Question

I have an NSDictionary in which I use my own classes (NSObject subclasses) as keys and would like to make sure that I do not include the same key twice. However, because NSDictionary copies its keys, if I try to check whether an object is in the dictionary, it never thinks it is.
For example,

MyClass* obj = [[MyClass alloc] init];
NSMutableDictionary* dict = [NSMutableDictionary dictionary];
[dict setObject:someObj forKey:obj];
if ([[dict allKeys] contains:obj]) // always returns false
    // obj is already in dict
else
    // add obj to dict etc.

Similarly, if I want to change the object associated with this key, it seems to create a new entry.

// dict is empty
// say [obj description] gives 'MyClass : 0x1' - (impossible address?)
[dict setObject:someObj forKey:obj];
// dict: { 'MyClass : 0x2' = someObjDesc }
[dict setObject:someOtherObj forKey:obj];
// dict: { 'MyClass : 0x2' = someObjDesc , 'MyClass : 0x3' = someOtherObjDesc }

Also, this same thing leads to not being able to access the items in the dictionary from the original object

[dict setObject:someObj forKey:obj];
[dict objectForKey:obj]; // returns null

So, as far as the uniqueness is concerned, would I be best off keeping track of the keys in a separate array or is there a better way of doing this.
I considered implementing an isEqual method based on a unique variable (such as a name) but didn’t think that was the Right Thing to do.

Background (in case it turns out that maybe I’m just using the wrong thing entirely):
I want to keep track of information about a group of people going to different places. So each person at each place has some info. What I’ve done is used nested dictionaries so the key to the main dictionary is a Person object and the object another dictionary. This latter dictionary has key Place and info as the object. I think this is Java syntax but something like > (the array holds the info). I want to be able to add a Person only if the don’t already exist, add a Place (for each person), change the array.

Any help on any of this would be greatly appreciated!

Answer 1

You should always use NSStrings as keys for dictionaries, especially if you are new at objective-C. There are a few things that I can see you are doing wrong with your current implementation – you would need to read up on key requirements for NSDictionaries.

You can do what you want with strings as keys – person’s name, etc.

The objects in a dictionary have all the info about a certain person:

NSDictionary* personsInfo = [mainDict objectForKey:@"Jane Smith"];

NSString* addressLine1 = [personsInfo objectForKey@"addressLine1"];

–Tom