When creating a custom key in Objective C for NSDictionary or NSCache or the like, you need to create an object which does the following:

Implements <NSCopying> protocol.

If your key is invariant, you can implement the method copyWithZone: as follows:

- (id)copyWithZone:(NSZone *)zone
{
    return self;
}

Of course if your key is invariant, ideally you would create the key entirely using a custom init function, and mark all the properties (readonly).

Implement the isEqual: method.

This is part of the NSObject protocol. Note that any class (or nil) could be passed in as the argument to isEqual: method, so you may want to use the method isKindOfClass: to verify that you got what you expected as the parameter.

Implement the hash method.

This is also part of the NSObject protocol, and returns a NSUInteger value which provides a hash function of the data you passed in.

The hash function doesn’t need to be complicated. For example, if your key is three integers, your hash function could be as simple as:

- (NSUInteger)hash
{
    return (self.a << 8) ^ (self.b << 4) ^ self.c;
}

What is important is that two keys passed into your system are unlikely to have a similar value.

Also note that many of the classes that you see used routinely as keys (such as NSString or NSNumber) also follow this protocol. Meaning if your custom key has a string in it, you can use the NSString’s hash function as one of the inputs to your own hashing function:

- (NSUInteger)hash
{
    return (self.intVal << 16) ^ [self.stringVal hash];
}