Re: Objects as keys NSMutableDictionary

FROM : Adam R. Maxwell
DATE : Sun Jul 09 15:51:28 2006

On Jul 9, 2006, at 06:11, Matthias Winkelmann wrote:

> I'm having a little problem with NSMutableDictionary and would like 
> to find out what "best practice" for this is.
>
> Here's a contrived example: A class "exam" should store the result 
> for each student. So we have 20 students and an NSNumber for each. 
> I'd like to store them like this:
>
> in init:
> NSMutableDictionary *result = [NSMutableDictionary dictionary];
>
>
> - (void)setResult:(NSNumber *)aResult forStudent:(Student *)aStudent
> {
>     [results setObject:aResult forKey:aStudent];
> }
>
> Now firstly, NSDictionary likes NSStrings as keys for key-value 
> coding. I don't depend on that, so I'm willing to ignore it. 
> However, it also requires the keys to be immutable, so I'd have to 
> copy the student object. I don't want to do that, since I want to 
> later be able to go back and retrieve the result for that specific 
> student.


You could implement -[Student copyWithZone:] to return [self retain], 
but that may have side effects you don't want.

Another solution would be to create a CFMutableDictionary with custom 
callbacks, where you can specify the dictionary's retain/release/
equality/hash behavior explicitly.  A recent post to this list from 
Mike Ash notes that you have to use the CF functions to add values to 
the dictionary if you do this, but that's not too painful.

-- Adam