Interface-like behavior 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>

class IShape {
public:
    virtual void draw() const = 0;
    virtual ~IShape() = default;
};

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

int main() {
    IShape* shape = new Circle();
    shape->draw();
    delete shape;
    return 0;
}

A Circle

B IShape

C Compilation error due to abstract class instantiation

D Runtime error due to pure virtual function call

💡 Hint

Think about how virtual functions work when called through a base class pointer.

The pointer is of type IShape*, but it points to a Circle object. The draw() function is virtual and overridden in Circle, so Circle's draw() is called, printing "Circle".

❓ Predict Output

intermediate

Output of multiple inheritance with interface classes

What will this program print?

C++

#include <iostream>

class IReadable {
public:
    virtual void read() const = 0;
    virtual ~IReadable() = default;
};

class IWritable {
public:
    virtual void write() const = 0;
    virtual ~IWritable() = default;
};

class File : public IReadable, public IWritable {
public:
    void read() const override {
        std::cout << "Reading file" << std::endl;
    }
    void write() const override {
        std::cout << "Writing file" << std::endl;
    }
};

int main() {
    File f;
    IReadable* r = &f;
    IWritable* w = &f;
    r->read();
    w->write();
    return 0;
}

A Compilation error due to ambiguous base classes

B Reading file\nWriting file

C Writing file\nReading file

D Runtime error due to multiple inheritance

💡 Hint

Each interface has a pure virtual function. The class File implements both.

The File class implements both read() and write(). The pointers r and w call the respective functions, printing "Reading file" then "Writing file".

❓ Predict Output

advanced

Output of interface pointer to derived object with overridden method

What is the output of this code?

C++

#include <iostream>

class IAnimal {
public:
    virtual void speak() const = 0;
    virtual ~IAnimal() = default;
};

class Dog : public IAnimal {
public:
    void speak() const override {
        std::cout << "Woof" << std::endl;
    }
};

class Cat : public IAnimal {
public:
    void speak() const override {
        std::cout << "Meow" << std::endl;
    }
};

void makeSpeak(const IAnimal* animal) {
    animal->speak();
}

int main() {
    Dog dog;
    Cat cat;
    makeSpeak(&dog);
    makeSpeak(&cat);
    return 0;
}

A Runtime error due to calling pure virtual function

B Meow\nWoof

C Compilation error due to missing virtual destructor

D Woof\nMeow

💡 Hint

The function makeSpeak calls speak() on the interface pointer.

Dog and Cat override speak(). The calls print "Woof" then "Meow" in order.

❓ Predict Output

advanced

Output of interface with default method and override

What will this program print?

C++

#include <iostream>

class IPrinter {
public:
    virtual void print() const {
        std::cout << "Default print" << std::endl;
    }
    virtual ~IPrinter() = default;
};

class CustomPrinter : public IPrinter {
public:
    void print() const override {
        std::cout << "Custom print" << std::endl;
    }
};

int main() {
    IPrinter* p1 = new IPrinter();
    IPrinter* p2 = new CustomPrinter();
    p1->print();
    p2->print();
    delete p1;
    delete p2;
    return 0;
}

A Default print\nCustom print

B Custom print\nDefault print

C Compilation error: cannot instantiate abstract class IPrinter

D Runtime error due to calling virtual function on deleted object

💡 Hint

IPrinter has a default implementation of print(), so it can be instantiated.

IPrinter's print() prints "Default print". CustomPrinter overrides print() to print "Custom print". The calls print those lines in order.

❓ Predict Output

expert

Output of diamond inheritance with virtual inheritance

What is the output of this program?

C++

#include <iostream>

class IDevice {
public:
    virtual void info() const = 0;
    virtual ~IDevice() = default;
};

class Scanner : virtual public IDevice {
public:
    void info() const override {
        std::cout << "Scanner" << std::endl;
    }
};

class Printer : virtual public IDevice {
public:
    void info() const override {
        std::cout << "Printer" << std::endl;
    }
};

class AllInOne : public Scanner, public Printer {
public:
    void info() const override {
        std::cout << "AllInOne" << std::endl;
    }
};

int main() {
    AllInOne aio;
    IDevice* device = &aio;
    device->info();
    return 0;
}

A Printer

B Scanner

C AllInOne

D Compilation error due to ambiguous inheritance

💡 Hint

Virtual inheritance avoids duplicate IDevice base. The AllInOne class overrides info().

AllInOne overrides info(), so calling info() through IDevice* calls AllInOne::info(), printing "AllInOne".

Practice

1. What is the main purpose of an interface-like class in C++?

easy

A. To store data members for derived classes

B. To provide full function implementations for reuse

C. To define functions that derived classes must implement

D. To create objects directly without inheritance

Solution

1. Step 1: Understand interface concept

   Interfaces specify what functions a class must have but do not provide their code.

