Question

What is the easiest way to create a global object. I have tried declaring the object outside the method with no luck.

@implementation UV_TouchpadViewController;
NSMutableString *string = [NSMutableString stringWithFormat:@"text"];

Answer 1

Very close — you can’t initialize a non-local variable with a non-const expression, and a method call is inherently non-const, even if it looks like it should be. So basically, change it to

NSMutableString *string;

but if it’s only going to be used inside the implementation file (eg. other classes would only get at it through UV_TouchpadViewController, not get/set it directly (this is also the recommended pattern)), then qualify it as static, like so

static NSMutableString *string;

If on the other hand you do want to be able to access it directly from outside UV_TouchpadViewController, leave off the static, but add

extern NSMutableString *string;

to your header file (outside the class @interface), and whomever includes the header will be able to access it. (Note that you could instead just put NSMutableString *string; in your header file, however this is quickly becomes unclear)

Also, if you are trying to do this for a singleton class, (I can’t think of a good reason to have a global mutable string — you know they’re not thread safe right?) I recommend reading Apple’s docs on singletons first, where they suggest you use ivars, not global variables, even for singletons. However, UV_TouchpadViewController should not even be a singleton (if it is in any way a view controller), it should just have a single instance, if that’s all you want.

If on the other hand you just want all UV_TouchpadViewControllers to have access to this one variable, note that across almost all languages this is considered a bad design pattern (globals are bad), and that you should instead stick it in, say, your app delegate (which is guaranteed to have a single globally accessible instance), where it can be an ivar+accessors, and generally considered a setting and (with a little extra code) persisted.

EDIT:

If you want to have a singleton that maintains global state, which I still recommend against — you should create a class, like for instance ApplicationState, which handles all of the application’s global state as a model object in the traditional model-view-controller pattern. I wont go into detail here because that would be highly redundant of a google search.

In your Application Delegate, somewhere, add an ivar ApplicationState *state, and a corresponding @property (and @synthesize in the implementation file).