How to Pass a C++ Interview: Top Questions from A to Z

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How to Pass a C++ Interview: Top Questions from A to Z

An interview for a C++ developer position is a test not only of syntax knowledge, but also of a deep understanding of how the compiler works, memory management, and the features of object-oriented programming. In this article, we have broken down the top questions from A to Z that are most often asked in interviews. The material will be useful for both junior developers and those preparing for a middle level.

1. Basics of Syntax and Data Types

Let's start with the basics — questions that test your understanding of fundamental C++ concepts.

1.1. What is the difference between a reference and a pointer?

This is a classic question. The main differences are:

A reference cannot be nullptr, a pointer can.
A reference cannot be rebound to another object after initialization.
Pointers support arithmetic, references do not.

int a = 5;int& ref = a;  // referenceint* ptr = &a; // pointer

ref = 10;      // a = 10*ptr = 20;     // a = 20

1.2. What is `const` and how to use it correctly?

const in C++ is used to protect data from modification. It is important to distinguish between const before the type and after it:

const int* ptr — pointer to a constant integer (cannot change the value at the address).
int* const ptr — constant pointer to an integer (cannot change the address).

2. Object-Oriented Programming (OOP)

OOP questions are a mandatory part of a C++ interview. Let's break down the key topics.

2.1. Virtual Functions and Polymorphism

Virtual functions provide late binding (dynamic polymorphism). Example:

class Base {public:    virtual void show() { std::cout << "Base"; }    virtual ~Base() = default;};

class Derived : public Base {public:    void show() override { std::cout << "Derived"; }};

int main() {    Base* b = new Derived();    b->show(); // Prints "Derived"    delete b;}

It is important to remember the virtual destructor — without it, when deleting an object through a base class pointer, only the base class destructor will be called, leading to a memory leak.

2.2. Pure Virtual Functions and Abstract Classes

A class containing at least one pure virtual function (= 0) becomes abstract. You cannot create an instance of such a class — only a derived class that implements all pure virtual methods.

2.3. What are vtable and vptr?

To implement polymorphism, the compiler creates a virtual table (vtable) for each class with virtual functions. Each object contains a hidden pointer (vptr) to this table. Calling a virtual function occurs through indirect access via vptr, which adds a small overhead.

3. Memory Management and Smart Pointers

C++ gives the developer full control over memory, which requires special care.

3.1. Difference between `new`/`delete` and `malloc`/`free`

malloc allocates memory but does not call a constructor. free frees memory without calling a destructor.
new allocates memory and calls a constructor. delete calls a destructor and frees memory.