400 C++ Interview Questions with Answers 2026

OOP & Advanced Features: Inheritance, Virtual Functions, Templates, and Lambda Expressions.

Memory & Performance: Stack vs. Heap, Move Semantics, Smart Pointers, and Cache Locality.

STL & Algorithms: Containers, Iterators, Custom Comparators, and Time Complexity.

Concurrency & Systems: Mutexes, Atomics, Design Patterns, CMake, and Secure Coding.

B) A deep copy of the underlying object is performed automatically.

C) The compilation fails because std::unique_ptr cannot be copied.

D) The ownership is automatically moved using move semantics.

E) The reference count is incremented, similar to std::shared_ptr.

F) The program crashes at runtime due to a double-delete.

Correct Answer: C

Overall Explanation: std::unique_ptr is designed for exclusive ownership. To prevent multiple pointers from managing the same resource, its copy constructor is explicitly deleted.

Option Detail:

B) Incorrect: C++ does not perform "automatic" deep copies for smart pointers.

C) Correct: The copy constructor is deleted; you must use std::move() or pass by reference.

D) Incorrect: Move semantics are not "automatic" when the parameter expects a copy; it requires an explicit std::move.

E) Incorrect: unique_ptr does not have a reference counter.

F) Incorrect: The compiler prevents this scenario from ever reaching runtime.

Question 2: In C++, what is the primary purpose of a virtual destructor in a base class?

B) To ensure the derived class destructor is called when deleting via a base pointer.

C) To increase the performance of object deallocation on the stack.

D) To prevent the base class from having any member variables.

E) To allow the destructor to be overloaded with different parameters.

F) To force the compiler to use static binding during destruction.

Correct Answer: B

Overall Explanation: When a base class pointer points to a derived class object, deleting that pointer requires a virtual destructor to trigger the correct cleanup chain.

Option Detail:

B) Correct: Without it, only the base destructor runs, causing potential memory leaks in the derived part.

C) Incorrect: Virtual functions actually add a slight overhead due to the vtable lookup.

D) Incorrect: Destructors have no impact on whether a class can have member variables.

E) Incorrect: Destructors cannot be overloaded; they take no arguments.

F) Incorrect: virtual specifically enables dynamic binding, the opposite of static binding.

Question 3: Which keyword is used to indicate that a function does not throw any exceptions, potentially allowing for compiler optimizations?

B) final

C) static_assert

D) noexcept

E) override

F) volatile

Correct Answer: D

Overall Explanation: The noexcept specifier informs the compiler (and the developer) that a function is guaranteed not to exit via an exception.

Option Detail:

B) Incorrect: final prevents further inheritance or virtual function overriding.

C) Incorrect: static_assert is for compile-time logical checks.

D) Correct: noexcept is the modern standard for exception guarantees and enables optimizations in STL containers.

E) Incorrect: override ensures a member function correctly overrides a base class virtual function.

F) Incorrect: volatile tells the compiler that a variable's value may change unexpectedly (e.g., hardware mapping).

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