Classes and Objects in C++

# CHAPTER 15

Classes and Objects #

1. Introduction #

2. Learning Objectives #

• Define a Class and instantiate Objects.

• Understand and apply Access Modifiers (public, private, protected).

• Define Member Functions inside and outside a class.

• Understand static class members.

3. Defining a Class and Object #

#include <iostream>
#include <string>
using namespace std;

// 1. Define the Class (The Blueprint)
class Car {
  public: // Access modifier (explained below)
    string brand;
    int year;
    
    // Member function (Method)
    void honk() {
        cout << "Beep beep!" << endl;
    }
};

int main() {
    // 2. Create an Object (The physical car)
    Car myCar;
    
    // 3. Access attributes and methods using the dot (.) operator
    myCar.brand = "Toyota";
    myCar.year = 2022;
    
    cout << myCar.brand << " (" << myCar.year << ")" << endl;
    myCar.honk();
    
    return 0;
}

4. Access Modifiers #

1. 1. public: Members are accessible from *outside* the class (e.g., inside main()).

1. 2. private: Members are ONLY accessible from *inside* the class itself. (By default, all members in a C++ class are private).

1. 3. protected: Similar to private, but can also be accessed by inherited classes (Covered in Chapter 17).

class BankAccount {
  private:
    double balance; // Hidden from the outside!
    
  public:
    string accountHolder;
    
    // Public method to safely interact with private data
    void deposit(double amount) {
        if (amount > 0) {
            balance += amount;
        }
    }
    
    void checkBalance() {
        cout << "Balance: $" << balance << endl;
    }
};

int main() {
    BankAccount acc;
    acc.accountHolder = "John";
    // acc.balance = 1000000; // ERROR! Cannot access private member
    
    acc.deposit(500); // Safe way to modify data
    acc.checkBalance();
    
    return 0;
}

5. Member Functions: Inside vs. Outside #

class Calculator {
  public:
    int add(int a, int b); // Declaration only
};

// Definition outside the class
int Calculator::add(int a, int b) {
    return a + b;
}

6. Static Members #

class Employee {
  public:
    static int employeeCount; // Shared by ALL Employee objects
    
    Employee() {
        employeeCount++; // Increment count when a new object is created
    }
};

// Static variables must be initialized outside the class!
int Employee::employeeCount = 0;

int main() {
    Employee e1;
    Employee e2;
    
    // Access static members using the class name, not the object
    cout << "Total Employees: " << Employee::employeeCount << endl; // Prints 2
    return 0;
}

7. Mini Project: Employee Management #

#include <iostream>
#include <string>
using namespace std;

class Employee {
  private:
    int id;
    double salary;
    
  public:
    string name;
    
    // Method to set private data
    void setDetails(int empId, double empSalary) {
        id = empId;
        salary = empSalary;
    }
    
    // Method to display data
    void display() {
        cout << "ID: " << id << " | Name: " << name << " | Salary: $" << salary << endl;
    }
};

int main() {
    Employee emp1;
    emp1.name = "Alice";
    emp1.setDetails(101, 75000);
    
    Employee emp2;
    emp2.name = "Bob";
    emp2.setDetails(102, 68000);
    
    emp1.display();
    emp2.display();
    
    return 0;
}

8. Memory-Level Explanation #

9. Common Mistakes #

• Forgetting public:: Because classes default to private, if you forget the public: label, you won't be able to use your class in main().

• Calling non-static functions via Class Name: Car::honk() is invalid unless honk() is a static function. You must call it on an object: myCar.honk().

10. Exercises #

1. 1. Create a Book class with private attributes for title, author, and pages. Create public methods to set and get these values.

1. 2. In the BankAccount example, add a withdraw() method that ensures the balance doesn't go below 0.

11. MCQ Quiz with Answers #

Q8. Are member functions duplicated in memory for every object created? a) Yes b) No, objects share the same function code in memory Answer: b) No, objects share the same function code in memory

12. Interview Questions #

• Q: What is the exact difference between a struct and a class in C++?

• Q: Explain how static member variables differ from normal member variables.

• Q: Why do we define functions outside the class using :: instead of just writing them inside the class?

13. Summary #

14. Next Chapter Recommendation #