It is a best practice for a $h$ header file to begin with a preprocessor directive like

#ifndef SHAPE$\_$H$\_$

and end with another directive like

#endif $//$ SHAPE$\_$H$\_$

Why is this a best practice?

It prevents compile errors in the event that the header is included multiple times.

It allows the compiler to optimize the code, making programs run faster.

It prevents runtime errors in the event that an undefined function is called.

It prevents compile errors in the event that a project has two variables with the same identifier.

It prevents compile errors in the event that a project has two headers with the same filename.