Pure virtual functions in C++ - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a class with pure virtual function

What is the output of this C++ program?

C++

#include <iostream>
using namespace std;

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

class Circle : public Shape {
public:
    void draw() override {
        cout << "Drawing Circle" << endl;
    }
};

int main() {
    Circle c;
    c.draw();
    return 0;
}

A Shape::draw called

B Drawing Circle

C Compilation error: cannot instantiate abstract class

D No output

💡 Hint

Look at which class implements the pure virtual function and which object is created.

The class Shape has a pure virtual function draw(), making it abstract. Circle overrides draw(), so creating Circle object and calling draw() prints "Drawing Circle".

❓ Predict Output

intermediate

Output when calling pure virtual function from base pointer

What will be the output of this program?

C++

#include <iostream>
using namespace std;

class Animal {
public:
    virtual void sound() = 0;
};

class Dog : public Animal {
public:
    void sound() override {
        cout << "Woof" << endl;
    }
};

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

A Runtime error: pure virtual function call

B Compilation error: cannot instantiate abstract class

C Woof

D No output

💡 Hint

Check which class implements the pure virtual function and how the pointer is used.

The pointer of type Animal points to a Dog object. Dog overrides sound(), so calling sound() prints "Woof".

🔧 Debug

advanced

Identify the error in this pure virtual destructor example

What error will this code produce when compiled?

C++

#include <iostream>
using namespace std;

class Base {
public:
    virtual ~Base() = 0;
};

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

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

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

A

Output:
Derived destructor
Base destructor

B Runtime error: pure virtual function call

C Compilation error: pure virtual destructor must have a definition

D No output

💡 Hint

Pure virtual destructors must have a body to allow proper cleanup.

The pure virtual destructor Base::~Base() has a definition, so the code compiles. Deleting through base pointer calls Derived destructor then Base destructor, printing both lines.

📝 Syntax

advanced

Which option correctly declares a pure virtual function?

Which of the following is the correct syntax to declare a pure virtual function named `foo` returning int?

A int virtual foo() = 0;

B int foo() = 0 virtual;

C virtual int foo() {} = 0;

D virtual int foo() = 0;

💡 Hint

Remember the order: 'virtual' keyword before return type, and '= 0;' at the end.

The correct syntax is 'virtual int foo() = 0;'. Other options have wrong order or invalid syntax.

🚀 Application

expert

Number of objects instantiated with pure virtual functions

Given the following code, how many objects of type `Base` or its derived classes are instantiated in `main`?

C++

#include <iostream>
using namespace std;

class Base {
public:
    virtual void display() = 0;
};

class Derived1 : public Base {
public:
    void display() override {
        cout << "Derived1" << endl;
    }
};

class Derived2 : public Base {
public:
    void display() override {
        cout << "Derived2" << endl;
    }
};

int main() {
    Derived1 d1;
    Base* b = new Derived2();
    d1.display();
    b->display();
    delete b;
    return 0;
}

A 2

B 1

C 3

D 0

💡 Hint

Count all objects created, including those created with new.

One object of Derived1 is created on stack (d1), and one object of Derived2 is created on heap (b). Total 2 objects.