Object interaction in C++ - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of method interaction between objects

What is the output of the following C++ code involving two interacting objects?

C++

#include <iostream>
using namespace std;

class Engine {
public:
    int power;
    Engine(int p) : power(p) {}
    int getPower() { return power; }
};

class Car {
public:
    Engine engine;
    Car(int p) : engine(p) {}
    void showPower() {
        cout << "Power: " << engine.getPower() << endl;
    }
};

int main() {
    Car car(150);
    car.showPower();
    return 0;
}

A

Power: 150

B

Power: 0

C

Compilation error due to missing default constructor

D

Runtime error due to uninitialized engine

💡 Hint

Check how the Engine object is initialized inside the Car constructor.

The Car constructor initializes its Engine member with power 150. The showPower method calls Engine's getPower, printing 150.

🧠 Conceptual

intermediate

Understanding object references in method calls

Consider two classes where one object calls a method on another object passed by reference. What is the effect of modifying the passed object inside the method?

C++

class Box {
public:
    int length;
    Box(int l) : length(l) {}
    void doubleLength() { length *= 2; }
};

class Worker {
public:
    void process(Box &b) {
        b.doubleLength();
    }
};

int main() {
    Box box(5);
    Worker w;
    w.process(box);
    // What is box.length now?
}

A 10

B 5

C Compilation error due to missing const

D Undefined behavior due to dangling reference

💡 Hint

Passing by reference allows the method to modify the original object.

The process method receives Box by reference and calls doubleLength, which doubles length from 5 to 10.

🔧 Debug

advanced

Identify the cause of runtime error in object interaction

This code compiles but crashes at runtime. What causes the crash?

C++

#include <iostream>
using namespace std;

class Node {
public:
    int value;
    Node* next;
    Node(int v) : value(v), next(nullptr) {}
};

class List {
public:
    Node* head;
    List() : head(nullptr) {}
    void add(int v) {
        Node n(v);
        n.next = head;
        head = &n;
    }
    void print() {
        Node* current = head;
        while(current) {
            cout << current->value << " ";
            current = current->next;
        }
        cout << endl;
    }
};

int main() {
    List l;
    l.add(10);
    l.add(20);
    l.print();
    return 0;
}

A The Node constructor is missing initialization of next pointer

B The head pointer is never initialized causing null pointer dereference

C The add method stores address of a local variable which is destroyed after add returns

D The print method modifies the list causing infinite loop

💡 Hint

Local variables inside functions are destroyed when the function ends.

In add(), Node n is a local variable. Its address is assigned to head, but after add() returns, n is destroyed. Accessing head causes undefined behavior and crash.

📝 Syntax

advanced

Correct syntax for object interaction with pointers

Which option correctly creates two objects and makes one object call a method on the other using pointers?

C++

class Printer {
public:
    void printMessage() { std::cout << "Hello" << std::endl; }
};

int main() {
    Printer* p1 = new Printer();
    Printer* p2 = new Printer();
    // Call printMessage of p1 using p2
}

A p2->printMessage();

B p1->printMessage();

C p2->p1->printMessage();

D (*p2).p1.printMessage();

💡 Hint

Use the arrow operator to call methods on pointers.

To call printMessage on p1, use p1->printMessage(). p2->printMessage() calls p2's method, not p1's. Options B and C are invalid syntax.

🚀 Application

expert

Determine the final state of interacting objects after method calls

Given these classes, what is the final value of the counter in the Manager object after main runs?

C++

#include <iostream>
using namespace std;

class Worker {
public:
    int id;
    Worker(int i) : id(i) {}
    void doWork(int &counter) {
        counter += id;
    }
};

class Manager {
public:
    int counter = 0;
    void assignWork(Worker &w) {
        w.doWork(counter);
    }
};

int main() {
    Manager m;
    Worker w1(3), w2(7);
    m.assignWork(w1);
    m.assignWork(w2);
    cout << m.counter << endl;
    return 0;
}

A 3

B 0

C 7

D 10

💡 Hint

The Manager's counter is updated by adding each Worker's id.

assignWork calls doWork which adds Worker's id to Manager's counter. After both calls, counter = 3 + 7 = 10.

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