iOS, NSMutableDictionary

Question

I had a problem in my project where I declared in the header file an NSMutableDictionary property like so:

@property (copy, nonatomic) NSMutableDictionary *DataDict ;

Now in the implementation file, I go ahead and initialise this dictionary because I am gonna use it, like so:

DataDict = [[NSMutableDictionary alloc]init];

Now when I did this, the minute I try to add something to this dictionary I would get this error message:

-[__NSDictionaryI setObject:forKey:]: unrecognized selector sent to instance 0x885ae60 2012-10-19 16:51:56.040 testing[2297:c07] * Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[__NSDictionaryI setObject:forKey:]: unrecognized selector sent to instance 0x885ae60'

After a while and running through my project a thousand times, I decided to uncomment my initialization line, like so

 //DataDict = [[NSMutableDictionary alloc]init];

and that fixed the problem.

My question is: Why?

Answer 1

The problem is in the way you have defined your property. If you change it to:

@property (strong, nonatomic) NSMutableDictionary *DataDict ;

instead of copy everything should be fine.

This happens because you basically say that you want a copy of your object through your generated accessors, which returns an NSDictionary instead (an immutable copy).

You can find more on objective-c properties here .

Just as a sidenote: objective-c ivars usually start with a lowercase letter (uppercase names are used for classes), so dataDict should be preferred over DataDict .

Answer 2

It is because the property have "copy" attribute so NSMutableDictionary instance alloc/init-ed is "copy"'ed using "copy" method, and "copy" method create not NSMutableDictionary but NSDictionary. ("mutableCopy" will create NSMutableDictionary).

Probably, you can use "retain" instead of "copy" as attributes.

@property (retain, nonatomic) NSMutableDictionary *DataDict ;

Or, just without "copy"/"retain" but use ARC.(Automatic reference counting).

Answer 3

I have this exact problem. No combination of retain/copy/strong/weak, etc do the trick. What does work is to create a temporary Mutable Dictionary, load it up and then set my original equal to it.

 NSMutableDictionary * tempD = [[NSMutableDictionary alloc] init];

[tempD setObject: epsTapeCut forKey:LWPrintParameterKeyTapeCut];
[tempD setObject: epsCopies forKey:LWPrintParameterKeyCopies];
[tempD setObject: epsHalfCut forKey:LWPrintParameterKeyHalfCut];
[tempD setObject: epsPrintSpeed forKey:LWPrintParameterKeyPrintSpeed];
[tempD setObject: epsDensity forKey:LWPrintParameterKeyTapeWidth];

self.ePSprintSettings = tempD;

This fails:

 [self.ePSprintSettings setObject: epsTapeCut forKey:LWPrintParameterKeyTapeCut];

Declaration:

 @property (nonatomic, retain) NSMutableDictionary *ePSprintSettings;

(But again, no combination of attributes makes a difference.)

Initialization:

 self.ePSprintSettings = (NSMutableDictionary *)[myUserDefaults dictionaryForKey:kEpsPrintSettings];

Thank you for helping me understand.

I'm happy enough that this works, but I'd like to understand why.