OOP MCQs - Polymorphism, Virtual Functions and Dynamic Dispatch (Set 4)

Q1. Which of the following is resolved at run time rather than at compile time?

a) Choosing between two overloaded functions with different parameter types

b) A call to an overridden virtual function through a base-class pointer

c) Selecting a template specialisation

d) Resolving an overloaded operator for built-in argument types

Q2. What is printed by this C++ program?

#include <iostream>
using namespace std;
struct B {
    void f() { cout << "Bf"; }
    virtual void g() { cout << "Bg"; }
};
struct D : B {
    void f() { cout << "Df"; }
    void g() { cout << "Dg"; }
};
int main() {
    B* p = new D;
    p->f();
    p->g();
}

a) DfDg

b) BfDg

c) BfBg

d) DfBg

Q3. Using the same B and D as above (g virtual in B and overridden in D), what does this C++ code print?

B b = D();
b.g();

a) Bg

b) Dg

c) Compilation error

d) Undefined behaviour

Q4. What is printed by this C++ program?

#include <iostream>
using namespace std;
struct B {
    virtual void f(int x = 1) { cout << "B" << x; }
};
struct D : B {
    void f(int x = 2) override { cout << "D" << x; }
};
int main() {
    B* p = new D;
    p->f();
}

a) D2

b) B1

c) D1

d) B2

Q5. What is printed by this C++ program?

#include <iostream>
using namespace std;
void f(int)    { cout << "int"; }
void f(double) { cout << "double"; }
int main() { f('a'); }

a) int

b) double

c) Compilation error: ambiguous call

d) Run-time error

Q6. What is printed by this C++ program?

#include <iostream>
using namespace std;
struct B { virtual ~B() { cout << "~B"; } };
struct D : B { ~D() { cout << "~D"; } };
int main() {
    B* p = new D;
    delete p;
}

a) ~B

b) ~D~B

c) ~B~D

d) ~D

Q7. What is printed by this C++ program?

#include <iostream>
using namespace std;
struct Base {
    Base() { show(); }
    virtual void show() { cout << "Base"; }
};
struct Der : Base {
    void show() override { cout << "Der"; }
};
int main() { Der d; }

a) Der

b) BaseDer

c) Compilation error

d) Base

Q8. What does this Java program print?

class P {
    P() { show(); }
    void show() { System.out.print("P"); }
}
class Q extends P {
    int v = 5;
    Q() { show(); }
    void show() { System.out.print(v); }
}
public class Main {
    public static void main(String[] args) { new Q(); }
}

a) P5

b) 55

c) 05

d) PP

Q9. What does this Java program print?

public class Main {
    static void m(Object o) { System.out.print("Object"); }
    static void m(String s) { System.out.print("String"); }
    public static void main(String[] args) { m(null); }
}

a) Object

b) Compilation error: ambiguous call

c) String

d) NullPointerException

Q10. What does this Java program print?

class P { static String s() { return "P"; } }
class Q extends P { static String s() { return "Q"; } }
public class Main {
    public static void main(String[] args) {
        P r = new Q();
        System.out.print(r.s());
    }
}

a) Q

b) PQ

c) Compilation error

d) P

Q11. What does this Java program print?

public class Main {
    static void m(long x)    { System.out.print("long"); }
    static void m(Integer x) { System.out.print("Integer"); }
    static void m(int... x)  { System.out.print("varargs"); }
    public static void main(String[] args) { m(5); }
}

a) Integer

b) varargs

c) long

d) Compilation error: ambiguous call

Q12. In a typical C++ implementation of virtual functions, which statement is true?

a) Each polymorphic class has one virtual table, and each object of such a class holds a hidden pointer to its class's table

b) Every object stores its own full copy of the virtual table

c) Virtual tables are created only for abstract classes

d) The virtual table stores the values of an object's data members

Q13. Which pair of C++ functions in the same scope is NOT a valid overload?

a) void put(int); and void put(double);

b) void put(int); and void put(int, int);

c) void put(int&); and void put(const int&);

d) int get(); and double get();

Q14. Which Java method can be overridden in a subclass?

a) A protected instance method

b) A private method

c) A static method

d) A final method

Q15. A Java superclass declares the method Animal create(). In subclass Dog (where Dog extends Animal), which declaration is a valid override?

a) void create()

b) Dog create()

c) Object create()

d) Animal create(int n)

Q16. Why is a virtual function call usually slightly more expensive than a non-virtual call?

a) Virtual functions are always interpreted rather than compiled

b) Each call copies the entire object onto the stack

c) The target is found through the object's vptr and vtable at run time, which adds an indirection and usually prevents inlining

d) Virtual functions must allocate heap memory on every call

Q17. What is the purpose of the C++11 override specifier on a member function?

a) It makes the compiler report an error if the function does not actually override a virtual function of a base class

b) It makes a non-virtual base function virtual

c) It prevents further overriding in derived classes

d) It forces the function to be inlined

Q18. A Java superclass declares public void run(). Which declaration of run() in a subclass compiles as a valid override?

a) void run()

b) protected void run()

c) private void run()

d) public final void run()