How vtable and vptr is working when virtual method is called from non-virtual method

Question

#include <bits/stdc++.h>
using namespace std;
class Base
{
public:
    virtual void print()
    {
        cout << "Base class print function \n";
    }
    void invoke()
    {
        cout << "Base class invoke function \n";
        this -> print(); // case: 1
        print();         // case: 2
        Base::print();   // case: 3
    }
};
 
class Derived: public Base
{
public:
    void print()
    {
        cout << "Derived class print function \n" ;
    }
    void invoke()
    {
        cout << "Derived class invoke function \n";
        this -> print(); 
    }
};
 
int main()
{
    Base *b = new Derived;
    b -> invoke();
    return 0;
}

For the above code I have hard time understanding how vptr and vtable of Derived and Base class is working for comments 1, 2 and 3. Why in case 1 and case 2 derived print is called ? A little arrow diagram will be very appreciated.

Answer 1

this -> print(); is just print() and uses the vtable to call the member of the actual type, as the function is virtual. That's what virtual functions are used for.

Base::print(); just calls the indicated function, whether it is virtual or not. Often used by derived classes to first call the base class function and then do it own thing.