Virtual functions in C++ - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of virtual function call through base pointer

What is the output of this C++ code?

C++

#include <iostream>
using namespace std;

class Base {
public:
    virtual void show() { cout << "Base show" << endl; }
};

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

int main() {
    Base* ptr = new Derived();
    ptr->show();
    delete ptr;
    return 0;
}

A Compilation error

B Base show

C Runtime error

D Derived show

💡 Hint

Remember how virtual functions behave when called through a base class pointer.

Because show() is virtual, the call ptr->show() invokes Derived's version, printing "Derived show".

❓ Predict Output

intermediate

Output when virtual function is not overridden

What will this program print?

C++

#include <iostream>
using namespace std;

class Animal {
public:
    virtual void speak() { cout << "Animal speaks" << endl; }
};

class Dog : public Animal {
    // No override of speak()
};

int main() {
    Animal* a = new Dog();
    a->speak();
    delete a;
    return 0;
}

A Animal speaks

B Compilation error

C Dog speaks

D Runtime error

💡 Hint

If a derived class does not override a virtual function, which version is called?

Since Dog does not override speak(), the base class Animal's speak() is called, printing "Animal speaks".

🔧 Debug

advanced

Identify the error in virtual destructor usage

What error will this code produce when compiled or run?

C++

#include <iostream>
using namespace std;

class Base {
public:
    ~Base() { cout << "Base destructor" << endl; }
};

class Derived : public Base {
public:
    ~Derived() { cout << "Derived destructor" << endl; }
};

int main() {
    Base* ptr = new Derived();
    delete ptr;
    return 0;
}

A Both Derived and Base destructors are called

B Only Base destructor is called, Derived destructor is skipped

C Compilation error due to missing virtual keyword

D Runtime crash due to invalid delete

💡 Hint

Think about what happens if the base class destructor is not virtual and you delete a derived object through a base pointer.

Because Base's destructor is not virtual, deleting a Derived object through a Base pointer calls only Base's destructor, skipping Derived's destructor, which can cause resource leaks.

📝 Syntax

advanced

Which option causes a compilation error?

Given the following class, which option will cause a compilation error?

C++

class Shape {
public:
    virtual void draw() = 0;
};

A class Square : public Shape {};

B class Rectangle : public Shape { void draw() override { /* code */ } };

C class Triangle : public Shape { void draw() override; };

D class Circle : public Shape { void draw() override { /* code */ } };

💡 Hint

What happens if a class inherits a pure virtual function but does not override it?

Square does not override the pure virtual function draw(), so it remains abstract and cannot be instantiated, causing a compilation error if instantiated.

🚀 Application

expert

Determine the number of virtual function calls

How many times is the virtual function 'action' called in this program?

C++

#include <iostream>
using namespace std;

class A {
public:
    virtual void action() { cout << "A"; }
};

class B : public A {
public:
    void action() override { cout << "B"; }
};

class C : public B {
public:
    void action() override { cout << "C"; }
};

int main() {
    A* arr[3];
    arr[0] = new A();
    arr[1] = new B();
    arr[2] = new C();

    for (int i = 0; i < 3; ++i) {
        arr[i]->action();
    }

    for (int i = 0; i < 3; ++i) {
        delete arr[i];
    }
    return 0;
}

A 6

B 1

C 3

D 0

💡 Hint

Count how many times the loop calls the virtual function.

The loop calls action() exactly 3 times, once for each pointer in the array. Each call is virtual and calls the correct overridden version.