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What are Classes and How to Use Them in C++

  • 5 min
  • Beginner

In C++, a CLASS is a data type that groups data and functions under a single name and represents a real-world concept or entity.

Classes provide a template for creating instances of the class. Each instance has its own state *(stored in fields) and behavior (defined in methods).

Classes allow us to encapsulate related data and functionality, which facilitates code reuse and maintenance of our applications. They are the foundation of object-oriented programming.

If you want to learn more, check out the Object-Oriented Programming Course

Classes in C++

The basic syntax for defining a class in C++ is as follows:

class ClassName
{
public:
    // Field definitions
    DataType fieldName;

    // Property definitions (access methods)
    DataType getPropertyName() const;
    void setPropertyName(DataType value);

    // Method definitions
    ReturnType methodName(Parameters);
};
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  • ClassName: The name of the class, which must be unique.
  • DataType: The data type of the fields and properties.
  • fieldName: The name of the field.
  • methodName: The name of the method.
  • ReturnType: The data type that the method returns.
  • Parameters: The variables used to pass information to the method.

Classes are generally defined in files with the .cpp or .h (header) extension.

Composition of a Class in C++

A class in C++ is composed of three main elements: fields (attributes), properties (accessor methods), and methods (member functions).

Persona
  • stringfechaNacimiento
  • stringdni
  • stringSaludar()
  • stringMandarEmail()

Fields represent the variables or data that are part of a class. These can be of different types (such as integers, strings, booleans, etc.)

Fields are how we store information within a class and allow us to manipulate it according to our needs.

Methods are the actions or behaviors that a class can perform. These can be functions that return a value or procedures that return nothing.

Methods allow us to modularize code and break tasks into smaller, more manageable parts.

Basic Example

To create a class in C++, we use the class keyword followed by the class name and a list of members (fields and methods).

#include <iostream>
#include <string>

class Person
{
public:
    // Fields
    std::string Name;
    int Age;

    // Constructor
    Person(const std::string& name, int age)
        : Name(name), Age(age) {}

    // Method
    void Greet()
    {
        std::cout << "Hello, I'm " << Name << " and I'm " << Age << " years old!" << std::endl;
    }
};

int main()
{
    // Create an object of the Person class
    Person person("Luis", 25);

    // Call the Greet method
    person.Greet();

    return 0;
}
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In this example, we have:

  • Defined a Person class with two fields (Name, Age).
  • Implemented a constructor that initializes these fields.
  • Defined a Greet method that prints a greeting with the person’s name and age.
  • Created an instance of Person and called the Greet method.

Using Classes

Creating Objects

To create an object of a class, we simply declare a variable of the class type and, optionally, call the constructor.

Person person("Ana", 30);
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Accessing Fields and Properties

The fields and properties of a class are accessed using the dot notation (.).

std::cout << person.Name << std::endl;
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Calling Methods

Methods of a class are also called using the dot notation (.).

person.Greet();
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Inheritance and Polymorphism

C++ supports inheritance, which allows creating a new class based on an existing class, and polymorphism, which allows using methods from base and derived classes in a flexible way.

We will see these concepts in the rest of the course

Practical Examples

Representing a Person

Here we define a Person class that contains properties for name and age, a constructor to initialize these values, and a method to greet.

#include <iostream>
#include <string>

class Person
{
private:
    std::string name;
    int age;

public:
    // Constructor
    Person(const std::string& name, int age)
        : name(name), age(age) {}

    // Method
    void Greet() const
    {
        std::cout << "Hello, I'm " << name << " and I'm " << age << " years old!" << std::endl;
    }
};
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Representing a Product

The Product class represents a product with properties for name and price. It includes a constructor to initialize these fields and a method to display the product information.

#include <iostream>
#include <string>

class Product
{
private:
    std::string name;
    double price;

public:
    // Constructor
    Product(const std::string& name, double price)
        : name(name), price(price) {}

    // Method
    void ShowInformation() const
    {
        std::cout << "Product: " << name << ", Price: " << price << std::endl;
    }
};
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