Exposing only some inherited methods in the derived class

Question

I stumbled across an interview question related to OOPS. Here is the question: There is a base class A with 5 methods. Now how should I design the class such that if a class B inherits class A , only 3 methods are exposed. And if a class C inherits class A , the rest of the 2 methods are exposed.

Any thoughts ??

Answer 1

It should not be possible in any object-oriented language, otherwise it would break the Liskov substitution principle . Substituting a B for an A should not reduce its correctness (meaning methods should not suddenly be unavailable)

However, there is still some ambiguity in the question that allows for some "out-of-the-box" thinking. Here are questions I would pose back to the interviewer:

• What do you mean by "exposed"?
• Do the 5 methods in A have to be public?
• Does the "exposition" by C need to be implicit or can the be explicitly exposed (eg pass-through)

Based on those answers you could either come up with possible options using internal , explicit interface implementations, etc.

Answer 2

if A is partial and you have 2 namespaces then:

namespace the_impossible
{
class Program
{
    static void Main(string[] args)
    {
        B b = new B();
        C c = new C();

        b.m1();
        b.m2();
        b.m3();

        c.m4();
        c.m5();
    }
}

namespace A_1
{
    public partial class A
    {
        public  void m1() { }
        public  void m2() { }
        public  void m3() { }
    }
}

namespace A_2
{
    public partial class A
    {
        public void m4() { }
        public void m5() { }
    }
}

class B : A_1.A 
{

}

class C : A_2.A
{ 

}
}

Answer 3

I think it was a trick or even dumb question. To achieve this, we must break the Liskov substitution principle. You shouldn't preseve the hierarchy of the classes.

Maybe you just should use interfaces instead:

public class A {} //why we even need this class?

public class B : A, I3Methods
{
    public void Method1() { }
    public void Method2() { }   
    public void Method3() { }
}

public class C : A, I2Methods
{
    public void Method4() { }
    public void Method5() { }
}

public interface I3Methods
{
    void Method1();
    void Method2();
    void Method3();
}

public interface I2Methods
{
    void Method4();
    void Method5();
}

Answer 4

我能想到的唯一方法就是让它们在A中都是私有的 ，然后通过B和C中的封装来暴露它们......但是它们没有暴露，只是被执行......所以它是半对的。

Answer 5

I also think that's impossible.

But to give an approximate answer:

Make 3 methods in A virtual, then implement them in B. Then override those 2 methods in C.

Answer 6

Nobody says that the 5 methods of class A should be exposed when writing them. In C# you could simply write 5 protected methods in class A and expose those you wish to be accessible by writing some hiding methods with the new modifier like this - although this wouldn't actually expose the methods directly they are merely wrapped.

class A
{
    protected void M1() { }
    protected void M2() { }
    protected void M3() { }
    protected void M4() { }
    protected void M5() { }
}

class B : A
{
    public new void M1()
    {
        base.M1();
    }

    public new void M2()
    {
        base.M2();
    }

    public new void M3()
    {
        base.M3();
    }

}

class C : A
{
    public new void M4()
    {
        base.M4();
    }
    public new void M5()
    {
        base.M5();
    }
}

Answer 7

In your comments, you mentioned that you were interested if this could be done in any other language. You can kind of do it in C++ through the use of the using keyword. So, starting with class A:

class A {
public:
    int Method1() {     return 1;   }
    int Method2() {     return 2;   }
    int Method3() {     return 3;   }
    int Method4() {     return 4;   }
    int Method5() {     return 5;   }
};

Then you define class B, using private inheritance (essentially you can't auto cast from B to A and all public methods in A become private methods in B).

class B: private A {
public:
    // We want to expose methods 1,2,3 as public so change their accessibility
    // with the using keyword
    using A::Method1;
    using A::Method2;
    using A::Method3;

};

Do the same for class C, exposing the other two methods instead:

class C: private A {
public:
    using A::Method4;
    using A::Method5;

};

Or if you're supposed to expose all the methods through C, simply use public inheritance and everything exists:

class C: public A {
public:

};

For usage:

B *b = new B();
b->Method1();  // This works, Method1 is public
b->Method4();  // This fails to compile, Method4 is inaccessible

The reason I said kind of above is because you can work around it by explicitly casting the instance of B to an A:

A *brokena = b;  // This wouldn't compile because the typecast is inaccessible

A *a = (A*)b;    // This however does work because you're explicitly casting
a->Method4();    // And now you can call Method4 on b...

Answer 8

I know, it is to late to respond. Just thought of sharing my thoughts:

Define Class A as a base class. Have intermediate child classes A1 -> M1,M2,M3 and A2 -> M4, M5 deriving from Class A

Now, you can have 1) Class B inheriting A1 2) Class C inheriting A2

These two classes are still derived from Class A.

And also we are not breaking liskov substitution principle.

Hope, this gives clarity.