Introduction - If you have any usage issues, please Google them yourself
The key points of the program lies in the function declaration, function declaration can not be omitted here, if the third and fourth lines of the function declaration: void add (int x, int y) void add2 (float x, float y) omitted add inside the functions defined in the next call add2: [add2 (x, y) ] will prompt an error, because of the declaration, the system does not know call add2 or the add what