Question

Is there a way to determine if a class is suitable as a key and will work as you expect, for example I want to use NSIndexPath as a key in NSDictionary but I don’t know for certain if two different NSIndexPath instances with the same integer values will always return the same hash value.

Answer 1

Apple’s NSObject’s isEqual document says:

If two objects are equal, they must have the same hash value. This last point is particularly important
if you define isEqual: in a subclass and intend to put instances of that subclass into a collection. Make sure
you also define hash in your subclass.

Look the following code:

NSIndexPath *indexPath1 = [NSIndexPath indexPathForRow:0 inSection:0];

NSIndexPath *indexPath2 = [NSIndexPath indexPathForRow:0 inSection:0];

NSObject *obj1 = [[NSObject alloc] init];
NSObject *obj2 = [[NSObject alloc] init];

NSLog(@"NSIndexPath isEqual's Result: %d", [indexPath1 isEqual:indexPath2]);
NSLog(@"NSObject isEqual's Result: %d", [obj1 isEqual:obj2]);

Output Result:

NSIndexPath isEqual’s Result: 1

NSObject isEqual’s Result: 0

The implementation of NSObject isEqual is that comare the address of two objects,
and hash implementation is that return object’s address.

NSIndexPath is inherited from NSObject, according to NSIndexPath isEqual output result,
NSIndexPath’s isEqual implementation should override superclass’s isEqual method, and NSIndexPath also override superclass’s hash method.

In attition, NSIndexPath also conform to the NSCopying protocol.

So NSIndexPath can be used as the Key class of NSDictionary.