Cocoa: Dictionary with enum keys?

Since enums are integers, you can wrap the enum in an NSNumber. When you add/retreive something to/from the map, you pass the enum to the NSNumber constructor...

Assuming you've got an enum like...

enum ETest {
    FOO, BAR
};

You can use it in an NSDictionary like this...

NSMutableDictionary *dict = [[NSMutableDictionary alloc] init];
[dict setObject: @"Foo!" forKey:[NSNumber numberWithInt: FOO]];
NSLog(@"getting value for FOO -> %@", 
      [dict objectForKey: [NSNumber numberWithInt: FOO]]);  
[dict release]; 

Further extending on the suggestion from Graham Lee...

You could use an objective-c category in order to add a method to NSMutableDictionary that allows you to add a value with a key of your non NSObject type. This keeps your code free from the wrapping/unwrapping syntax.

Again, assuming

enum ETest { FOO, BAR };

First, we're adding a convince constructor to NSValue:

@interface NSValue (valueWithETest)
+(NSValue*) valueWithETest:(enum ETest)etest; 
@end

@implementation NSValue (valueWithETest)
+(NSValue*) valueWithETest:(enum ETest)etest
{
    return [NSValue value: &etest withObjCType: @encode(enum ETest)];
}
@end

Next we'll add 'enum ETest' support to NSMutableDictionary

@interface NSMutableDictionary (objectForETest)
-(void) setObject:(id)anObject forETest:(enum ETest)key;
-(id) objectForETest:(enum ETest)key;
@end

@implementation NSMutableDictionary (objectForETest)
-(void) setObject:(id)anObject forETest:(enum ETest)key
{
    [self setObject: anObject forKey:[NSValue valueWithETest:key]];
}
-(id) objectForETest:(enum ETest)key
{
    return [self objectForKey:[NSValue valueWithETest:key]];
}
@end

The original Example can thus be transformed to

NSMutableDictionary *dict = [[NSMutableDictionary alloc] init];
[dict setObject: @"Bar!" forETest:BAR];
NSLog(@"getting value Bar -> %@", [dict objectForETest: BAR]);      
[dict release];

Depending on how much you use your enum to access the dictionary this may ease readability of your code quite a bit.

With VoidPointer's suggestion, it may be better to use NSValue for those times when enums turn out not to be integers (such as when -fshort-enums is in play, which should be never as you'd probably break compatibility with Foundation).

NSValue *value = [NSValue value: &myEnum withObjCType: @encode(enum ETest)];

That's not going to add much here but gives you the general "I want to use <name of non-ObjC type> in a collection class" technique.

Notice that with modern compilers you can tell enums to use a fixed underlying type. This means you can control what storage is used for the enum, but as the above solution is general it still applies even when you know this.