type-safe enum pattern

Hi everyone,

Does anybody has ever implemented some kind of type-safe enum pattern
in Objective-C, like the one used in Java before the introduction of
Enums in the language.

The idea is to have something similar to C enums but with object
members instead of integers. For instance, I'd like to have a enum for
length units whose member are instances of a LengthUnit class.

The requirements are :

1) The enums should allow access a enum member by name like :

LengthUnit *mile = [LengthUnit mile];

2) The enums should allow user to retrieve all members like :

NSArray *lengthUnits = [LengthUnit members];

3) It should not be possible to create new members of an enum type.

// should generate a compile time error
LengthUnit *newUnit = [[LengthUnit alloc] initWithName:@"my unit"
symbol:@"U" toMeters:0.0.254"];

I've looked at the Class Cluster pattern but it seems better suited for
"value" objects.

I've come up with a partial solution which is not really satisfying :

1) I defined protocols Unit and LengthUnit to prevent member object creation :

@protocal Unit
- (NSString*)name;
- (NSString*)symbol;
@end

@protocol LengthUnit <Unit>
- (double)toMeters:(double)value;
- (double)fromMeters:(double)value;
@end

2) I defined a class LengthUnits as follow :

@interface LengthUnits : NSObject
+ (id<LengthUnit>)meter;
+ (id<LengthUnit>)foot;
+ (id<LengthUnit>)mile:
+ (id<LengthUnit>)fathom;
....
@end

3) The implementations of LengthUnits static methods meter, foot, ....
return instances of a "private" class to conform to the LengthUnit
protocol

However I not quite satisfied with this ( the use involved ids instead
of typed pointer to LengthUnit ), it doesn't implement the members
method, and it might have a lot of default I've overlooked...

If anyone as a better solution, I would really appreciate if he could share it.

thanks

Brieuc