如何在Objective-C中使用对象作为键

Question

I would like to use a custom object as a key in a hash-like structure. I've tried using NS[Mutable]Dictionary but in order for my object to be a key it has to implement the NSCopying protocol. NSDictionary is sending a copy message to all of it's keys as far as I've read. I don't want to implement the protocol (my object is quite complex) nor do I want it to be copied. What are my options? Do I have any?

Answer 1

NSDictionary is toll-free bridged with CFDictionaryRef , but they actually differ in behavior when adding objects. Specifically, NSDictionary 's -setObject:forKey: will copy the key, but CFDictionaryRef 's CFDictionarySetValue() will not copy the key. This means that if you want to use non-copyable keys, you can use CFDictionarySetValue() instead to add it to the dictionary.

CFDictionarySetValue((CFMutableDictionaryRef)myDict, myKey, myValue);

This will still retain the key, but it won't copy it. And you can use the normal NSDictionary methods for everything else.

Answer 2

Do you need the NSDictionary to retain the object? If not, you can turn it into an NSValue and use that as the key:

NSValue *value = [NSValue valueWithNonretainedObject:yourCustomObject];
[dictionary setObject:someObject forKey:value];

This can get a bit messy but is in alternative to implementing NSCopying .

Answer 3

You can roll your own dictionary. Not really that hard.

Another option is to use a surrogate object, containing a pointer to "the" object. The surrogate would implement the hash and either copy or reference the fields to be compared for isEqual. It could do a basic sanity check to assure the compared fields have not been changed when it's referenced.

Answer 4

You could just do this:

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