection of a vtable pointer into your object. VC++ is putting the vtable pointer at the beginning of the object (which is typical, but purely an internal detail).
Let's add a new field to A so that it's easier to explain.
class A {
public:
int a;
};
// other classes unchanged
Now, in memory, your `pA` and `A` look something like this:
pA --> | a | 0x0000004
Once you add B and C into the mix, you end up with this:
pC --> | vtable | 0x0000000
pA --> | a | 0x0000004
| i | 0x0000008
| c | 0x000000C
As you can see, `pA` is pointing to the data after the vtable, because it doesn't know anything about the vtable or how to use it, or even that it's there. `pC` does know about the vtable, so it points directly to the table, which simplifies its use.
Problem
In the code below, pC == pA: ``` class A { }; class B : public A { public: int i; }; class C : public B { public: char c; }; int main() { C* pC = new C; A* pA = (A*)pC; return 0; } ``` But when I add a pure virtual function to B and implement it in C, pA != pC: ``` class A { }; class B : public A { public: int i; virtual void Func() = 0; }; class C : public B { public: char c; void Func() {} }; int main() { C* pC = new C; A* pA = (A*)pC; return 0; } ``` Why is pA not equal to pC in this case? Don't they both still point to the same "C" object in memory?