Question

I’m new to Objective-C, but I am curious about something that I haven’t really seen addressed anywhere else.

Could anyone tell me what is the difference between a private variable that is declared at the @interface block versus a variable that is declared within the @implementation block outside of the class methods, i.e:

@interface Someclass : NSObject {

 NSString *forExample;

}

@end

vs.

@implementation Someclass

 NSString *anotherExample;

-(void)methodsAndSuch {}

@end

It seems both variables ( forExample, anotherExample ) are equally accessible throughout the class and I can’t really find a difference in their behaviour. Is the second form also called an instance variable?

Answer 1

The latter is not defining an instance variable. Rather, it is defining a global variable in the .m file. Such a variable is not unique to or part of any object instance.

Such globals have their uses (roughly equivalent C++ static members; e.g. storing a singleton instance), but normally you would define them at the top of the file before the @implementation directive.