who can tell differ from "int" and "extern int"? "the C++ Primer" says ,"extern int" is always written in the header files?..
thanks!!

I believe that External (or Global) Integers (extern int) are able to be used outside of a function, whereas a standard integer is supposed to be used inside of a function.

(keep in mind, I'm in no way a cpp programmer, i simply know how to google things properly to have a proper outcome)

Also I still haven't really got it,but THANKS indeed!

extern int a;
means the 'a' was defined in some other files .