Keys in an NSDictionary

[Steve Ketcham]

I'd like to implement my own class for use as keys in an NSDictionary.
My first try was to subclass NSObject and implement isEqual: and hash,
which is what the NSDictionary documentation implies is necessary.

When that didn't work, I subclassed NSString, but had problems
initializing super with the text I wanted.

Does anyone know what the requirements are, or where they might be
documented?

(Or is this not possible? I note that the GNUstep documentation says
that the key must be an NSString. Apple's documentation just says that
the key is an object.)

Thanks,
Steve

[Tom Harrington]

In article <160920031326401042%sgketcham@comcast.net>,
Steve Ketcham <sgketcham@comcast.net> wrote:

> I'd like to implement my own class for use as keys in an NSDictionary.
> My first try was to subclass NSObject and implement isEqual: and hash,
> which is what the NSDictionary documentation implies is necessary.
>
> When that didn't work, I subclassed NSString, but had problems
> initializing super with the text I wanted.
>
> Does anyone know what the requirements are, or where they might be
> documented?
>
> (Or is this not possible? I note that the GNUstep documentation says
> that the key must be an NSString. Apple's documentation just says that
> the key is an object.)

The docs for -[NSObject isEqual] note that:

"If two objects are equal, they must have the same hash value. This
last point is particularly important if you define isEqual: in a
subclass and intend to put instances of that subclass into a
collection. Make sure you also define hash in your subclass."

Did you implement -hash?

--
Tom "Tom" Harrington
Macaroni, Automated System Maintenance for Mac OS X.
Version 1.4: Best cleanup yet, gets files other tools miss.
See [url]http://www.atomicbird.com/[/url]

[matt neuburg]

In <tph-ABDB93.12000616092003@localhost> Tom Harrington wrote:

> In article <160920031326401042%sgketcham@comcast.net>,
> Steve Ketcham <sgketcham@comcast.net> wrote:
>

>> I'd like to implement my own class for use as keys in an NSDictionary.
>> My first try was to subclass NSObject and implement isEqual: and hash,

>
> Did you implement -hash?

He just told you he did. :) Okay, I haven't tried this, but I find the
problem interesting. What didn't work about your first attempt? Did you
remember to make the key class implement NSCopying? m.

--
matt neuburg, phd = [email]matt@tidbits.com[/email], [url]http://www.tidbits.com/matt[/url]
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[Michael Ash]

In article <160920031326401042%sgketcham@comcast.net>,
Steve Ketcham <sgketcham@comcast.net> wrote:

> I'd like to implement my own class for use as keys in an NSDictionary.
> My first try was to subclass NSObject and implement isEqual: and hash,
> which is what the NSDictionary documentation implies is necessary.
>
> When that didn't work, I subclassed NSString, but had problems
> initializing super with the text I wanted.
>
> Does anyone know what the requirements are, or where they might be
> documented?

According to the NSDictionary docs:

Methods that add entries to dictionaries‹whether as part of
initialization (for all dictionaries) or during modification (for
mutable dictionaries)‹copy each key argument (keys must conform to
the NSCopying protocol) and add the copies to the dictionary.

So your objects must implement NSCopying. This is a small part of the
docs and easy to miss but if your objects don't do it, you'll have all
kinds of problems.

On a side note, you are properly subclassing NSString (since it's a
class cluster you have to subclass it in a special way), right?

[Steve Ketcham]

I'm going to reply to all three of you in one message:

The dictionary keys will consist of (i,j) indices into a
two-dimensional array. I could convert them into a string "iijj" and
use that, but I thought it would be more interesting (ie a better
learning experience) to keep them as ints and add convenience
functions to retrieve them, so I don't have to decode the string over
and over.

I didn't implement NSCopying, and that's probably my problem. But let
me answer the questions anyway.

Tom asked if implemented -hash. I did, by creating the NSString as
described above, and using its hash value.

Matt asked what didn't work. The program hung at the addition of the
key to the dictionary. (It's really a mutable dictionary, of course.)
Since that's when the key copy is done, it makes sense that copying is
the problem.

Michael asked if I'd subclassed NSString "in a special way." I didn't
-- what is the special way.

