C++ Classes and Objects

• Classes and Objects are fundamental concepts in object-oriented programming (OOP).
• The topics covered are Class Definitions and Objects, Member Functions, Data Members, Get and Set functions and Constructors.

C++ Classes/Objects

• C++ supports an object-oriented style of programming, allowing complex problems to be divided into smaller sets via object creation.
• C++ is an object-oriented programming language.
• Everything in C++ is associated with classes and objects, along with their attributes and methods.
• Real-world example: a car is an object with attributes like weight and color, and methods like drive and brake.
• Attributes and methods are basically variables and functions that belong to a class and are often referred to as "class members."
• An object is a collection of data and functions that act on that data.
• A class is a user-defined data type and acts as an object constructor (a "blueprint" for object creation).

Class Definitions

• A class needs to be defined before an object can be created.
• A class serves as a blueprint for the object.
• Classes are defined in C++ using the class keyword, followed by the class name.
• The class body is defined inside curly brackets {} and terminated by a semicolon ;.

Example Class

class MyClass
{ 
    // The class
public: 
    // Access specifier
    int myNum; 
    // Attribute
    string myString; 
    // Attribute
};

• class keyword creates a class named MyClass.
• The public keyword is an access specifier that determines the accessibility of members (attributes and methods) from outside the class.
• myNum is an integer variable, and myString is a string variable; both are declared as attributes within the class.
• The class definition ends with a semicolon ;.

Object Creation

• In C++, an object is created from a class.
• An object of MyClass has been created, so it can be used to create other objects.
• To create a MyClass object, specify the class name followed by the object name, e.g., MyClass myObj;.
• To access class attributes, such as myNum and myString, use the dot syntax (.) on the object, e.g., myObj.myNum.

Example Object

class MyClass {
public:
  int myNum;
  string myString;
};

int main() {
  MyClass myObj; // Create an object of MyClass

  // Access attributes and set values
  myObj.myNum = 15;
  myObj.myString = "Welcome to OOP";

  // Print attribute values
  cout << myObj.myNum << "\n"; // Output: 15
  cout << myObj.myString; // Output: Welcome to OOP
  return 0;
}

Multiple Objects

• You can create multiple objects from one class.

class Car {
public:
  string brand;
  string model;
  int year;
};

int main() {
  // Create an object of Car
  Car Obj1;
  Obj1.brand = "Mercedes";
  Obj1.model = "E class";
  Obj1.year = 2019;

  // Create another object of Car
  Car Obj2;
  Obj2.brand = "Toyota";
  Obj2.model = "Fielder";
  Obj2.year = 2015;

  // Print attribute values
  cout << Obj1.brand << " " << Obj1.model << " " << Obj1.year
       << "\n"; // Output: Mercedes E class 2019
  cout << Obj2.brand << " " << Obj2.model << " " << Obj2.year
       << "\n"; // Output: Toyota Fielder 2015
  return 0;
}

Class Methods

• Methods are functions that belong to the class.
• Functions that belong to a class can be defined in two ways:
  • Inside the class definition.
  • Outside the class definition.
• Methods access attributes just like attributes, by creating an object of the class and by using the dot syntax (.).

Inside Class Definition

class MyClass {
public:
  void Display() { // Method defined inside the class
    cout << "WELCOME TO OOP PROGRAMMING!";
  }
};

int main() {
  MyClass myObj;        // Create an object of MyClass
  myObj.Display(); // Call the method
  return 0;
}

Outside Class Definition

class MyClass {
public:
  void Display(); // Method declaration
};

// Method definition outside the class
void MyClass::Display() { cout << "WELCOME TO OOP PROGRAMMING!"; }

int main() {
  MyClass myObj;       // Create an object of MyClass
  myObj.Display(); // Call the method
  return 0;
}

Example Calling methods with arguments

#include <iostream>
using namespace std;

class Car {
public:
  int speed(int maxSpeed);
};

int Car::speed(int maxSpeed) { return maxSpeed; }

int main() {
  Car myObj;                  // Create an object of Car
  cout << myObj.speed(200); // Call the method with an argument
  return 0;
}

Exercises

• Write a program that calculates the area and perimeter of a triangle of sides 3, 4 and 5 by creating a Triangle class with a function that prints the area and perimeter.
• Write a C++ program that implements a class called sphere. The program accepts the radius and specifies the volume. The program outputs the volume. Use pie as 3.14. formula: volume=4/3πr³.