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OOPs Concepts — Questions and Answers

Object-oriented programming in C++ rests on four pillars — encapsulation, abstraction, inheritance and polymorphism — and interviewers test whether you can distinguish them with an example rather than recite the list. The C++-specific depth comes from compile-time versus run-time polymorphism, virtual functions and the vtable, and why a base class destructor must be virtual.

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OOPs Concepts— Concepts, Formulas & Shortcuts

  • Encapsulation: bundling data and the methods operating on it, with access controlled by private/protected/public.
  • Abstraction: exposing what an object does while hiding how — achieved with abstract classes and pure virtual functions.
  • Inheritance: deriving a class from another to reuse and extend behaviour.
  • Polymorphism: compile-time (function/operator overloading) versus run-time (virtual functions).
  • A pure virtual function (virtual void f() = 0;) makes the class abstract — it cannot be instantiated.
  • Deleting a derived object through a base pointer requires a virtual destructor, or the derived destructor never runs.

OOPs Concepts Practice Questions with Answers

Attempt each question first, then open the explanation. All 8 questions below are free to read and require no signup.

  1. Q1.Which of the following is NOT one of the four pillars of OOP?

    Easy
    • AEncapsulation
    • BInheritance
    • CCompilation
    • DPolymorphism
    +Show Answer & Explanation

    Answer: C. Compilation

    Explanation: The four pillars are encapsulation, abstraction, inheritance and polymorphism. Compilation is a build step, not an OOP concept.

    OOPs Concepts question 1 of 8

  2. Q2.Run-time polymorphism in C++ is achieved through:

    Moderate
    • AFunction overloading
    • BOperator overloading
    • CVirtual functions
    • DTemplates
    +Show Answer & Explanation

    Answer: C. Virtual functions

    Explanation: Virtual functions are dispatched via the vtable at run time. Overloading and templates are resolved at compile time.

    OOPs Concepts question 2 of 8

  3. Q3.A class containing at least one pure virtual function is called:

    Easy
    • AA friend class
    • BAn abstract class
    • CA static class
    • DA sealed class
    +Show Answer & Explanation

    Answer: B. An abstract class

    Explanation: A pure virtual function makes the class abstract, so it cannot be instantiated and must be inherited from.

    OOPs Concepts question 3 of 8

  4. Q4.What happens if a base class destructor is not declared virtual and a derived object is deleted through a base pointer?

    Difficult
    • ACompilation error
    • BOnly the base destructor runs, leaking derived resources
    • CBoth destructors run normally
    • DThe program always crashes
    +Show Answer & Explanation

    Answer: B. Only the base destructor runs, leaking derived resources

    Explanation: Without a virtual destructor the call is statically bound to the base destructor, so the derived class part is never cleaned up.

    OOPs Concepts question 4 of 8

  5. Q5.Which access specifier makes members accessible to derived classes but not to outside code?

    Easy
    • Aprivate
    • Bprotected
    • Cpublic
    • Dfriend
    +Show Answer & Explanation

    Answer: B. protected

    Explanation: protected members are visible inside the class and to its derived classes, but not to unrelated external code.

    OOPs Concepts question 5 of 8

  6. Q6.A friend function in C++:

    Moderate
    • AIs a member of the class
    • BCan access private members without being a member
    • CIs inherited by derived classes
    • DMust be declared public
    +Show Answer & Explanation

    Answer: B. Can access private members without being a member

    Explanation: A friend is a non-member function granted access to private and protected members. Friendship is neither inherited nor reciprocal.

    OOPs Concepts question 6 of 8

  7. Q7.Which feature allows the same operator to behave differently for user-defined types?

    Easy
    • AInheritance
    • BOperator overloading
    • CEncapsulation
    • DAbstraction
    +Show Answer & Explanation

    Answer: B. Operator overloading

    Explanation: Operator overloading gives operators like + or << a custom meaning for a class — a form of compile-time polymorphism.

    OOPs Concepts question 7 of 8

  8. Q8.In multiple inheritance, the ambiguity when a class inherits the same base twice is solved by:

    Difficult
    • AVirtual base classes
    • BFriend functions
    • CStatic members
    • DTemplates
    +Show Answer & Explanation

    Answer: A. Virtual base classes

    Explanation: Declaring the common base virtual ensures only one shared sub-object exists, resolving the diamond problem.

    OOPs Concepts question 8 of 8

OOPs Concepts — Frequently Asked Questions

What is the difference between overloading and overriding?+

Overloading means several functions share a name but differ in parameters, resolved at compile time. Overriding means a derived class redefines a base class virtual function with the same signature, resolved at run time through the vtable.

Why should a base class destructor be virtual?+

So that deleting a derived object through a base class pointer calls the derived destructor first. Without it the derived part is never destroyed, leaking resources.

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