§2024-12-03
OOP stands for Object-Oriented Programming.
Everything in C++ is associated with classes and objects, along with its attributes and methods. For example: in real life, a car is an object. The car has attributes, such as weight and color, and methods, such as drive and brake.
Attributes and methods are basically variables and functions that belongs to the class. These are often referred to as "class members".
A class is a user-defined data type that we can use in our program, and it works as an object constructor, or a "blueprint" for creating objects.
// Online C++ compiler to run C++ program online
#include <iostream>
using namespace std;
class MyClass { // The class
public: // Access specifier
int myNum; // Attribute (int variable)
string myString; // Attribute (string variable)
};
int main() {
MyClass myObj; // Create an object of MyClass
// Access attributes and set values
myObj.myNum = 15;
myObj.myString = "Some text";
// Print attribute values
cout << myObj.myNum << "\n";
cout << myObj.myString;
return 0;
}
/*
15
Some text
*/
¶Class Methods
Methods are functions that belongs to the class.
There are two ways to define functions that belongs to a class:
- Inside class definition
- Outside class definition
- inside Example
class MyClass { // The class
public: // Access specifier
void myMethod() { // Method/function defined inside the class
cout << "Hello World!";
}
};
int main() {
MyClass myObj; // Create an object of MyClass
myObj.myMethod(); // Call the method
return 0;
}
- Outside Example
class MyClass { // The class
public: // Access specifier
void myMethod(); // Method/function declaration
};
// Method/function definition outside the class
void MyClass::myMethod() {
cout << "Hello World!";
}
int main() {
MyClass myObj; // Create an object of MyClass
myObj.myMethod(); // Call the method
return 0;
}
- Parameters, You can also add parameters:
#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;
}
¶C++ Constructors
A constructor in C++ is a special method that is automatically called when an object of a class is created.
To create a constructor, use the same name as the class, followed by parentheses ():
class MyClass { // The class
public: // Access specifier
// The constructor has the same name as the class, it is always public, and it does not have any return value.
MyClass() { // Constructor
cout << "Hello World!";
}
};
int main() {
MyClass myObj; // Create an object of MyClass (this will call the constructor)
return 0;
} // Hello World!
¶Constructor Parameters
Constructors can also take parameters (just like regular functions), which can be useful for setting initial values for attributes.
class Car { // The class
public: // Access specifier
string brand; // Attribute
string model; // Attribute
int year; // Attribute
Car(string x, string y, int z) { // Constructor with parameters
brand = x;
model = y;
year = z;
}
};
int main() {
// Create Car objects and call the constructor with different values
Car carObj1("BMW", "X5", 1999);
Car carObj2("Ford", "Mustang", 1969);
// Print values
cout << carObj1.brand << " " << carObj1.model << " " << carObj1.year << "\n";
cout << carObj2.brand << " " << carObj2.model << " " << carObj2.year << "\n";
return 0;
}
Just like functions, constructors can also be defined outside the class. First, declare the constructor inside the class, and then define it outside of the class by specifying the name of the class, followed by the scope resolution :: operator, followed by the name of the constructor (which is the same as the class):
class Car { // The class
public: // Access specifier
string brand; // Attribute
string model; // Attribute
int year; // Attribute
Car(string x, string y, int z); // Constructor declaration
};
// Constructor definition outside the class
Car::Car(string x, string y, int z) {
brand = x;
model = y;
year = z;
}
int main() {
// Create Car objects and call the constructor with different values
Car carObj1("BMW", "X5", 1999);
Car carObj2("Ford", "Mustang", 1969);
// Print values
cout << carObj1.brand << " " << carObj1.model << " " << carObj1.year << "\n";
cout << carObj2.brand << " " << carObj2.model << " " << carObj2.year << "\n";
return 0;
}
¶C++ Access Specifiers
class MyClass {
public: // Public access specifier
int x; // Public attribute
private: // Private access specifier
int y; // Private attribute
};
int main() {
MyClass myObj;
myObj.x = 25; // Allowed (public)
myObj.y = 50; // Not allowed (private)
return 0;
}
Note: By default, all members of a class are private if you don't specify an access specifier:
class MyClass {
int x; // Private attribute
int y; // Private attribute
};
¶Encapsulation
The meaning of Encapsulation, is to make sure that "sensitive" data is hidden from users. To achieve this, you must declare class variables/attributes as private (cannot be accessed from outside the class). If you want others to read or modify the value of a private member, you can provide public get and set methods.
