Each function that you declare for your class must have a definition. The definition is also called the function implementation. Like other functions, the definition of a class method has a function header and a function body.
The definition must be in a file that the compiler can find. Most C++ compilers want that file to end with .C, or .CPP. This book uses .CPP, but check your compiler to see what it prefers.
Many compilers assume that files ending with .C are C programs, and that C++ program files end with .CPP. You can use any extension, but .CPP will minimize confusion.
You will need ...