Objective C Static Class Level variables

Question

I have a class Film, each of which stores a unique ID. In C#, Java etc I can define a static int currentID and each time i set the ID i can increase the currentID and the change occurs at the class level not object level. Can this be done in Objective C? I've found it very hard to find an answer for this.

Answer 1

On your .m file, you can declare a variable as static:

static ClassName *variableName = nil;

Then you can initialize it on your +(id)initialize method.

Please not that this is a plain C static variable, not static in the sense Java or C# consider it, but will yield similar results.

Answer 2

As pgb said, there are no "class variables," only "instance variables." The objective-c way of doing class variables is a static global variable inside the .m file of the class. The "static" ensures that the variable can not be used outside of that file (i.e. it can't be extern).

Answer 3

In your .m file, declare a file global variable:

static int currentID = 1;

then in your init routine, refernce that:

- (id) init
{
    self = [super init];
    if (self != nil) {
     _myID = currentID++; // not thread safe
    }
    return self;
}

or if it needs to change at some other time (eg in your openConnection method), then increment it there. Remember it is not thread safe as is, you'll need to do syncronization (or better yet, use an atomic add) if there may be any threading issues.

Answer 4

Issue Description:

1. You want your ClassA to have a ClassB class variable.
2. You are using Objective-C as programming language.
3. Objective-C does not support class variables as C++ does.

One Alternative:

Simulate a class variable behavior using Objective-C features

1. Declare/Define an static variable within the classA.m so it will be only accessible for the classA methods (and everything you put inside classA.m).

2. Overwrite the NSObject initialize class method to initialize just once the static variable with an instance of ClassB.

3. You will be wondering, why should I overwrite the NSObject initialize method. Apple documentation about this method has the answer: "The runtime sends initialize to each class in a program exactly one time just before the class, or any class that inherits from it, is sent its first message from within the program. (Thus the method may never be invoked if the class is not used.)".

4. Feel free to use the static variable within any ClassA class/instance method.

Code sample:

file: classA.m

static ClassB *classVariableName = nil;

@implementation ClassA

...

+(void) initialize
{
    if (! classVariableName)
     classVariableName = [[ClassB alloc] init];
}

+(void) classMethodName
{
    [classVariableName doSomething]; 
}

-(void) instanceMethodName
{
    [classVariableName doSomething]; 
}

...

@end

References:

1. Class variables explained comparing Objective-C and C++ approaches