usage of extern pg 209

[Mike]

I'm really not sure about extern.  I guess to start from the beginning, the @interface section declares instance variables and methods while the @implementation section defines instance variables and methods.  At this point it sounds like extern is used to declare a instance variable that has been defined or declared already?   Given that we have inheritance in OOP I don't understand the concept of it.

Thanks
Mike

[skochan]

extern is not used for instance variables.  It's used to reference variables defined outside of any method or function, possibly even in another source file.  This allows you to create variables whose is scope is global.  These global variables don't belong to any method or function (that is, they're not local) and therefore their values can be shared among multiple methods and functions.

Cheers,

Steve

[Mike]

On page 210 I see

 int gGlobalVar = 5 

Since this is defined then any method or function can use it.  So if I wanted to use gGlobalVar in my implentation file I would add extern gGlobalVar at the top of that file?

Thanks
Mike

 

[skochan]

Yes, or from a self-documenting point of view, put individual extern declarations inside the particular method(s) that wanted to access the variable.

Cheers,

Steve