Mutable Object within an Immutable object

Question

Is it acceptable to have a NSMutableArray within an NSDictionary? Or does the NSDictionary also have to be mutable?

The NSMutableArray will have values added to it at runtime, the NSDictionary will always have the same 2 NSMutableArrays.

Thanks,

Dan

Answer 1

Yes, it's perfectly acceptable. Keep in mind, the contents of the array are the pointers to your NSMutableArrays--those are what can't change in the immutable dictionary structure. What the pointers point to can change all you want. To wit:

NSMutableArray *arr = [[NSMutableArray alloc] init];
NSDictionary *dict = [NSDictionary dictionaryWithObject:arr forKey:@"test"];

[arr addObject:@"Hello"];
NSString *str = [[dict objectForKey:@"test"] objectAtIndex:0];
NSLog("%@", str);

Answer 2

It's quite acceptable. But, it's precisely the sort of setup that suggests you should seriously consider replacing the dictionary with an NSObject subclass that sports two properties for accessing the arrays.