static variables in Objective-C – what do they do?

Because Objective-C simply extends C:

In both C and Objective-C, a static variable is a variable that is allocated for the entire lifetime of a program. This is in contrast to automatic variables, whose lifetime exists during a single function call; and dynamically-allocated variables like objects, which can be released from memory when no longer used. More simply put, a static variable’s value is maintained throughout all function/method calls. When declared outside of a function, a static variable is visible to everything within the file in which it is declared; when declared inside a function or method, it is visible only within that function or method, but the value is retained between calls.

Say you have this:

int f(void)
{
    int i = 5;
    i += 10;
    return i;
}

Every call to f() will return the value 15.

Now say you have this:

int g(void)
{
    static int i = 5;
    i += 10;
    return i;
}

The first time g() is called, the value 15 will be returned. The second time, 25 will be returned, as i maintained its value of 15 and then incremented itself by 10. The third call, 35 will be returned. And so on.

In the context of Objective-C classes, static variables are often used to mimic class variables, as Objective-C does not have class variables (other languages, such as Java, do). For instance, say you want to lazily initialize an object, and only return that object. You might see this:

static MyObject *obj = nil;

@implementation MyObject

+ (id)sharedObject
{
    if (obj == nil) obj = [[MyObject alloc] init];
    return obj;
}

@end

obj will be initialized the first time classObject is called; subsequent invocations of classObject will return the same object. You could check this by logging the address of the object:

NSLog(@"obj is at %p", [MyObject sharedObject]);
NSLog(@"obj is at %p", [MyObject sharedObject]);    // Will print the same address both times

Furthermore, obj will be visible to all methods in MyObject.

This technique is used to implemented singleton classes in Objective-C as well.

Note that putting “static” variables inside methods is not thread-safe, at least not in all compilers.

And even in those compilers where it’s safe, it’s a little expensive (because of hidden auto locking, yes, it uses something like a “global mutex” without asking the developer).