C++ new keyword in inheritance with different types

Question

I recently started learning OOP. Forgive me if this is a noob question. My question is, I have thought new keyword is used with only same datatypes such as:

char* p = new char; // OR
int* myArr = new int[i] //etc...

While studying inheritance and virtual functions I've come across this:

#include <iostream>
using namespace std;
class Human {
public:
    virtual void className() {
        cout << "Human" << endl;
    }
};
class Asian : public Human {
public:
    void className() {
        cout << "Asian" << endl;
    }
};
int main() {
    Human* h1 = new Asian(); 
    h1->className();
}

In the main function, we initialize the pointer with the base class and then there is the derived class after new keyword? What do those 2 datatypes represent, how am I supposed to use them?

Answer 1

Note that in your case, the class Human is accessible class of Asian and

$11.2/5 states -

If a base class is accessible, one can implicitly convert a pointer to a derived class to a pointer to that base class (4.10, 4.11). [ Note: it follows that members and friends of a class X can implicitly convert an X* to a pointer to a private or protected immediate base class of X. —end note ]

According to the above quoted statement, you can convert a pointer to the derived class Asian to a pointer to the base class Human(which means from Asian* to Human*), which is what you did when you wrote:

Human* h1 = new Asian(); 

In the above statement, on the left hand side we have a pointer named h1 of type Human and on the right hand side by using the keyword new we are creating an object of type Asian on the heap. But note that using new has a second effect which is that after the object of type Asian is created on the heap then a pointer to that object is returned. So essentially, on the right hand side we get a pointer to Asian (Asian*).

Now as i have already explained(quoted) at the beginning of the answer, you can convert a pointer to Asian to a pointer to Human which is what has happened here.

Now when you wrote:

h1->className();

There are 3 important facts that you have to consider:

1. className is a virtual function/method
2. h1 is a pointer
3. Asian inherit Human publicly

and according to virtual function documentation

a virtual function is a member function you may redefine for other derived classes, and can ensure that the compiler will call the redefined virtual function for an object of the corresponding derived class, even if you call that function with a pointer or reference to a base class of the object.

And so the result is that the derive class' function named className is called at run-time and we see Asian printed on the console.