Top 15 C++ Interview Questions for Freshers with Answers

Q1. What is the difference between C and C++?

C is procedural; C++ adds object-oriented programming (classes, inheritance, polymorphism) plus references, overloading, STL and exceptions on top of C. C++ is practically a superset of C.

Q2. What is a class and an object?

A class is a blueprint defining data and functions together; an object is a real instance with its own memory. One class, many independent objects — like one house map, many houses.

Q3. What is the only difference between struct and class in C++?

Default access: struct is public by default, class is private by default (same for default inheritance). Everything else — constructors, functions, inheritance — works identically in both.

Q4. What are the 4 pillars of OOP?

Encapsulation (data + functions bundled, private by default), Inheritance (reuse via base→derived), Polymorphism (overloading at compile time, virtual overriding at runtime), Abstraction (expose what, hide how — abstract classes, interfaces).

Q5. Reference vs pointer?

Reference: permanent alias, must be initialized, never null, never re-seated. Pointer: separate variable holding an address — nullable, reassignable, supports arithmetic.

Q6. What are constructor and destructor?

Constructor (class name, no return type) runs automatically at object creation to initialize; destructor (~ClassName, no parameters, only one) runs at destruction to release resources. Destruction order is reverse of creation (LIFO).

Q7. What is a copy constructor?

A constructor taking a reference to the same class — ClassName(const ClassName &other) — called when an object is initialized from another object, passed by value, or returned by value. Needed for deep copy when the class owns pointers.

Q8. new vs malloc?

new = operator, allocates AND calls constructor, typed pointer, throws bad_alloc. malloc = C function, raw bytes only, void*, returns NULL. Pairs never mix: new↔delete, new[]↔delete[], malloc↔free.

Q9. What is a virtual function?

A function whose call is resolved at runtime by the actual object (via vtable), not at compile time by pointer type — enabling runtime polymorphism. Without virtual, a base pointer to a child object calls the base version.

Q10. Why should a base class destructor be virtual?

So delete on a base pointer runs the derived destructor too; otherwise only the base part is destroyed and derived resources leak.

Q11. What is a pure virtual function and abstract class?

virtual void f() = 0; makes a function pure virtual; a class with at least one becomes abstract — cannot be instantiated, exists to force derived classes to implement the function.

Q12. What is function overloading and its biggest rule?

Same name, different parameter lists, resolved at compile time. Biggest rule: return type alone cannot distinguish overloads — that is a compile error.

Q13. What does inline do?

Requests the compiler to paste the function body at the call site, removing call overhead for tiny functions. It is only a hint — the compiler can refuse (recursion, loops), and functions defined inside a class are implicitly inline.

Q14. Predict the output:

class A { public: A(){cout<<"A";} ~A(){cout<<"~A";} };
class B { public: B(){cout<<"B";} ~B(){cout<<"~B";} };
int main() { A a; B b; return 0; }

Output: AB~B~A — constructors in creation order, destructors in reverse (LIFO). The classic order question.

Q15. What is this pointer?

A hidden pointer available inside every non-static member function, holding the address of the calling object — it is how the function knows whose data to use, and enables method chaining via return *this.