C++ is an object-oriented programming language, which means that it allows you to define your own data types, called classes, that can contain data and functions. These classes can be used to create objects that represent real-world entities or concepts.

Here's an example of a simple class definition in C++:

class Person {
public:
    // data members
    std::string name;
    int age;

    // member functions
    void say_hello() {
        std::cout << "Hello, my name is " << name << " and I am " << age << " years old.\n";
    }
};

This class defines a Person type that has two data members (name and age) and one member function (say_hello). The public keyword indicates that these members can be accessed from outside the class.

To create an object of this class, you can use the following code:

Person john;
john.name = "John";
john.age = 30;
john.say_hello();

This creates a Person object named john and sets its name and age members. The say_hello() function is then called on the john object, which prints a greeting message to the console.

In addition to member functions, C++ also allows you to define global functions that can operate on objects of a given class. These functions are called non-member functions or friend functions, and they can be declared as follows:

class Rectangle {
public:
    // data members
    double width, height;

    // constructor
    Rectangle(double w, double h) : width(w), height(h) {}

    // friend function declaration
    friend double area(const Rectangle& rect);
};

// friend function definition
double area(const Rectangle& rect) {
    return rect.width * rect.height;
}

In this example, the Rectangle class has a friend function named area() that calculates the area of a rectangle. The friend keyword allows the function to access the private data members of the class.

You can then use this function to calculate the area of a Rectangle object as follows:

Rectangle rect(5.0, 10.0);
double rect_area = area(rect);

This creates a Rectangle object with a width of 5.0 and a height of 10.0, and then calculates its area using the area() function.