Object interaction in C++ - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

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

C++

class Car {};

int main() {
    Car [1];
    return 0;
}

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

AmyCar

Bcar1

CCar

Dobject

Attempts:

3 left

💡 Hint

Common Mistakes

Using the class name as the object name.

Forgetting to name the object.

The correct way to create an object named myCar of class Car is Car myCar;.

2fill in blank

medium

Complete the code to call the startEngine method on the object myCar.

C++

class Car {
public:
    void startEngine() {}
};

int main() {
    Car myCar;
    myCar.[1]();
    return 0;
}

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

ArunEngine

BstopEngine

CengineStart

DstartEngine

Attempts:

3 left

💡 Hint

Common Mistakes

Using a wrong method name.

Forgetting the parentheses after the method name.

To call the method startEngine on the object myCar, use myCar.startEngine();.

3fill in blank

hard

Fix the error in the code to access the speed attribute of the object car1.

C++

class Car {
public:
    int speed;
};

int main() {
    Car car1;
    car1.[1] = 100;
    return 0;
}

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

Acar1.speed

BSpeed

Cspeed

Dthis.speed

Attempts:

3 left

💡 Hint

Common Mistakes

Using uppercase 'Speed' instead of 'speed'.

Trying to use 'this' pointer outside class methods.

The attribute speed is accessed directly by car1.speed. Inside main, to assign a value, use car1.speed = 100;. The blank is for the attribute name after the dot.

4fill in blank

hard

Fill both blanks to create a method setSpeed that sets the speed attribute.

C++

class Car {
public:
    int speed;
    void [1](int [2]) {
        speed = [2];
    }
};

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

AsetSpeed

Bspeed

CnewSpeed

DupdateSpeed

Attempts:

3 left

💡 Hint

Common Mistakes

Using the attribute name as parameter name without distinction.

Using wrong method names.

The method name is setSpeed and the parameter name is newSpeed. Inside the method, speed = newSpeed; sets the attribute.

5fill in blank

hard

Fill all three blanks to create an object car2, set its speed to 80, and print it.

C++

#include <iostream>

class Car {
public:
    int speed;
    void setSpeed(int s) { speed = s; }
    int getSpeed() { return speed; }
};

int main() {
    Car [1];
    [1].[2](80);
    std::cout << [1].[3]() << std::endl;
    return 0;
}

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

Acar2

BsetSpeed

CgetSpeed

Dspeed

Attempts:

3 left

💡 Hint

Common Mistakes

Using different object names in different places.

Calling methods with wrong names.

Create the object named car2. Call setSpeed(80) on it to set speed. Then call getSpeed() to get the speed and print it.

Practice

1. What does it mean when one object calls a method on another object in C++?

easy

A. The first object is asking the second object to perform an action.

B. The first object copies the second object's data.

C. The first object deletes the second object.

D. The first object creates a new object inside the second object.

Solution

1. Step 1: Understand method calls between objects

   When one object calls a method on another, it requests that object to do something or provide information.

2. Step 2: Differentiate from other actions

   Copying data, deleting, or creating objects are different operations, not the same as calling a method.

3. Final Answer:

   The first object is asking the second object to perform an action. -> Option A

4. Quick Check:

   Object method call = request action [OK]

Hint: Method call means asking another object to do something [OK]

Common Mistakes:

• Confusing method call with copying data
• Thinking method call deletes objects
• Assuming method call creates new objects inside others

2. Which of the following is the correct syntax to pass an object obj of class Car to a method drive of another object driver?

easy

A. driver.drive(obj);

B. drive.driver(obj);

C. driver->drive(obj);

D. drive(obj).driver;

Solution

1. Step 1: Identify correct method call syntax

   For an object driver, calling method drive with argument obj uses dot notation: driver.drive(obj);

2. Step 2: Eliminate incorrect syntax

   drive.driver(obj); reverses object and method, driver->drive(obj); is for pointers, and drive(obj).driver; is invalid.

3. Final Answer:

   driver.drive(obj); -> Option A

4. Quick Check:

   Object.method(argument) = driver.drive(obj) [OK]

Hint: Use object.method(argument) syntax for calling methods [OK]

Common Mistakes:

• Using arrow operator on non-pointer objects
• Swapping method and object names
• Incorrect punctuation or order

3. What is the output of this code?

#include <iostream>

class Light {
public:
    void turnOn() { std::cout << "Light is ON" << std::endl; }
};

class Switch {
public:
    void press(Light &l) { l.turnOn(); }
};

int main() {
    Light roomLight;
    Switch wallSwitch;
    wallSwitch.press(roomLight);
    return 0;
}

medium

A. Compilation error

B. Light is OFF

C. No output

D. Light is ON

Solution

1. Step 1: Understand method call inside press()

   The press method calls turnOn() on the passed Light object, which prints "Light is ON".

2. Step 2: Trace main function calls

   wallSwitch.press(roomLight); calls turnOn() on roomLight, so output is "Light is ON".

3. Final Answer:

   Light is ON -> Option D

4. Quick Check:

   Method call prints "Light is ON" [OK]

Hint: Method calls inside other methods print output [OK]

Common Mistakes:

• Assuming no output because press() returns void
• Confusing reference passing with copying
• Thinking code causes compilation error

4. Identify the error in this code snippet:

#include <iostream>

class Engine {
public:
    void start() { std::cout << "Engine started" << std::endl; }
};

class Car {
public:
    void ignite(Engine e) { e.start(); }
};

int main() {
    Engine myEngine;
    Car myCar;
    myCar.ignite(myEngine);
    return 0;
}

medium

A. Method start() is not declared correctly.

B. Car class should inherit from Engine.

C. Passing Engine by value causes unnecessary copying.

D. Cannot call start() on Engine object.

Solution

1. Step 1: Analyze parameter passing in ignite()

   The method ignite takes Engine e by value, which copies the object unnecessarily.

2. Step 2: Suggest improvement

   Passing by reference (Engine &e) avoids copying and is more efficient.

3. Final Answer:

   Passing Engine by value causes unnecessary copying. -> Option C

4. Quick Check:

   Pass objects by reference to avoid copying [OK]

Hint: Pass objects by reference to avoid copies [OK]

Common Mistakes:

• Thinking method start() is incorrect
• Assuming inheritance is required
• Believing start() cannot be called on Engine

5. Given classes Teacher and Student, how can you design a method in Teacher that lets a teacher assign a grade to a student object? Choose the best approach.

hard

A. Make Student inherit from Teacher and override a grade method.

B. Define a method in Teacher that takes a Student reference and an int grade, then calls a Student method to set the grade.

C. Store grades only inside Teacher without involving Student objects.

D. Use global variables to store grades for all students.

Solution

1. Step 1: Understand object interaction for assigning grades

   The Teacher should interact with Student by calling a method on Student to set the grade, passing the grade value.

2. Step 2: Evaluate options

   Define a method in Teacher that takes a Student reference and an int grade, then calls a Student method to set the grade. correctly uses a method in Teacher that takes a Student reference and grade, then calls Student's method to set the grade. Other options misuse inheritance, ignore object interaction, or use bad global state.

3. Final Answer:

   Define a method in Teacher that takes a Student reference and an int grade, then calls a Student method to set the grade. -> Option B

4. Quick Check:

   Teacher calls Student method to assign grade [OK]

Hint: Teacher calls Student method with grade parameter [OK]

Common Mistakes:

• Misusing inheritance for unrelated roles
• Ignoring encapsulation by using globals
• Storing grades only in Teacher without Student update