Le 23 août 08 à 15:39, Graham Cox a écrit :
On 23 Aug 2008, at 9:52 pm, Jean-Daniel Dupas wrote:
Le 23 août 08 à 13:41, Graham Cox a écrit :
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 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 defer to the -hash
returned by the string in question?
Yes, that the way to do it.
To understand what an hash is and how it is used, I suggest you
read the hash table article in wikipedia.
http://en.wikipedia.org/wiki/Hash_table
Hash tables are used in Cocoa in NSDictionary, NSSet, NSHashTable,
NSHashMap and maybe more. When you understand how it works,
choosing the implementation should be obvious.
Thanks for that. I'm pretty familiar with hash tables in general
(and in quite a bit of theory too) but I wasn't able to find out
what Cocoa uses for its hashing function or how "good" this needed
to be to work well with the built-in classes. However, by deferring
to the string I can avoid the issue altogether.
For curiosity's sake, I would be interested to know what sort of
hashing functions Cocoa does use. I haven't come across the need to
generate one for any of my custom classes before so it's not
something I need right now, but every bit of knowledge is worth
having.
For example, here's a very crude hash function I used in a very
simple symbol table implementation from a long while ago. I imagine
most hash functions in Cocoa would be more sophisticated.
unsigned long ZHashTable::Hash( const char* name )
{
register unsigned long h = 1;
register char* p = (char*) name;
register char c;
while(( c = *p++ ) != 0 )
h *= (unsigned long) c;
return h % kHashTableSize;
}
Look into the CoreFoundation sources. as Cocoa primitive are tool free
bridged, they use the same hash functions. They is even a special case
for NSString into the CFString.c file.
http://www.opensource.apple.com/darwinsource/10.5.4/CF-476.14/
You can find Integer and double hash function into ForFoundationOnly.h.
And there is another generic hash function into CFData.c
According to the Leopard release notes, we can guess that thoses
function have been updated into Leopard:
"Order of values in hashing-based collections
The CoreFoundation and Foundation framework-provided implementations
of hashing-based collections such as dictionaries have changed how
they store elements, so elements may be retrieved in a different order
than in previous releases. The order of elements in hashing-based
collections is undefined and can change at any time, so developers
must never rely on the order that elements are enumerated, or returned
from a function like CFDictionaryGetKeysAndValues(). This is true even
for cases where a developer may be trying to manipulate the hash codes
of the objects in order to achieve some particular ordering."
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