Runtime polymorphism 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 when run?

C++

#include <iostream>
using namespace std;

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

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

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

A Animal sound

B Bark

C Compilation error

D Runtime error

💡 Hint

Think about which version of the function is called when using a base class pointer to a derived object.

Because the function sound() is virtual, the call a->sound() uses the derived class Dog's override, printing "Bark".

❓ Predict Output

intermediate

Output of non-virtual function call through base pointer

What will this C++ program print when executed?

C++

#include <iostream>
using namespace std;

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

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

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

A Compilation error

B Derived show

C Base show

D Runtime error

💡 Hint

Is the function show() declared virtual? How does that affect which function is called?

Since show() is not virtual, the call b->show() calls Base's version, printing "Base show".

🔧 Debug

advanced

Identify the runtime error in polymorphic code

This code compiles but causes a runtime error. What is the cause?

C++

#include <iostream>
using namespace std;

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

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

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

A No runtime error occurs

B Virtual function display() is not overridden correctly

C Base destructor is virtual causing double delete

D Derived destructor is not called causing resource leak

💡 Hint

Check if the destructor in Base is virtual and how that affects deleting derived objects.

Base destructor is not virtual, so deleting Base* pointing to Derived object calls only Base destructor, skipping Derived destructor and causing resource leaks.

📝 Syntax

advanced

Identify the syntax error in polymorphic class declaration

Which option contains a syntax error in this polymorphic class code?

C++

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

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

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

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

C class Square : public Shape { public: void draw() { cout << "Square" << endl; } };

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

💡 Hint

Check if the function declared with override has a body or is defined outside the class.

Option A declares draw() with override but does not provide a function body or definition, causing a linker error (not a syntax error strictly, but incomplete definition).

🚀 Application

expert

Determine the number of objects created and destructed

Consider this C++ program using runtime polymorphism. How many times are constructors and destructors called in total?

C++

#include <iostream>
using namespace std;

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

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

int main() {
    Base* arr[3];
    arr[0] = new Base();
    arr[1] = new Derived();
    arr[2] = new Derived();

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

A 5 constructors and 5 destructors called

B 6 constructors and 3 destructors called

C 3 constructors and 3 destructors called

D 6 constructors and 6 destructors called

💡 Hint

Remember each Derived object calls Base constructor first. Also destructors are called in reverse order.

Each Derived object calls Base constructor then Derived constructor (2 constructors per Derived). There are 2 Derived and 1 Base objects: total 3 Base constructors + 2 Derived constructors = 5 constructors. Destructors called similarly: Derived destructors for 2 Derived objects + Base destructors for all 3 objects = 5 destructors total. So total constructors and destructors are 5 each.

Practice

1. What is the main purpose of virtual functions in C++ runtime polymorphism?

easy

A. To allocate memory for objects dynamically

B. To make functions run faster by avoiding function calls

C. To prevent any function from being overridden in derived classes

D. To allow derived classes to override base class methods and decide at runtime which method to call

Solution

1. Step 1: Understand virtual functions role

   Virtual functions allow a base class to declare a function that derived classes can override.

2. Step 2: Explain runtime decision

   At runtime, the program decides which overridden function to call based on the actual object type.

3. Final Answer:

   To allow derived classes to override base class methods and decide at runtime which method to call -> Option D

4. Quick Check:

   Virtual functions enable runtime method selection [OK]

Hint: Virtual means choose function at runtime, not compile time [OK]

Common Mistakes:

• Thinking virtual speeds up function calls
• Believing virtual prevents overriding
• Confusing virtual with memory allocation

2. Which of the following is the correct syntax to declare a virtual function in a base class?

easy

A. void display() virtual;

B. void virtual display();

C. virtual void display();

D. virtual: void display();

Solution

1. Step 1: Recall virtual function syntax

   The keyword virtual comes before the return type in the function declaration.

2. Step 2: Match correct syntax

   virtual void display(); correctly places virtual before void display();

3. Final Answer:

   virtual void display(); -> Option C

4. Quick Check:

   Virtual keyword precedes return type [OK]

Hint: Virtual always goes before return type in declaration [OK]

Common Mistakes:

• Placing virtual after function name
• Using virtual as a label or modifier incorrectly
• Missing semicolon after declaration

3. What will be the output of the following code?

class Base {
public:
    virtual void show() { std::cout << "Base"; }
};
class Derived : public Base {
public:
    void show() override { std::cout << "Derived"; }
};
int main() {
    Base* ptr = new Derived();
    ptr->show();
    delete ptr;
    return 0;
}

medium

A. Derived

B. Base

C. Compilation error

D. Runtime error

Solution

1. Step 1: Identify virtual function usage

   The show() function in Base is virtual and overridden in Derived.

2. Step 2: Understand pointer type and object type

   The pointer is of type Base* but points to a Derived object, so the overridden show() in Derived is called at runtime.

3. Final Answer:

   Derived -> Option A

4. Quick Check:

   Virtual function calls derived version [OK]

Hint: Virtual functions call derived method via base pointer [OK]

Common Mistakes:

• Expecting base class method output
• Confusing pointer type with object type
• Ignoring virtual keyword effect

4. Identify the error in the following code snippet related to runtime polymorphism:

class Animal {
public:
    void sound() { std::cout << "Animal sound"; }
};
class Dog : public Animal {
public:
    void sound() { std::cout << "Bark"; }
};
int main() {
    Animal* a = new Dog();
    a->sound();
    delete a;
    return 0;
}

medium

A. Incorrect pointer type declaration

B. Missing virtual keyword in base class function

C. Derived class function should be static

D. Memory leak due to missing delete

Solution

1. Step 1: Check base class function declaration

   The base class function sound() is not declared virtual, so no runtime polymorphism occurs.

2. Step 2: Effect on function call

   Because sound() is not virtual, calling a->sound() calls the base class version, not the derived one.

3. Final Answer:

   Missing virtual keyword in base class function -> Option B

4. Quick Check:

   Virtual needed for runtime polymorphism [OK]

Hint: Base function must be virtual for polymorphism [OK]

Common Mistakes:

• Assuming override works without virtual
• Thinking pointer type causes error
• Ignoring virtual keyword importance

5. Given the classes below, what will be the output when ptr->identify() is called?

class Shape {
public:
    virtual void identify() { std::cout << "Shape"; }
};
class Circle : public Shape {
public:
    void identify() override { std::cout << "Circle"; }
};
class ColoredCircle : public Circle {
public:
    void identify() override { std::cout << "Colored Circle"; }
};
int main() {
    Shape* ptr = new ColoredCircle();
    ptr->identify();
    delete ptr;
    return 0;
}

hard

A. Colored Circle

B. Circle

C. Shape

D. Compilation error due to multiple overrides

Solution

1. Step 1: Trace virtual function overrides

   identify() is virtual in Shape, overridden in Circle, and again in ColoredCircle.

2. Step 2: Determine actual object type at runtime

   The pointer ptr is of type Shape* but points to a ColoredCircle object, so the most derived override runs.

3. Final Answer:

   Colored Circle -> Option A

4. Quick Check:

   Most derived override called at runtime [OK]

Hint: Runtime calls most derived override in inheritance chain [OK]

Common Mistakes:

• Expecting base or intermediate class output
• Thinking multiple overrides cause errors
• Ignoring virtual override chain