2. Step 2: Identify interface role in C++

   In C++, interfaces are created using pure virtual functions that derived classes must implement.

3. Final Answer:

   To define functions that derived classes must implement -> Option C

4. Quick Check:

   Interface = function declarations only [OK]

Hint: Interfaces declare required functions without code [OK]

Common Mistakes:

• Thinking interfaces provide function code
• Assuming interfaces hold data members
• Believing interfaces can be instantiated

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

easy

A. virtual void func() = 0;

B. virtual void func();

C. void func() override;

D. virtual void func() = 1;

Solution

1. Step 1: Recall pure virtual function syntax

   Pure virtual functions are declared with '= 0' after the function signature.

2. Step 2: Check options for correct syntax

   virtual void func() = 0; uses '= 0' correctly, others do not match pure virtual syntax.

3. Final Answer:

   virtual void func() = 0; -> Option A

4. Quick Check:

   Pure virtual = '= 0' [OK]

Hint: Pure virtual functions end with '= 0;' [OK]

Common Mistakes:

• Using '= 1' instead of '= 0'
• Omitting 'virtual' keyword
• Confusing override with pure virtual

3. Consider the following code:

class IShape {
public:
    virtual double area() const = 0;
};

class Circle : public IShape {
    double radius;
public:
    Circle(double r) : radius(r) {}
    double area() const override { return 3.14 * radius * radius; }
};

int main() {
    Circle c(2);
    std::cout << c.area();
    return 0;
}

What is the output of this program?

medium

A. 4

B. Compilation error

C. 3.14

D. 12.56

Solution

1. Step 1: Calculate area of circle with radius 2

   Area = π * r² = 3.14 * 2 * 2 = 12.56

2. Step 2: Confirm code compiles and runs correctly

   Circle implements pure virtual function area(), so no error; output is area value.

3. Final Answer:

   12.56 -> Option D

4. Quick Check:

   Area = 3.14 * 4 = 12.56 [OK]

Hint: Calculate area formula carefully [OK]

Common Mistakes:

• Forgetting to implement pure virtual function
• Using wrong radius value
• Expecting compilation error

4. What is wrong with this code snippet?

class IAnimal {
public:
    virtual void speak() = 0;
};

class Dog : public IAnimal {
public:
    void speak() { std::cout << "Woof"; }
};

int main() {
    IAnimal a;
    a.speak();
    return 0;
}

medium

A. Missing override keyword in Dog::speak()

B. Cannot create object of abstract class IAnimal

C. speak() should return a value

D. Dog class must be abstract

Solution

1. Step 1: Identify abstract class usage

   IAnimal has a pure virtual function, so it is abstract and cannot be instantiated.

2. Step 2: Check main() for object creation

   Code tries to create IAnimal a; which is illegal and causes compilation error.

3. Final Answer:

   Cannot create object of abstract class IAnimal -> Option B

4. Quick Check:

   Abstract class objects cannot be created [OK]

Hint: Abstract classes cannot be instantiated [OK]

Common Mistakes:

• Ignoring abstract class instantiation rules
• Thinking override keyword is mandatory
• Assuming pure virtual functions must return values

5. You want to create an interface-like class IPrinter with a pure virtual function print(). Then create two classes ConsolePrinter and FilePrinter that implement print(). Which of the following code snippets correctly demonstrates this?

hard

A. class IPrinter {
public:
virtual void print() = 0;
};

class ConsolePrinter : public IPrinter {
public:
void print() override { std::cout << "Console output"; }
};

class FilePrinter : public IPrinter {
public:
void print() override { /* write to file */ }
};

B. class IPrinter {
public:
void print() { }
};

class ConsolePrinter : public IPrinter {
public:
void print() { std::cout << "Console output"; }
};

C. class IPrinter {
public:
virtual void print();
};

class ConsolePrinter : public IPrinter {
};

D. class IPrinter {
public:
virtual void print() = 1;
};

Solution

1. Step 1: Define interface with pure virtual function

   IPrinter declares print() as pure virtual with '= 0', making it an interface.

2. Step 2: Implement print() in derived classes

   ConsolePrinter and FilePrinter override print() with their own code, fulfilling interface contract.

3. Final Answer:

   Code snippet D correctly defines interface and implementations -> Option A

4. Quick Check:

   Pure virtual = 0 + override implementations [OK]

Hint: Use '= 0' for pure virtual and override in derived [OK]

Common Mistakes:

• Using '= 1' instead of '= 0' for pure virtual
• Not implementing pure virtual functions in derived classes
• Defining interface functions with bodies