Thanks to all of you,
Steve

[Michael Ash]

In article <160920031536570101%sgketcham@comcast.net>,
Steve Ketcham <sgketcham@comcast.net> wrote:

> I'm going to reply to all three of you in one message:
>
> The dictionary keys will consist of (i,j) indices into a
> two-dimensional array. I could convert them into a string "iijj" and
> use that, but I thought it would be more interesting (ie a better
> learning experience) to keep them as ints and add convenience
> functions to retrieve them, so I don't have to decode the string over
> and over.

Did you look at NSValue? You can use that to store ints, and NSValue
objects work as keys to NSDictionary. Of course you don't have all the
fun of implementing your own class.

> Matt asked what didn't work. The program hung at the addition of the
> key to the dictionary. (It's really a mutable dictionary, of course.)
> Since that's when the key copy is done, it makes sense that copying is
> the problem.

What do you mean by "hung"? Did you poke around with the debugger?

> Michael asked if I'd subclassed NSString "in a special way." I didn't
> -- what is the special way.

Have a look at
<[url]http://developer.apple.com/documentation/Cocoa/Conceptual/Foundation/Con[/url]
cepts/ClassClusters.html>.

To summarize: Several Cocoa classes are actually class clusters. What
this means is that, for example, NSString is just an abstract class.
When you do [[NSString alloc] init], the object you get is not an
instance of NSString, but an instance of a private subclass thereof. So
there are special considerations when you subclass these classes. You
can't just subclass and override what you feel like, there are specific
methods that must be implemented to make the thing work at all.

[Tom Harrington]

In article <20030916111155552-0700@news.la.sbcglobal.net>,
matt neuburg <matt@tidbits.com> wrote:

> In <tph-ABDB93.12000616092003@localhost> Tom Harrington wrote:

> > In article <160920031326401042%sgketcham@comcast.net>,
> > Steve Ketcham <sgketcham@comcast.net> wrote:
> >

> >> I'd like to implement my own class for use as keys in an NSDictionary.
> >> My first try was to subclass NSObject and implement isEqual: and hash,

> >
> > Did you implement -hash?

>
> He just told you he did. :) Okay, I haven't tried this, but I find the
> problem interesting. What didn't work about your first attempt? Did you
> remember to make the key class implement NSCopying? m.

D'oh. That's what I get for posting before I've had enough coffee.
Nevermind.

--
Tom "Tom" Harrington
Macaroni, Automated System Maintenance for Mac OS X.
Version 1.4: Best cleanup yet, gets files other tools miss.
See [url]http://www.atomicbird.com/[/url]

[Steve Ketcham]

In article <mail-D14D9F.21472216092003@localhost>, Michael Ash
<mail@mikeash.com> wrote:

> Did you look at NSValue? You can use that to store ints, and NSValue
> objects work as keys to NSDictionary. Of course you don't have all the
> fun of implementing your own class.

Yes, NSValue would work. It was more of an exercise to learn about
keys. By the way, implementing the copyWithZone: solved the problem

> What do you mean by "hung"? Did you poke around with the debugger?

I meant that I attempted to step through the call to setObject:forKey:
and it never returned to the debugger. I didn't poke further.

>

> > Michael asked if I'd subclassed NSString "in a special way." I didn't
> > -- what is the special way.

>
> Have a look at
> <[url]http://developer.apple.com/documentation/Cocoa/Conceptual/Foundation/Con[/url]
> cepts/ClassClusters.html>.
>
> To summarize: Several Cocoa classes are actually class clusters. What
> this means is that, for example, NSString is just an abstract class.
> When you do [[NSString alloc] init], the object you get is not an
> instance of NSString, but an instance of a private subclass thereof. So
> there are special considerations when you subclass these classes. You
> can't just subclass and override what you feel like, there are specific
> methods that must be implemented to make the thing work at all.

Thanks for this. I'll read it.

S

[matt neuburg]

In <160920031923335824%sgketcham@comcast.net> Steve Ketcham wrote:

> implementing the copyWithZone: solved the problem

You're welcome. m.

--
matt neuburg, phd = [email]matt@tidbits.com[/email], [url]http://www.tidbits.com/matt[/url]
REALbasic: The Definitive Guide! 2nd edition!
[url]http://www.amazon.com/exec/obidos/ASIN/0596001770/somethingsbymatt[/url]
Subscribe to TidBITS. It's free and smart.