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POElegant and 'correct' multiton implementation in Objective C?
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  1. POElegant and 'correct' multiton implementation in Objective C?
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    copied!<p>Would you call this implementation of a multiton in objective-c 'elegant'? I have programmatically 'disallowed' use of <code>alloc</code> and <code>allocWithZone:</code> because the decision to allocate or not allocate memory needs to be done based on a key.</p> <p>I know for sure that I need to work with only two instances, so I'm using 'switch-case' instead of a map.</p> <pre><code>#import "Multiton.h" static Multiton *firstInstance = nil; static Multiton *secondInstance = nil; @implementation Multiton + (Multiton *) sharedInstanceForDirection:(enum KeyName)direction { return [[self allocWithKey:direction] init]; } + (id) allocWithKey:(enum KeyName)key { return [self allocWithZone:nil andKey:key]; } + (id) allocWithZone:(NSZone *)zone andKey:(enum KeyName)key { Multiton **sharedInstance; @synchronized(self) { switch (key) { case KEY_1: sharedInstance = &amp;firstInstance; break; case KEY_2: sharedInstance = &amp;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; } @end </code></pre> <p>PS: I've not tried out if this works as yet, but its compiling cleanly :)</p>
 

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