Welcome to Programming and Data Structures

Like a struct, a class defines a new type of object. That datatype consists of other pieces of data that we refer to as its member variables or as instance variables. However, a class will also have member functions - functions that are a part of the class.

class Point {
    ...member functions...
    ...member variables...
};

Think of the class definition as a “blueprint” that describes what a type of data is. The class Point does not actually create a point anymore than the blueprint of a house gives you something to live in. To work with the Point data type, we must make a Point object from the blueprint. We call the object an instance of the class. For example, we might create a Point object named p1 like this:

class Point {
    ...member functions...

    ...member variables...
    double m_x, m_y;
};

int main() {
    Point p1;
}

Here, we assume that a point has two member variables m_x and m_y. That means any time we make a Point object, we are creating a package that has two named parts:

As usual, the name of the variable p1 appears outside the box, and its data appears inside the box. In this case, the data is two separate member variables, which are represented with two boxes. However, unlike with a struct, we can’t access those values by saying p1.m_x or p1.m_y. That is because the members of a class are inaccessible outside of the class by default. Why are the inaccessible? How do we use them if they are inaccessible? We will tackle that next...