Polymorphism? C++ vs Java

Question

Trying out polymorphism as a beginner. I think I am trying the same code in different languages, but the result is not the same :

C++

#include <iostream>
using namespace std;

class A {
    public:

    void whereami() {
        cout << "You're in A" << endl;
    }
};

class B : public A {
    public:

    void whereami() {
        cout << "You're in B" << endl;
    }
};

int main(int argc, char** argv) {
    A a;
    B b;
    a.whereami();
    b.whereami();
    A* c = new B();
    c->whereami();
    return 0;
}

Results :

You're in A
You're in B
You're in A

Java :

public class B extends A{

    void whereami() {
        System.out.println("You're in B");
    }

}

//and same for A without extends
//...


public static void main(String[] args) {
    A a = new A();
    a.whereami();
    B b = new B();
    b.whereami();
    A c = new B();
    c.whereami();
}

Results :

You're in A
You're in B
You're in B

Not sure if i'm doing something wrong, or this has to do with the languages themselves?

youre missing the `virtual` keyword in `whereami` in the 1st program... — Reimeus, Dec 24 '15 at 00:06
@Reimeus: The OP also forgets to provide virtual destructor in Base class & write delete c; in the main() function. — Destructor, Dec 24 '15 at 05:13

score 10 · Answer 1 · answered Dec 24 '15 at 00:08

10

You need to read up about the virtual keyword for C++. Without that qualifier, what you have are member functions in your objects. They don't follow the rules of polymorphism (dynamic binding). With the virtual qualifier you get methods, which use dynamic binding. In Java, all instance functions are methods. You always get polymorphism.

answered Dec 24 '15 at 00:08

LIProf

436
2
5

2

With the virtual qualifier they're still member functions. C++ does not use the term "method". There are virtual member functions and non-virtual member functions. Terminology aside, though, the answer is correct. – Pete Becker Dec 24 '15 at 01:05
I would never argue about C++ with Pete Becker. – LIProf Dec 24 '15 at 01:16
I forgot to say that aside from the terminology, this answer is correct. – Pete Becker Dec 24 '15 at 01:18

score 5 · Answer 2 · answered Dec 24 '15 at 00:09

Use "virtual functions".

#include <iostream>
using namespace std;

class A {
    public:

    virtual void whereami() { // add keyword virtual here
        cout << "You're in A" << endl;
    }
};

class B : public A {
    public:

    void whereami() {
        cout << "You're in B" << endl;
    }
};

int main(int argc, char** argv) {
    A a;
    B b;
    a.whereami();
    b.whereami();
    A* c = new B();
    c->whereami();
    return 0;
}

In Java, all instance methods are virtual functions in default.

score 0 · Answer 3 · answered Apr 17 '21 at 05:24

0

It's a bit late to answer but use override keyword in derived classes to let compiler know that you are overriding.

answered Apr 17 '21 at 05:24

vishal

62
7

Polymorphism? C++ vs Java

3 Answers3