On Sep 19, 2008, at 6:32 AM, Keith Duncan wrote:
you [...] aren't allowed to mutate objects that are in collections
Where is that documented?
It is documented (parenthetically) in the documentation for the -hash
method on NSObject.
It is also called out in the Collections programming guide in the
NSSet section.
I've been doing this without consideration and nothing's blown up yet.
For a collection which uses the result of -hash to determine the
storage location, if you have side effects on hash after storage, you
won't be able to correctly locate the object within the collection.
Consider:
#import <Foundation/Foundation.h>
int main (int argc, const char * argv[])
{
NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
NSMutableString *string = [NSMutableString string];
NSMutableSet *set = [NSMutableSet set];
[string appendString:@"Fred"];
[set addObject: string];
NSLog(@"Before mutation, [set containsObject: string] = %d", [set
containsObject: string]);
[string appendString: @" Flintstone"];
NSLog(@"After mutation, [set containsObject: string] = %d", [set
containsObject: string]);
NSLog(@"After mutation, [[set allObjects] containsObject: string]
= %d", [[set allObjects] containsObject: string]);
[pool drain];
return 0;
}
Different frameworks/platforms have subtly different behaviors related
to equality/hash value. You can run into this behavior in Java too,
depending upon the Object subclass, and its hash/equals behavior. (You
won't run into it with StringBuffer because equals() is defined as
identity, so a direct port of the above example will not demonstrate
the problem.)
Python also takes a somewhat different tact, refusing to store certain
mutable objects (raises a TypeError: xxx objects are unhashable) in
collections which depend on the hash value.
If this is so fundamental, why haven't I been tripped up by it?
It is an edge case, but one worth knowing about. You have to be
storing mutable objects in a collection whose internal storage
position depends on the objects hash code.
Jim
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