How to use global variables in Objective-C?

Question

How should I declare a global variable in my Objective-C project?

Answer 1

Don't. Global variables are often a sign of poor design. A common replacement in Objective-C is a class method that returns an object (that may or may not be a singleton), such as [NSUserDefaults standardUserDefaults] or [UIDevice currentDevice].

However, if you must use a global variable, read on.

In your header:

extern NSString *someString;
extern NSInteger someInteger;

In your implementation file:

NSString *someString = @"DEFAULT_VALUE";
NSInteger someInteger = DEFAULT_VALUE;

Answer 2

Traditionally, global variables are declared in a header, and defined in a source file. Other source files only need to know how it is declared to use it (i.e. its type and its name). As long as the variable is defined somewhere in a source file, the linker will be able to find it and appropriately link all the references in other source files to the definition.

Somewhere in your header, you would declare a global variable like this:

extern int GlobalInt;

The extern part tells the compiler that this is just a declaration that an object of type int identified by GlobalInt exists. It may be defined later or it may not (it is not the compiler's responsibility to ensure it exists, that is the linker's job). It is similar to a function prototype in this regard.

In one of your source files, you define the GlobalInt integer:

int GlobalInt = 4;

Now, each file that includes the header will have access to GlobalInt, because the header says it exists, so the compiler is happy, and the linker will see it in one of your source files, so it too will be happy. Just don't define it twice!

However

---

You should consider whether or not this approach is useful. Global variables get messy for a number of reasons (trying to find out exactly where it is defined or declared, threading issues), there is usually not a need for global variables. You should perhaps consider using a singleton approach.

Answer 3

In my experience there are few instances when a program doesn't need, at least, some sort of data or utility/helper methods that can be accessed throughout the program.

They way I deal with this, rather than using global variables is to create what I call a 'project applicance', which is essentially just a class with a bunch of static methods.

It could be implemented multiple ways, but I use a singleton and just have the static methods call through to the single instance of the appliance class. For example, in my project Oovium I have:

Oovium.h:

@interface Oovium : NSObject {
    UIWindow* _window;
}

+ (UIWindow*) window;

Oovium.m:

@implementation Oovium

static Oovium* oovium;

- (UIWindow*) window {return _window;}

+ (void) initialize {
    oovium = [[Oovium alloc] init];
}

+ (UIWindow*) window {return [oovium window];}

I then include Oovium.h in my Oovium_Prefix.pch file so that it is automatically included in all of my files.