Question

I know these 3 concepts.

But I remember there was another definition:
Say a base class has 2 virtual methods: Foo() and Foo(int a).
Is there any rule that when a derived class overrides Foo(int a) has to override all other overloads of Foo ?

Was it in Java? I believe it dosn’t exist in C#.

Thanks

Answer 1

C++:

What you are referring to is name hiding in C++. When you have a class with overrided methods, and you extend this class an override one of the overrided methods, you need to override all the overloaded methods. If not, calls to non-overridden overloaded in the extended class won’t work.

For example:

class Base {
    public:
        virtual void A (int);
        virtual void A (int, int);
};
void Base::A(int i) {
    std::cout << “Hi\n”;
}
void Base::A (int i, int j) {
    std::cout << “Bye!!\n”;
}

Suppose you only override one of the methods:

class Sub: public Base {
    public:
        void A(int);
};
void Sub::A(int i) {
    std::cout << “Hey, La!\n”;
}
void main () {
    Sub a;
    a.A(1);
    a.A(1, 1);//won't compile
}

The second call won’t work, as A(int, int) is not visible. This is name hiding.
If you want to circumvent this, you can use the using keyword as follows:

class Sub: public Base {
    public:
        void A(int);
        using Base::A;
};
void Sub::A(int i) {
    std::cout << “Hey, La!\n”;
}
void main () {
    Sub a;
    a.A(1);
    a.A(1, 1);//will compile
}

Java:

Java doesn’t have such a concept though. You can try this out yourself. Note that all Java methods are virtual by default as per virtual C++ methods.

public class Base {
    public void A() {
        System.out.println("Hi");
    }
    public void A(int i, int j) {
        System.out.println("Bye");
    }
}

public class Sub extends Base {
    public void A() {
        System.out.println("Hey, La!");
    }
}

public class Test {
    public static void main(String[] args) {
        Sub a = new Sub();
        a.A();
        a.A(1, 1);//perfectly fine
    }
}

Aside:

I hope you’re not referring to extending an abstract class- if you extend an abstract class, you need to override all abstract methods else your class has to be declared abstract.

All methods of an implemented interface need to be implemented though.