static NSDictionary* const letterValues = @{ ..... } in a method does not compile

Question

app screenshot

I'm trying to use the following code in my custom view Tile.m:

- (void)awakeFromNib
{
    [super awakeFromNib];

    static NSDictionary* const letterValues = @{
                                         @"A": @1,
                                         @"B": @4,
                                         @"C": @4,
                                         // ...
                                         @"X": @8,
                                         @"Y": @3,
                                         @"Z": @10,
                                         };

    NSString* randomLetter = [kLetters substringWithRange:[kLetters rangeOfComposedCharacterSequenceAtIndex:arc4random_uniform(kLetters.length)]];
    int letterValue = [letterValues[randomLetter] integerValue];

    _smallLetter.text = _bigLetter.text = randomLetter;
    _smallValue.text = _bigValue.text = [NSString stringWithFormat:@"%d", letterValue];
}

Unfortunately this gives me compile error Initializer element is not a compile-time constant and I have to remove the static keyword to get my app compile in Xcode (here fullscreen):

Xcode screenshot

I think I initialize the NSDictionary correctly - by using the new Objective-C Literals syntax.

But why can't I use static here?

I thought it would be appropriate here to make sure that my letterValues constant is set only once?

Which code is valid (the code in my question doesn't compile in Xcode 5.1) and what do you mean by Objective-**C++**? — Alexander Farber, Mar 20 '14 at 14:20
@AlexanderFarber: See http://stackoverflow.com/questions/3684112/what-is-objective-c - Objective-C++ is the "combination" of Objective-C with C++ (and is poorly documented). Objective-C++ files have the extension .mm. Your code should compile if you rename the file to "Tile.mm", because C++ has different rules for static initializers. - But this was just meant as a remark, not as *the* solution. - You can also have a look at http://stackoverflow.com/questions/21365532/meaning-of-static-nsmutablearray-and-multi-thread-usage, which is a different question with a similar answer. — Martin R, Mar 20 '14 at 14:32

Dave Wood · Accepted Answer · 2014-03-20T14:07:58.533

You can only set a static variable during initialization with a constant. @{} creates an object, thus not a constant.

Do this instead:

- (void)awakeFromNib
{
    [super awakeFromNib];

    static NSDictionary* letterValues = nil;

    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        letterValues = @{
          @"A": @1,
          @"B": @4,
          @"C": @4,
          // ...
          @"X": @8,
          @"Y": @3,
          @"Z": @10,
          };
    });


    ...
}

Some other answers here suggest a check for nil instead of dispatch once, but that can cause issues when creating multiple tiles at the same time (via threads). dispatch_once implements the required locking.

Using `dispatch_once` is the best and safest way to initialize this kind of data objects. This is the most correct answer here. — Léo Natan, Mar 21 '14 at 13:39

score 5 · Answer 2 · answered Mar 20 '14 at 13:24

You can use static, but the assignment can't be made on the same line. Try this:

- (void)awakeFromNib {
    [super awakeFromNib];

    static NSDictionary* letterValues = nil;
    if (!letterValues) {
        letterValues = @{@"A": @1,
                         @"B": @4,
                         @"C": @4,
                         // ...
                         @"X": @8,
                         @"Y": @3,
                         @"Z": @10};
    }
    ...
}

The reason is that the @{<key> : <value>} syntax is translated by the compiler into an Objective-C method ([[NSPlaceholderDictionary alloc] initWithObjects:forKeys:count:]), which cannot be resolved at compile-time.

MGY · Answer 3 · 2021-03-26T13:10:24.403

Another alternative is using the Macro Approach:

Macro (The Easiest) Approach (2021)

#define kScreenNames @{ \
        @"HOME":                     @"Home Page", \
        @"SEARCH":                   @"Search Page", \
        @"MYLISTS":                  @"My List", \
        @"MYACCOUNT":                @"My Aaccount" \
}

it's short and elegant.

Then, you can use it like any other NSDictionary.

I hope this will help someone on the planet.

Best

score 3 · Answer 4 · answered Mar 20 '14 at 13:28

3

NSDictionary objects can't be created at compile time. However, if you need a static object, you can create one. You can, for example, use the initialize method, like this:

static NSDictionary* letterValues;

+ (void)initialize
{
    if (self == [MyClass class]) {
        letterValues = @{
                         @"A": @1,
                         @"B": @4,
                         @"C": @4,
                         @"X": @8,
                         @"Y": @3,
                         @"Z": @10,
                         };
    }
}

The if statement is there to prevent multiple calls of your code in MyClass subclasses.

answered Mar 20 '14 at 13:28

Adam

26,549
8
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wouldn't `+ (void)load` be more appropriate? – dwery Jan 01 '15 at 14:51
@dwery It really depends on your use case. See [this question](http://stackoverflow.com/q/13326435/730701) to find out more about the difference between `load` and `initialize`. – Adam Jan 04 '15 at 21:49

score 2 · Answer 5 · answered Jan 02 '22 at 11:11

2

If you support iOS 14 (and its year-aligned releases) or later, you can now do this, assuming your keys are NSStrings. This is because Xcode 13 supports Clang's -fobjc-constant-literals flag, which is on by default.

answered Jan 02 '22 at 11:11

saagarjha

2,221
1
20
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static NSDictionary* const letterValues = @{ ..... } in a method does not compile

5 Answers5

Macro (The Easiest) Approach (2021)

Linked