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    <p>A few alternatives.</p> <h2>Keep using a string as a key, but do it faster:</h2> <p>Using a string is a bit more heavyweight than you really need, but it is at least simple. </p> <p>Using <code>-[NSString stringByAppendingString]</code> would be faster. Parsing format strings is a lot of work.</p> <pre><code>return [[NSStringFromClass(clazz) stringByAppendingString:@"_"] stringByAppendingString:NSStringFromSelector(selector)]; </code></pre> <p>It may be better to use a single <code>NSMutableString</code> instead of making intermediate strings. Profile it and see.</p> <pre><code>NSMutableString* result = [NSStringFromClass(clazz) mutableCopy]; [result appendString:@"_"]; [result appendString:NSStringFromSelector(selector)]; return result; </code></pre> <h2>Use a custom object as a key:</h2> <p>You can make a custom object as the key that refers to the class and selector. Implement <code>NSCopying</code> and <code>-isEqual:</code> and <code>-hash</code> on it, so you can use it as a key in a dictionary.</p> <pre><code>@interface MyKey : NSObject &lt;NSCopying&gt; { Class _clazz; SEL _selector; } - (id)initWithClass:(Class)clazz andSelector:(SEL)selector; @end @implementation MyKey - (id)initWithClass:(Class)clazz andSelector:(SEL)selector { if ((self = [super init])) { _clazz = clazz; _selector = selector; } return self; } - (id)copyWithZone:(NSZone*)zone { return self; // this object is immutable, so no need to actually copy it } - (BOOL)isEqual:(id)other { if ([other isKindOfClass:[MyKey class]]) { MyKey* otherKey = (MyKey*)other; return _clazz == otherKey-&gt;_clazz &amp;&amp; _selector == otherKey-&gt;_selector; } else { return NO; } } // Hash combining method from http://www.mikeash.com/pyblog/friday-qa-2010-06-18-implementing-equality-and-hashing.html #define NSUINT_BIT (CHAR_BIT * sizeof(NSUInteger)) #define NSUINTROTATE(val, howmuch) ((((NSUInteger)val) &lt;&lt; howmuch) | (((NSUInteger)val) &gt;&gt; (NSUINT_BIT - howmuch))) - (NSUInteger)hash { return NSUINTROTATE([_clazz hash], NSUINT_BIT / 2) ^ (NSUInteger)_selector; } @end + (MyKey*)keyForClass:(Class)clazz andSelector:(SEL)selector { return [[MyKey alloc] initWithClass:clazz andSelector:selector]; } </code></pre> <h2>Eliminate the middleman:</h2> <p>If you never need to pull the class and selector out of your key object, then you can just use the hash as computed above, stored in an <code>NSNumber</code>.</p> <pre><code>// Hash combining method from http://www.mikeash.com/pyblog/friday-qa-2010-06-18-implementing-equality-and-hashing.html #define NSUINT_BIT (CHAR_BIT * sizeof(NSUInteger)) #define NSUINTROTATE(val, howmuch) ((((NSUInteger)val) &lt;&lt; howmuch) | (((NSUInteger)val) &gt;&gt; (NSUINT_BIT - howmuch))) + (NSNumber*)keyForClass:(Class)clazz andSelector:(SEL)selector { NSUInteger hash = NSUINTROTATE([clazz hash], NSUINT_BIT / 2) ^ (NSUInteger)selector; return [NSNumber numberWithUnsignedInteger:hash]; } </code></pre>
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    1. POHow to Create hash from a class and selector? (objective c)
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    1. POHow to Create hash from a class and selector? (objective c)
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    1. COEdited to remove the `get` prefix.
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    2. COOne thing... And correct me if I'm wrong here, but doesn't NSDictionary look at the hash first when it is storing/retrieving something? (Instead of doing a bunch of isEquals first). To me that would mean you have a collision and a slowdown for any entries with the same class with the above code. One way around that would be to add the class hash to the hash of the NSStringFromSelector'd selector in the hash method.
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    3. COYes, why is why I had to implement `-hash`. The rule is: if two objects are equal, they must have the same hash. (But if two objects are *not* equal, they may still have the same hash.) I could have made `hash` return 0 every time, and it would be technically correct, although probably poor for performance. Returning just `[_clazz hash]` is easy, correct, and probably good enough. If necessary, you can do better by combining `[_clazz hash]` and the SEL. [More discussion here.](http://www.mikeash.com/pyblog/friday-qa-2010-06-18-implementing-equality-and-hashing.html)
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