I was reading that NSArray is just such a thing. Sounds heavy. I have 7 really fat books here on my desk about Objective-C, Cocoa and C. None of them mention Class Cluster at all, at least I can't find it in the Index at the back of the books. So what's that?
I don't know what is in the CDP that Steve referenced but basically the Objective-C Class Cluster is a construct that supports implementing the abstract Factory pattern.
The idea is simple: You want to provide a Factory (Cluster) interface that, with minimal description, manufactures and returns a specific concrete instance of a Factory Object that satisfies the behavior of the cluster family described by the Factory (Cluster) interface.
A simple concrete example: This example provides a Laugh factory that produces concrete classes of specific laughter types (e.g. Guffaw, Giggle). Pay attention to the Laugh initWithLaughter: method.
In Laugh.h:
#define kLaughWithGuffaw 1
#define kLaughWithGiggle 2
@interface Laugh: NSObject {}
- (Laugh *) initWithLaughter:(NSUInteger) laughterType;
- (void) laugh;
@end
In Laugh.m:
@interface Guffaws:Laugh {}
- (void) laugh;
@end
@interface Giggles:Laugh {}
- (void) laugh;
@end
@implementation Laugh
- (Laugh *) initWithLaughter:(NSUInteger) laugherType {
id instanceReturn=nil;
; // Removed for ARC [self release]
if ( laughterType == kLaughWithGuffaw )
instanceReturn = [[Guffaws alloc]init];
else if( laughterType == kLaughWithGiggle )
instanceReturn = [[Giggles alloc]init];
else
; // deal with this
return instanceReturn;
}
- (void) laugh {
NSLog(@"Humbug");
}
@end
@implementation Guffaws
- (void) laugh {
NSLog(@"OH HA HA HOWAH HA HA HA");
}
@end
@implementation Giggles
- (void) laugh {
NSLog(@"Tee hee");
}
@end