Commit 4de2a68

The declaration tells the compiler whether is an **int**, a **double**, a **function**, a **class** or some other thing. Furthermore, each name must be declared (directly or indirectly) in every .cpp file in which it is used. When you compile a program, each .cpp file is compiled independently into a compilation unit. The compiler has no knowledge of what names are declared in other compilation units. That means that if you define a class or function or global variable, you must provide a declaration of that thing in each additional .cpp file that uses it. Each declaration of that thing must be exactly identical in all files. A slight inconsistency will cause errors, or unintended behavior, when the linker attempts to merge all the compilation units into a single program.

To minimize the potential for errors, C++ has adopted the convention of using *header files* to contain declarations. You make the declarations in a header file, then use the #include directive in every .cpp file or other header file requires that declaration. The #include directive inserts a copy of the header file directly into the .cpp file prior to compilation.