Question

If I have something like this:

SomeObject *obj = [[SomeObject alloc] init];
obj.someIvar = 100;

NSMuteableArray *arr = [[NSMutableArray alloc] initWithCapacity:10];
[arr addObject:obj];

NSMutableDictionary *dict = [[NSMutableDictionary alloc] initWithCapacity:50];
[dict setValue:obj forKey:@"key"];

[obj release];

Can I update obj like so:

SomeObject *objFromDict = [dict objectForKey:@"key"];
objFromDict.someIvar = 5200;

…and expect the object in arr to be updated as well? I’m assuming collections are storing and giving out pointers.

Answer 1

Yes, you are right.

In NSMuteableArray *arr and NSMutableDictionary *dict will be stored reference to object SomeObject *obj. When you are calling [dict objectForKey:@"key"]; you get this reference and in expression objFromDict.someIvar = 5200; you are modifying property someIvar.

When you will try to get the same object from arr the value of someIvar will be also changed because of arrays and dictionaries just store references to instances.