NSMutableDictionary Memory management.

Question

Hi, please go through the following code and please explain why is it crashes in last line?

NSMutableDictionary *dic1 = [[NSMutableDictionary alloc] initWithCapacity:10];
NSString *val = [[NSString alloc] initWithFormat:@"Deepak"];
NSString *key = [[NSString alloc] initWithFormat:@"First Name"];    

int a = [val retainCount];
a = [key retainCount];

[dic1 setObject:val forKey:key];
a = [val retainCount];
a = [key retainCount];

//self.mainDic = [dic1 copy];
self.mainDic = [dic1 mutableCopy];//mainDic is like @property(copy) NSMutableDictionary *
[self.mainDic setObject:@"Hi" forKey:@"Good"];//Problem

Thanks.

Answer 1

copy properties are not suitable for mutable classes as they don't respect mutability and just send the copy message to the instances - what happens in the second last line is basically:

// ... release previous mainDic, if any
mainDic = [[dic1 mutableCopy] copy];

As the copy message results in an immutable version, NSDictionary, you are probably getting exception for an unrecognized selector -setObject:forKey: while debugging.

If you want to use copy properties you should provide your own setter instead and remove that manual mutableCopy - see e.g. Apples docs on the copy semantics.

Answer 2

I've almost never used a property with a modifier other than (nonatomic, retain) this has saved me of a lot of problems, that and on the dealloc method set'em to nil

this is the way I'd do it

NSMutableDictionary *dic1 = [[NSMutableDictionary alloc] initWithCapacity:10];

NSString *val = [[NSString alloc] initWithFormat:@"Deepak"]; 
NSString *key = [[NSString alloc] initWithFormat:@"First Name"];    

[dic1 setObject:val forKey:key];

self.mainDic = dic1; //where mainDic is like @property (nonatomic, retain) NSMutableDictionary *

[self.mainDic setObject:@"Hi" forKey:@"Good"];

//let's clean this mess up 
[val release]; 
[key release]; 
[dic1 release];

Answer 3

@Deepak ! your code itself works well. And there is no static analysis erro except un-referring a and memory leaking of dic1.

self.mainDic = [dic1 mutableCopy]; works well. Please run your code block again. I created a new project and put your code and tested it. There was no problem.

@Georg Fritzsche your code is making crash.