Class variable defined at @implementation rather than @interface?

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

Answer 1

They're very different! The one in @implementation is a global variable not unique to each instance. Imagine there were accessors for both variables, written in the obvious way. Then the difference in behavior is shown here:

Someclass* firstObject = [[Someclass alloc] init];
Someclass* secondObject = [[Someclass alloc] init];

//forExample is an instance variable, and is unique to each instance.
[firstObject setForExample:@"One"];
[secondObject setForExample:@"Two"];
NSLog(@"%@",[firstObject forExample]); //Result: "One"
NSLog(@"%@",[secondObject forExample]); //Result: "Two"

//anotherExample is a global variable, and is NOT unique to each instance.
[firstObject setAnotherExample:@"One"];
[secondObject setAnotherExample:@"Two"];
NSLog(@"%@",[firstObject anotherExample]); //Result: "Two" (!)
NSLog(@"%@",[secondObject anotherExample]); //Result: "Two"

//Both instances return "Two" because there is only ONE variable this time.
//When secondObject set it, it replaced the value that firstObject set.

If you are looking for this sort of behavior, you might be better off using a class variable, like this:

static NSString* yetAnotherExample = nil;

Then you can use class methods to interact with the variable, and it's clearly class-specific (as opposed to instance-specific or global).