How to invoke a function from another .cpp source file

Senario:

There are two source files a.cpp and b.cpp both in a project. a.cpp includes a function called viod show(). Now I wanna invoke that function in b.cpp.

As I know, there are two ways we can use to invoke a function from another .cpp source file:

1. In b.cpp, use "extern void show()" statement to declare an extern function.

2. add a new header file(we call a.h for example), in that header file, we declare that function we want other source files to see and use by adding these codes:

#ifndef A_H #define A_H void show(); #endif 

then, we need to add "include "a.h"" statement at the beginning of b.cpp, after "include <iostream>" statement. After that, we can invoke show() in b.cpp now.

PS: there is no restriction about the name of the header file, I use "b.h" as its name here is just to say that the function included in it is defined in a.cpp file.

PPS: Does anyone know the differences between these two methods?