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".