IOS, NSMutableDictionary

Question

I had a problem in my project where I declared the NSMutableDictionary property in the header file as follows:

@property (copy, nonatomic) NSMutableDictionary *DataDict ; 

Now, in the implementation file, I continue and initialize this dictionary, because I will use it, for example:

 DataDict = [[NSMutableDictionary alloc]init]; 

Now that I have done this, the minute I try to add something to this dictionary, I would get an error message:

- [__ NSDictionaryI setObject: forKey:]: unrecognized selector sent to instance 0x885ae60 2012-10-19 16: 51: 56.040 testing [2297: c07] * Application terminated due to an uncaught exception 'NSInvalidArgumentException', reason: '- [__ NSDictionaryI setObject: forKey:]: unrecognized selector sent to instance 0x885ae60 '

After some time and through my project a thousand times, I decided to uncomment my initialization line, for example:

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

and this fixed the problem.

My question is why?

Answer 1

The problem is how you defined your property. If you change it to:

 @property (strong, nonatomic) NSMutableDictionary *DataDict ; 

instead of copy everything should be fine.

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

More information about objective-c properties can be found here .

Just like a side element: objective-c ivars usually start with a lowercase letter (upper case names are used for classes), so dataDict should be preferable to dataDict .

Answer 2

This is because the property has the attribute “copy”, so the instance of the NSMutableDictionary alloc / init-ed instance “copies” ed using the “copy” method, and the “copy” method creates not NSMutableDictionary, but NSDictionary. ("mutableCopy" will create an NSMutableDictionary).

Perhaps you can use “save” instead of “copy” as attributes.

 @property (retain, nonatomic) NSMutableDictionary *DataDict ; 

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