Welcome to Programming and Data Structures

Like a struct, a class defines a new type of data defined in terms of other pieces of data. Something like “A Point is a thing that stores two double values”:

class Point {
    //member variables
    double m_x;
    double m_y;
};

The class declares the state of a given Point is defined by two pieces of data: the m_x and m_y. The variables declared in the class are member variables. (The variables that are “members”, or a part, of the class.)

We have named the member variables with the prefix m_ (e.g. m_x instead of x). This is a convention used to indicate that these variables are member variables (“m_” for “member”) of a class. As we will see later, we will often also have functions that use a mix of member variables and local variables. The m_ prefix helps us to distinguish between the member variables that are a permanent part of the object and temporary local variables.

Think of this 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:

int main() {
    Point p1;
}

Another term for creating an instance of a class is instantiating the class. When we say Point p1; we are instantiating the Point class to create an instance of a Point object named p1.

Here is a memory diagram of p1:

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. We don’t know what values the member variables hold as we have not initialized them.

So far, this feels just like using a struct, only with more emphasis on terminology. However, unlike with a struct, we can’t access those values by saying p1.m_x or p1.m_y. Try compiling this code and notice what happens.

Why is that an error? We will tackle that next.