Interface-like behavior in C++ - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a pure virtual function in a C++ class.

C++

class Shape {
public:
    virtual void draw() [1];
};

Drag options to blanks, or click blank then click option'

A= 0

B{}

C;

D()

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting the = 0 and just ending with a semicolon.

Adding an empty function body instead of making it pure virtual.

In C++, a pure virtual function is declared by assigning = 0 at the end of the function declaration.

2fill in blank

medium

Complete the code to override the pure virtual function in the derived class.

C++

class Circle : public Shape {
public:
    void draw() [1] {
        // drawing code
    }
};

Drag options to blanks, or click blank then click option'

A() override

B()

C(int)

D(void)

Attempts:

3 left

💡 Hint

Common Mistakes

Changing the function parameters.

Forgetting to add override keyword.

When overriding a virtual function, use the same signature and add override to indicate it overrides a base class method.

3fill in blank

hard

Fix the error in the code to prevent instantiation of the abstract class.

C++

Shape s[1];

Drag options to blanks, or click blank then click option'

A[]

B;

C{}

D()

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to create an object of an abstract class.

Not using pointers or references for abstract types.

You cannot create an object of an abstract class. The error is trying to instantiate Shape with parentheses. To fix, remove the instantiation or use a pointer/reference.

4fill in blank

hard

Fill both blanks to declare an interface-like class with a pure virtual destructor.

C++

class Interface {
public:
    virtual ~Interface() [1];
    virtual void execute() [2];
};

Drag options to blanks, or click blank then click option'

A= 0

B{}

C;

D()

Attempts:

3 left

💡 Hint

Common Mistakes

Not declaring the destructor as pure virtual.

Forgetting to provide a destructor body if pure virtual.

To make a class interface-like, declare all functions, including the destructor, as pure virtual by using = 0.

5fill in blank

hard

Fill all three blanks to implement a derived class overriding interface methods correctly.

C++

class Impl : public Interface {
public:
    ~Impl() [1] override [2] {
        // destructor code
    }
    void execute() [3] override {
        // execution code
    }
};

Drag options to blanks, or click blank then click option'

A()

B{}

C;

D(void)

Attempts:

3 left

💡 Hint

Common Mistakes

Omitting function bodies.

Using semicolons instead of braces for function definitions.

Destructor and methods must have matching signatures and bodies. Use () for parameters and {} for function bodies.

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