Classes and objects in C++ - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a class named Car.

C++

class [1] {
public:
    int speed;
};

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

ASpeed

Bcar

CVehicle

DCar

Attempts:

3 left

💡 Hint

Common Mistakes

Using lowercase names for class names.

Using unrelated names like Vehicle or Speed.

The class name should be Car to match the declaration.

2fill in blank

medium

Complete the code to create an object named myCar of class Car.

C++

Car [1];

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

ACar

BmyCar

Ccar1

Dobject

Attempts:

3 left

💡 Hint

Common Mistakes

Using the class name instead of the object name.

Using generic names like object.

The object name should be myCar as requested.

3fill in blank

hard

Fix the error in the code to access the speed attribute of myCar.

C++

myCar.[1] = 100;

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

Aspeed()

BSpeed

Cspeed

DgetSpeed

Attempts:

3 left

💡 Hint

Common Mistakes

Using uppercase attribute names.

Adding parentheses to access attributes.

The attribute name is speed and accessed without parentheses.

4fill in blank

hard

Fill both blanks to define a constructor that sets speed to 0.

C++

class Car {
public:
    int speed;
    [1] [2] {
        speed = 0;
    }
};

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

ACar()

Bvoid

C()

D{}

Attempts:

3 left

💡 Hint

Common Mistakes

Adding a return type like void to the constructor.

Using parentheses without braces for the body.

The constructor has the same name as the class Car() and uses braces {} for the body.

5fill in blank

hard

Fill all three blanks to create a method named accelerate that increases speed by 10.

C++

class Car {
public:
    int speed;
    void [1][2] [3] {
        speed += 10;
    }
};

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

Aaccelerate

B()

C{}

DspeedUp

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect method names.

Omitting parentheses or braces.

The method name is accelerate, followed by parentheses () and braces {} for the body.

Practice

1. What is the main purpose of a class in C++?

easy

A. To group data and functions that represent a real-world concept

B. To execute a program faster

C. To store only numbers

D. To replace variables with constants

Solution

1. Step 1: Understand what a class represents

   A class groups related data (variables) and functions (methods) to model real-world things.

2. Step 2: Compare options with the definition

   Only To group data and functions that represent a real-world concept correctly describes this purpose; others are unrelated to classes.

3. Final Answer:

   To group data and functions that represent a real-world concept -> Option A

4. Quick Check:

   Class = group data + functions [OK]

Hint: Classes bundle data and actions together [OK]

Common Mistakes:

• Thinking classes only store numbers
• Confusing classes with program speed
• Believing classes replace variables

2. Which of the following is the correct way to declare a class named Car in C++?

easy

A. class Car { };

B. Car class { };

C. class = Car { };

D. class Car();

Solution

1. Step 1: Recall C++ class declaration syntax

   The correct syntax is: class ClassName { }; with a semicolon at the end.

2. Step 2: Check each option

   class Car { }; matches the correct syntax. Options A, C, and D have syntax errors or wrong order.

3. Final Answer:

   class Car { }; -> Option A

4. Quick Check:

   class ClassName { }; syntax [OK]

Hint: Remember: class name follows 'class' keyword, ends with semicolon [OK]

Common Mistakes:

• Writing 'Car class' instead of 'class Car'
• Missing semicolon after class body
• Using parentheses after class name

3. What will be the output of this C++ code?

class Box {
public:
  int length;
  void setLength(int len) { length = len; }
  int getLength() { return length; }
};

int main() {
  Box box1;
  box1.setLength(10);
  std::cout << box1.getLength();
  return 0;
}

medium

A. length

B. 0

C. Compilation error

D. 10

Solution

1. Step 1: Understand the class methods

   The method setLength sets the length to 10 for box1. The method getLength returns this value.

2. Step 2: Trace the output statement

   std::cout << box1.getLength(); prints the value 10 stored in length.

3. Final Answer:

   10 -> Option D

4. Quick Check:

   setLength(10) then getLength() = 10 [OK]

Hint: Setter sets value, getter returns it [OK]

Common Mistakes:

• Forgetting to call setLength before getLength
• Expecting default value 0 without initialization
• Confusing method names

4. Identify the error in this class definition:

class Person {
  string name;
  int age;
public:
  void setName(string n) { name = n; }
  void setAge(int a) { age = a; }
};

medium

A. Missing semicolon after class definition

B. Missing public: before data members

C. Missing #include <string> header

D. Incorrect function return types

Solution

1. Step 1: Check class syntax

   The class ends with a semicolon, and functions have correct return types (void). Data members are private by default, which is allowed.

2. Step 2: Check for missing includes

   The code uses string but does not include <string> header, which is required for std::string.

3. Final Answer:

   Missing #include <string> header -> Option C

4. Quick Check:

   Use #include <string> for string type [OK]

Hint: Always include headers for standard types [OK]

Common Mistakes:

• Assuming private members need public keyword
• Forgetting to add semicolon after class
• Confusing function return types

5. Given this class:

class Student {
  int id;
  string name;
public:
  Student(int i, string n) : id(i), name(n) {}
  void display() {
    std::cout << "ID: " << id << ", Name: " << name << std::endl;
  }
};

int main() {
  Student s1(101, "Alice");
  s1.display();
  return 0;
}

What concept does this code demonstrate?

hard

A. Inheritance between classes

B. Constructor initializing object data

C. Function overloading

D. Static class members

Solution

1. Step 1: Identify the special function in the class

   The function Student(int i, string n) : id(i), name(n) {} is a constructor initializing the object's data members.

2. Step 2: Check other options

   There is no inheritance, no function overloading, and no static members shown.

3. Final Answer:

   Constructor initializing object data -> Option B

4. Quick Check:

   Constructor sets initial values [OK]

Hint: Look for function named as class for constructor [OK]

Common Mistakes:

• Confusing constructor with regular method
• Thinking this shows inheritance
• Assuming static members without keyword