When you include a file within your source file, the code within that header file will be accessible to your code. Your include file may contain whole function or class definitions (these will be covered in later chapters), but this will result in the problem mentioned previously: multiple definitions of a function or class. Instead, you can declare a class or function prototype, which indicates how calling code will call the function without actually defining it. Clearly, the code will have to be defined elsewhere, and this could be a source file or a library, but the compiler will be happy because it only sees one definition.
A library is code that has already been defined; it has been fully debugged and tested, and therefore, ...
