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copied!<p>Like you said, you can create a Pair class:</p> <pre><code>//Pair.h @interface Pair : NSOpject <NSCopying> { id<NSCopying> left; id<NSCopying> right; } @property (nonatomic, readonly) id<NSCopying> left; @property (nonatomic, readonly) id<NSCopying> right; + (id) pairWithLeft:(id<NSCopying>)l right:(id<NSCopying>)r; - (id) initWithLeft:(id<NSCopying>)l right:(id<NSCopying>)r; @end //Pair.m #import "Pair.h" @implementation Pair @synthesize left, right; + (id) pairWithLeft:(id<NSCopying>)l right:(id<NSCopying>)r { return [[[[self class] alloc] initWithLeft:l right:r] autorelease]; } - (id) initWithLeft:(id<NSCopying>)l right:(id<NSCopying>)r { if (self = [super init]) { left = [l copy]; right = [r copy]; } return self; } - (void) finalize { [left release], left = nil; [right release], right = nil; [super finalize]; } - (void) dealloc { [left release], left = nil; [right release], right = nil; [super dealloc]; } - (id) copyWithZone:(NSZone *)zone { Pair * copy = [[[self class] alloc] initWithLeft:[self left] right:[self right]]; return copy; } - (BOOL) isEqual:(id)other { if ([other isKindOfClass:[Pair class]] == NO) { return NO; } return ([[self left] isEqual:[other left]] && [[self right] isEqual:[other right]]); } - (NSUInteger) hash { //perhaps not "hashish" enough, but probably good enough return [[self left] hash] + [[self right] hash]; } @end </code></pre> <p>Edit:</p> <p>Some notes on creating objects that can be keys:</p> <ol> <li>They have to conform to the <code>NSCopying</code> protocol, since we don't want the key to change out from underneath us.</li> <li>Since the Pair itself is copied, I make sure the objects in the pair are also copied.</li> <li>Keys have to implement <code>isEqual:</code>. I implement the <code>hash</code> method for good measure, but it's probably not necessary.</li> </ol>

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