How to convert this to ARC

Question

I'm converting some older Obj-C code to ARC. I have this method:

+ (NSString **)sortKeys
{
    return sortKeys;
}

And the complaint I get from the automatic conversion is:

Cannot initialize return object of type 'NSString *__autoreleasing *' with an lvalue of type 'NSString *__strong [14]

The sortKeys are declared as:

static NSString *sortKeys[] = {
NAME_KEY,
CATNUM_KEY,
CATNUM_NAME_KEY,
MAG_KEY,
DISTANCE_KEY,
CONST_KEY,
RISE_KEY,
TRANSIT_KEY,
SET_KEY,
RA_KEY,
DEC_KEY,
AZM_KEY,
ALT_KEY,
DATE_KEY
};

There is also a complaint when called:

    NSString **sortKeys = [ListMgr sortKeys];

I don't want to transfer any ownership of the string, I just want the caller to be able to iterate through them.

How do I declare the method when using ARC?

Answer 1

This is not compatible with ARC. It can't safely memory-manage this. You're relying on behaviors that aren't promised (so this has always been unsafe). sortKeys is not retaining NAME_KEY, so there is no promise that NAME_KEY will be valid. You may happen to have special knowledge that it will be (because NAME_KEY is a static string for instance), but this is an implementation detail, not a language promise. ARC is about guarantees, not "we happen to get away with it."

The correct tool for this is an NSArray:

@interface MyClass : NSObject
+ (NSArray<NSString *>*)sortKeys;
@end

@implementation MyClass

static NSArray<NSString *>* sortKeys;

+ (void)initialize {
    if (self == [MyClass class] ) {
        sortKeys = @[ ... ];
    }
}

+ (NSArray<NSString *>*)sortKeys
{
    return sortKeys;
}

@end

If you can't change the interface, then you'll have to keep this definition in an non-ARC file.

I kept your caching here, but if you don't call +sortKeys very often, I'd get rid of the static and just construct a new array each time. It's quite cheap, especially in this case:

@interface MyClass : NSObject
+ (NSArray<NSString *>*)sortKeys;
@end

@implementation MyClass

+ (NSArray<NSString *>*)sortKeys
{
    return @[...]; // Just put your value here; then it's simple
}

@end

Answer 2

You can use NSArray's initWithObjects:count: initializer to convert from the C array into a Foundation one, and create an ARC, memory-safe array:

+ (NSArray<NSString *> *)sortKeys
{
    static NSArray<NSString *> *convertedKeys;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        convertedKeys = [[NSArray alloc] initWithObjects:sortKeys count:sizeof(sortKeys)];
    });
    return convertedKeys;
}

You'd get ARC-compatible code, and also you can access NSArray's helpers methods - like count, fast enumeration, etc.