50ptrtoderivedcl.cpp

#include <iostream>
using namespace std;

class BaseClass
{
public:
    int var_base;
    void display()
    {
        cout << "Base class variable var_base value is " << var_base << endl;
    }
};

class DerivedClass : public BaseClass
{
public:
    int var_derived;
    void display()
    {
        cout << "Base class variable var_base value is " << var_base << endl;
        cout << "Derived class variable var_derived value is " << var_derived << endl;
    }
};
int main()
{
    // Base class pointer can point to the derived class object

    BaseClass *base_class_pointer; // this is a base class pointer
    BaseClass obj_base;
    DerivedClass obj_derived; // this is a derived class object

    base_class_pointer = &obj_derived; // this will not throw any error as it is very legal to do

    DerivedClass *derived_class_pointer; // This is not legal or valid to do
    // derived_class_pointer = &obj_base; // derived class pointer cannot point to a base class object

    base_class_pointer->var_base = 12;
    base_class_pointer->display();

    /*
    SINCE THE POINTER IS OF BASE CLASS, IT WILL ONLY LET THE DERIVED OBJECT TO ACCESS BASE CLASS INHERITED MEMBERS
    THE POINTER CANNOT ACCESS THE DERIVED CLASS MEMBERS
    IT CAN ONLY ACCESS THE INHERITED BASE CLASS MEMBERS
    THE DERIVED OBJECT ONLY STORES THAT VALUE AND NOT GOVERN THE POINTER

    base_class_pointer->var_derived = 8; won't work and will give error
    It will only access the class function it belongs to in case of overloading
        This is called late binding
    */
   derived_class_pointer = &obj_derived;
   derived_class_pointer->var_base = 78;
   derived_class_pointer->var_derived = 783;
   derived_class_pointer->display();
   // This derived class pointer now points to the derived class display function
    return 0;
}
/*
        SO FROM THIS WE CAN SAY THAT
        IN RUN TIME POLYMORPHISM,

        The compiler checks the pointer and the class it is of to finally select the members to access with the pointer
        this is done in run time 
*/