Error when using setObject in NSMutableDictionary - objective-c

Question

I am getting the exception:
-[__NSCFDictionary setObject:forKey:]: mutating method sent to immutable object'
The offending line is:
[delegate.sharedData.dictFaves setObject:#"test" forKey:#"4"];
Delegate is initialized thus in MyViewController.m:
delegate = (AppDelegate *)[[UIApplication sharedApplication] delegate];
This is how my delegate is defined in AppDelegate.h:
#import "CommonData.h"
...
#interface AppDelegate : UIResponder <UIApplicationDelegate>
{
NSString *tempFave;
CommonData *sharedData;
}
#property (strong, nonatomic) NSString *tempFave;
#property (strong, nonatomic) CommonData *sharedData;
sharedData is initialized in AppDelegate.m thus:
#import "AppDelegate.h"
...
#implementation AppDelegate
#synthesize sharedData;
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
sharedData = [[CommonData alloc] init];
return YES;
}
sharedData is defined in CommonData.h:
#interface CommonData : NSObject
{
NSMutableDictionary *dictAffirms;
NSMutableDictionary *dictFaves;
}
#property (nonatomic, copy) NSMutableDictionary *dictAffirms;
#property (nonatomic, copy) NSMutableDictionary *dictFaves;
shared data implementation file CommonData.m:
#import "CommonData.h"
...
#implementation CommonData
#synthesize dictAffirms;
#synthesize dictFaves;
#end
I have declared the members of CommonData to be Mutable. Apparently that is insufficient. What else must I do in order to write to the Dictionaries inside CommonData?

Answer 1

I have seen this error before when trying to write to a dictionary that is filled from a plist. If you use
yourMutableDictionary = [someDataSource objectForKey:#"someKey"];
your dictionary will be immutable, even if it is declared mutable. Use instead
yourMutableDictionary = [someDataSource mutableArrayValueForKey:#"someKey"];
and your problem will go away, assuming this is in fact your problem. It might be something like:
yourMutableDictionary = [[NSDictionary alloc] init];
or
yourMutableDictionary = [NSDictionary new];
and you just are accidentally creating immutable objects, which is pretty much the same problem as above, just different.
It would be nice to see the code used to initialize the NSMutableDictionaries.
Edit: Maybe try something like this, as I'm curious as to what the results would be. Instead of using:
[delegate.sharedData.dictFaves setObject:#"test" forKey:#"4"];
try
NSMutableDictionary* dict = [[NSMutableDictionary alloc] initWithDictionary:delegate.sharedData.dictFaves];
[dict setObject:#"test" forKey:#"4"];
delegate.sharedData.dictFaves = dict;
[dict release];

Answer 2

You've declared the dictFaves to be mutable, but that doesn't mean you actually stored a mutable object in there. Check your initializer. You'll probably have something like the following:
dictFaves = [[NSDictionary alloc] init];
If so, you need to change that to NSMutableDictionary instead.

Answer 3

Your problem is that your property setter for the dictionary is declared as copy. NSMutableDictionary's copy method returns an immutable NSDictionary (in general, copy almost always returns an immutable object). So assuming you're using the standard synthesized setter, any time you set that property, you're assigning the wrong type behind the scenes. It should probably be strong instead.