Elegant and 'correct' multiton implementation in Objective C?
- by submachine
Would you call this implementation of a multiton in objective-c 'elegant'? I have programmatically 'disallowed' use of alloc and allocWithZone: because the decision to allocate or not allocate memory needs to be done based on a key.
I know for sure that I need to work with only two instances, so I'm using 'switch-case' instead of a map.
#import "Multiton.h"
static Multiton *firstInstance = nil;
static Multiton *secondInstance = nil;
@implementation Multiton
+ (Multiton *) sharedInstanceForDirection:(char)direction {
return [[self allocWithKey:direction] init];
}
+ (id) allocWithKey:(char)key {
return [self allocWithZone:nil andKey:key];
}
+ (id)allocWithZone:(NSZone *)zone andKey:(char)key {
Multiton **sharedInstance;
@synchronized(self) {
switch (key) {
case KEY_1:
sharedInstance = &firstInstance;
break;
case KEY_2:
sharedInstance = &secondInstance;
break;
default:
[NSException raise:NSInvalidArgumentException format:@"Invalid key"];
break;
}
if (*sharedInstance == nil)
*sharedInstance = [super allocWithZone:zone];
}
return *sharedInstance;
}
+ (id) allocWithZone:(NSZone *)zone {
//Do not allow use of alloc and allocWithZone
[NSException raise:NSObjectInaccessibleException format:@"Use allocWithZone:andKey: or allocWithKey:"];
return nil;
}
- (id) copyWithZone:(NSZone *)zone {
return self;
}
- (id) retain {
return self;
}
- (unsigned) retainCount {
return NSUIntegerMax;
}
- (void) release {
return;
}
- (id) autorelease {
return self;
}
- (id) init {
[super init];
return self;
}
PS: I've not tried out if this works as yet, but its compiling cleanly :)
