How can I use an NSArray as a global constant?

Question

I'm using a set of Constant.m files, one per target, to define specific things for each target. For example:

// Constants.h
extern NSString * const kDatabaseFileName;
//Constants.m
NSString * const kDatabaseFileName = @"target_one.sqlite";

I'd also like to define an NSArray for each of my targets:

NSArray * const kLabelNames = [[NSArray alloc] initWithObjects:
    @"nameLabel", @"addressLabel", nil];

But this gives "error: initializer element is not constant". Using 'arrayWithObjects` doesn't work either. Is this because the strings in my array are not constants?

How can I set up an array as a global constant? Thanks.

Answer 1

In Objective-C, objects can only be allocated in heap, so there's no way to create an NSArray in static memory. However, you can create a C array of pointers to NSString constants like so...

NSString * const kLabelNames[] = {
    @"Foo", @"Bar", @"Baz"
};

...and then you can write class methods like this one...

+ (NSArray *)labelNames
{
    static NSArray *names;
    if (names == nil) {
        names = [[NSArray alloc] initWithObjects:kLabelNames count:3];
    }
    return names;
}

Edit

Note that with the introduction of new technologies such as ARC, Grand Central Dispatch and the new literal syntax for arrays, there's now a more straightforward way to accomplish something similar. Note that the example below also provides greater thread safety, though the original example could have incorporated an @synchronized block, or one of several other mechanisms, to achieve similar results.

+ (NSArray *)labelNames
{
    static NSArray *names;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        names = @[@"Foo", @"Bar", @"Baz"];
    });

    return names;
}

However the above example doesn't completely address the original question though. If a global constant array is truly needed, the preceding example could be rewritten along similar lines as the original answer, while still taking advantage of GCD:

NSString * const kLabelNames[] = {
    @"Foo", @"Bar", @"Baz"
};

+ (NSArray *)labelNames
{
    static NSArray *names;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        names = [NSArray arrayWithObjects:kLabelNames count:3];
    });

    return names;
}

Answer 2

Use a macro:

#define SOME_ARRAY (@[@"blah", @"asdf", @"qwerty"])

Answer 3

Here's a much simpler approach:

Declare NSString with comma separated elements (or whatever delimiter you want)

NSString *const kLabelNames = @"Foo,Bar,Baz";

Then convert to NSArray whenever you need it:

NSArray *namesArray = [kLabelNames componentsSeparatedByString:@","];