#include <iostream>
using namespace std;
class Employee {
private:
// Private attribute
int salary;
public:
// Setter
void setSalary(int s) {
salary = s;
}
// Getter
int getSalary() {
return salary;
}
};
int main() {
Employee myObj;
myObj.setSalary(50000);
cout << myObj.getSalary();
return 0;
}
¶Inheritance
In C++, it is possible to inherit attributes and methods from one class to another. We group the "inheritance concept" into two categories:
derived class (child) - the class that inherits from another class
base class (parent) - the class being inherited from
To inherit from a class, use the : symbol.
It is useful for code reusability: reuse attributes and methods of an existing class when you create a new class.
// Base class
class Vehicle {
public:
string brand = "Ford";
void honk() {
cout << "Tuut, tuut! \n" ;
}
};
// Derived class
class Car: public Vehicle {
public:
string model = "Mustang";
};
int main() {
Car myCar;
myCar.honk();
cout << myCar.brand + " " + myCar.model;
return 0;
}
¶Multilevel Inheritance
A class can also be derived from one class, which is already derived from another class.
In the following example, MyGrandChild is derived from class MyChild (which is derived from MyClass).
// Base class (parent)
class MyClass {
public:
void myFunction() {
cout << "Some content in parent class." ;
}
};
// Derived class (child)
class MyChild: public MyClass {
};
// Derived class (grandchild)
class MyGrandChild: public MyChild {
};
int main() {
MyGrandChild myObj;
myObj.myFunction();
return 0;
}
¶Multiple Inheritance
A class can also be derived from more than one base class, using a comma-separated list:
// Base class
class MyClass {
public:
void myFunction() {
cout << "Some content in parent class." ;
}
};
// Another base class
class MyOtherClass {
public:
void myOtherFunction() {
cout << "Some content in another class." ;
}
};
// Derived class
class MyChildClass: public MyClass, public MyOtherClass {
};
int main() {
MyChildClass myObj;
myObj.myFunction();
myObj.myOtherFunction();
return 0;
}
¶Access Specifiers
You learned from the Access Specifiers chapter that there are three specifiers available in C++. Until now, we have only used public (members of a class are accessible from outside the class) and private (members can only be accessed within the class). The third specifier, protected, is similar to private, but it can also be accessed in the inherited class:
protected is to allow access within the class and in derived classes
// Base class
class Employee {
protected: // Protected access specifier
int salary;
};
// Derived class
class Programmer: public Employee {
public:
int bonus;
void setSalary(int s) {
salary = s;
}
int getSalary() {
return salary;
}
};
int main() {
Programmer myObj;
myObj.setSalary(50000);
myObj.bonus = 15000;
cout << "Salary: " << myObj.getSalary() << "\n";
cout << "Bonus: " << myObj.bonus << "\n";
return 0;
}
¶ C++ Polymorphism
Polymorphism means "many forms", and it occurs when we have many classes that are related to each other by inheritance.
Like we specified in the previous chapter; Inheritance lets us inherit attributes and methods from another class. Polymorphism uses those methods to perform different tasks. This allows us to perform a single action in different ways.
For example, think of a base class called Animal that has a method called animalSound(). Derived classes of Animals could be Pigs, Cats, Dogs, Birds - And they also have their own implementation of an animal sound (the pig oinks, and the cat meows, etc.):
// Base class
class Animal {
public:
void animalSound() {
cout << "The animal makes a sound \n";
}
};
// Derived class
class Pig : public Animal {
public:
void animalSound() {
cout << "The pig says: wee wee \n";
}
};
// Derived class
class Dog : public Animal {
public:
void animalSound() {
cout << "The dog says: bow wow \n";
}
};
// Base class
class Animal {
public:
void animalSound() {
cout << "The animal makes a sound \n";
}
};
// Derived class
class Pig : public Animal {
public:
void animalSound() {
cout << "The pig says: wee wee \n";
}
};
// Derived class
class Dog : public Animal {
public:
void animalSound() {
cout << "The dog says: bow wow \n";
}
};
int main() {
Animal myAnimal;
Pig myPig;
Dog myDog;
myAnimal.animalSound();
myPig.animalSound();
myDog.animalSound();
return 0;
}