C++ OOPs Interview Questions 2026: 28 Q&A With Code

Last Updated: June 2026 | Level: Freshers to Mid-Level | Read Time: ~16 min

C++ object orientation goes deeper than most languages because it exposes the machinery: vtables, object layout, and manual resource management. Candidates report that virtual functions, the rule of five, and RAII are guaranteed for product-company and systems roles. This guide covers 28 C++ OOP questions with full answers and predict-the-output traps. Behaviour reflects the official cppreference documentation; confirm any object-model detail on the standard.

Pair this with C++ Programming Placement Questions and C++ STL Interview Questions 2026.

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Table of Contents

1. Classes and Constructors (Q1 to Q9)
2. Inheritance and Virtual (Q10 to Q19)
3. Resource Management (Q20 to Q25)
4. Advanced (Q26 to Q28)
5. Predict the Output
6. Quick Revision Table
7. Frequently Asked Questions

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Classes and Constructors

Q1. What are the access specifiers in C++? Easy

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Q2. What is the difference between struct and class in C++? Easy

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Q3. What is a constructor and its types? Easy

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Q4. What is a copy constructor? Medium

class Buf {
  int* data;
public:
  Buf(const Buf& o) { data = new int[o.size]; /* deep copy */ }
};

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Q5. What is the difference between shallow and deep copy? Medium

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Q6. What is an initializer list in a constructor? Medium

class P { const int x; public: P(int v) : x(v) {} };

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Q7. What is a destructor and when is it called? Easy

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Q8. What is the difference between a constructor and a member function? Easy

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Q9. What does the explicit keyword do? Medium

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Inheritance and Virtual

Q10. What are the types of inheritance in C++? Easy

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Q11. What is a virtual function? Medium

struct Base { virtual void f() { cout << "B"; } };
struct Derived : Base { void f() override { cout << "D"; } };
Base* p = new Derived(); p->f();   // D

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Q12. What is a vtable and vptr? Hard

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Q13. What is a pure virtual function and an abstract class? Medium

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Q14. Why should a base class destructor be virtual? Hard

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Q15. What is the difference between overloading and overriding? Medium

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Q16. What is the diamond problem and virtual inheritance? Hard

struct A {};
struct B : virtual A {};
struct C : virtual A {};
struct D : B, C {};   // one shared A

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Q17. What does the override keyword do? Medium

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Q18. Can constructors be virtual? Medium

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Q19. What is object slicing? Hard

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Resource Management

Q20. What is RAII? Medium

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Q21. What is the rule of three? Medium

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Q22. What is the rule of five? Hard

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Q23. What are smart pointers? Medium

auto p = std::make_unique<int>(5);   // auto-freed

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Q24. What is the difference between unique_ptr and shared_ptr? Hard

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Q25. What is a memory leak and how do smart pointers prevent it? Medium

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Advanced

Q26. What are move semantics? Hard

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Q27. What is the difference between static and dynamic binding? Medium

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Q28. What is a friend function or class? Medium

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Predict the Output

Snippet 1

struct Base { ~Base() { cout << "B "; } };
struct Derived : Base { ~Derived() { cout << "D "; } };
Base* p = new Derived();
delete p;

Output:

B 

Why: The non-virtual base destructor means only ~Base runs, leaking the derived part (undefined behaviour). Make ~Base virtual to print "D B".

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Snippet 2

struct A { virtual void f() { cout << "A"; } };
struct B : A { void f() override { cout << "B"; } };
A a = B();
a.f();

Output:

A

Why: Assigning a B to an A value slices off the derived part, so a is a pure A and calls A::f.

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Snippet 3

struct A { A() { cout << "A"; } virtual void g() {} };
struct B : A { B() { cout << "B"; } };
B b;

Output:

AB

Why: Base constructor runs before the derived constructor, so "A" prints before "B".

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Quick Revision Table

Concept	Key takeaway
virtual function	runtime dispatch via vtable
pure virtual	abstract class, must override
virtual destructor	required for polymorphic base
rule of three / five	destructor implies copy and move
RAII	resource tied to object lifetime
smart pointers	unique exclusive, shared counted
object slicing	pass by value loses derived part

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Frequently Asked Questions

What C++ OOP topic is asked most in 2026?

Candidates report that virtual functions and the vtable, the difference between virtual and pure virtual, and why a base class destructor should be virtual are the three most repeated C++ OOP questions.

Is the rule of three or five asked in interviews?

Yes. Interviewers expect you to explain that if you define a destructor, copy constructor, or copy assignment you usually need all of them, and with move semantics the rule extends to five.

What is RAII and why does it matter?

RAII ties resource lifetime to object lifetime so destructors release resources automatically, even on exceptions. It is the foundation of safe C++ and a guaranteed interview topic for systems roles.

Why should a base destructor be virtual?

Deleting a derived object through a base pointer with a non-virtual destructor only runs the base destructor, leaking the derived part and causing undefined behaviour. A virtual destructor ensures the full destruction chain runs.

What is the difference between unique_ptr and shared_ptr?

unique_ptr is exclusive, non-copyable ownership with no overhead, while shared_ptr is reference-counted shared ownership that frees the object when the count reaches zero. Prefer unique_ptr unless sharing is genuinely required.

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Related Articles

• C++ Programming Placement Questions, the master set
• C++ STL Interview Questions 2026, containers and algorithms
• C++ Pointers Interview Questions 2026, memory and pointers
• Java OOPs Interview Questions 2026, compare with Java's model

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Confirm any object-model detail on the official cppreference documentation before your interview. This guide reflects candidate-reported patterns and public preparation resources as of June 2026.