In C, a class is a user-defined data type, a method to encapsulate and manipulate data, and an object is a specific instance of the class. 1. The class keyword is used to define the class, including member variables and member functions, and control visibility through access modifiers; 2. The object can be directly declared, defined or initialized by the array; 3. The constructor is used to initialize the object, without a return type, and can be overloaded; 4. Encapsulate the data protected through private, provide public method access and add verification logic. Mastering classes and objects is the basis for understanding OOP characteristics such as inheritance and polymorphism.

In C, classes and objects are the core concepts of object-oriented programming (OOP). Simply put, a class is like a blueprint, and an object is a concrete instance created based on this blueprint. If you are just starting to get involved in OOP, understanding the relationship between the two can help you write code that has clearer structure and easier maintenance.

What is a class? How to define a class?

A class is a user-defined data type that encapsulates data (variables) and functions (methods) that operate this data.

The basic syntax is as follows:

 class class name {
    // Member access permissions: public, private, protected
    public:
        // Member variable or member function};

For example, let's define a class that represents "human information":

 class Person {
    public:
        string name;
        int age;

        void introduce() {
            cout << "Hi, I&#39;m " << name << " and I&#39;m " << age << " years old." << endl;
        }
};

Here name and age are attributes of the class, introduce() is its behavior.

A few points to note:

• By default, members in the class are private, so you need to use public: to explicitly declare which ones are accessible externally.
• Class definitions usually end with semicolons.

How to create and use objects?

With classes, you can create objects like using built-in types.

 Person person1;
person1.name = "Alice";
person1.age = 30;
person1.introduce(); // Output: Hi, I'm Alice and I'm 30 years old.

You can also define multiple objects at once:

 Person person1, person2;

Or initialize the object array outside the class:

 Person people[2];
people[0].name = "John";
people[0].age = 25;

It should be noted that if the members in the class are private, they cannot be accessed directly like the one above and must be operated indirectly through public methods (getter/setter).

Constructor: Let the object have initial value as soon as it is born

The constructor is automatically called every time an object is created. We can define the constructor ourselves to set the initial value.

 class Person {
    public:
        string name;
        int age;

        // Constructor Person(string n, int a) {
            name = n;
            age = a;
        }

        void introduce() {
            cout << "Hi, I&#39;m " << name << " and I&#39;m " << age << " years old." << endl;
        }
};

Then create the object like this:

 Person person2("Bob", 28);
person2.introduce(); // Output: Hi, I'm Bob and I'm 28 years old.

Some details:

• The constructor has no return type (including void)
• There can be multiple constructors (overloaded)
• If not written, the compiler will generate a default constructor (no parameters)

Encapsulation and access modifier: Protect your data

C provides three types of access controls:

• public : externally accessible
• private : Only accessible inside the class
• protected : The derived class can be accessed (the inheritance section will be discussed later)

Usually we will set the data to private and provide public methods to access them:

 class Person {
    private:
        string name;
        int age;

    public:
        void setName(string n) { name = n; }
        void setAge(int a) { age = a; }

        void introduce() {
            cout << "Hi, I&#39;m " << name << " and I&#39;m " << age << " years old." << endl;
        }
};

The advantage of this is that you can add verification logic when setting values ??to prevent illegal data.

Basically that's it. Classes and objects are a very basic but also very powerful part of C programming. Only by mastering them well can we better understand and use advanced features such as inheritance and polymorphism.

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