Re: Implementing isEqual: and hash

[Adam R. Maxwell]

On Aug 23, 2008, at 5:13 AM, Michael Ash wrote:

On Sat, Aug 23, 2008 at 7:41 AM, Graham Cox <[EMAIL PROTECTED]> wrote:

I have a class for which equality can be defined as having the same internal

string value (which happens to be a UUID-turned-string). I can easily

defer to the -hash returned by the string in question?

implement isEqual: based on that but the docs say I also need to implement -hash. Any pointers on what is a good way to do that? Could I just safely

The implementation of -hash should *always* match the implementation
of -isEqual:.

And as Jim alluded, -hash must not depend on mutable properties; if the hash changes while the object is in a hashing collection, you'll end up with random crashes.

If you compare primitives in -isEqual:, you should combine them (using
xor or the like) in -hash. If you compare objects by calling -isEqual:
on them, you should combine their hashes (using xor or the like). If
you do some of each, combine them all.

thanks,
Adam

What's the motivation for combining hashes in this case? I've wondered what is the best thing to do when isEqual: is based on comparing multiple ivars; I typically just use one of them for the hash.

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