Cocoa：使用 NSSet 作为 NSMutableDictionary 中的键有什么缺点吗？
使用 NSSet 作为 NSMutableDictionary 中的键是否有任何缺点，需要注意的任何问题，是否有巨大的性能影响？
我认为键是在 Cocoa 容器中复制的，这是否意味着 NSSet 被复制到字典中？或者在这种情况下是否有一些保留 NSSet 的优化？
与 NSDictionary 可以接受 NSSet 作为键吗？ 相关
示例代码：
NSMutableDictionary * dict = [NSMutableDictionary dictionary];
NSSet * set;
set = [NSSet setWithObjects:@"a", @"b", @"c", @"d", nil];
[dict setObject:@"1" forKey:set];
set = [NSSet setWithObjects:@"b", @"c", @"d", @"e", nil];
[dict setObject:@"2" forKey:set];
id key;
NSEnumerator * enumerator = [dict keyEnumerator];
while ((key = [enumerator nextObject]))
NSLog(@"%@ : %@", key, [dict objectForKey:key]);
set = [NSSet setWithObjects:@"c", @"b", @"e", @"d", nil];
NSString * value = [dict objectForKey:set];
NSLog(@"set: %@ : key: %@", set, value);
输出：
2009-12-08 15:42:17.885 x[4989] (d, e, b, c) : 2
2009-12-08 15:42:17.887 x[4989] (d, a, b, c) : 1
2009-12-08 15:42:17.887 x[4989] set: (d, e, b, c) : key: 2
Is there any downside to using NSSet as key in NSMutableDictionary, any gotchas to be aware of, any huge performance hits?
I think keys are copied in Cocoa containers, does it mean NSSet is copied to dictionary? Or is there some optimization that retains the NSSet in this case?
Related to Can a NSDictionary take in NSSet as key?
Example code:
NSMutableDictionary * dict = [NSMutableDictionary dictionary];
NSSet * set;
set = [NSSet setWithObjects:@"a", @"b", @"c", @"d", nil];
[dict setObject:@"1" forKey:set];
set = [NSSet setWithObjects:@"b", @"c", @"d", @"e", nil];
[dict setObject:@"2" forKey:set];
id key;
NSEnumerator * enumerator = [dict keyEnumerator];
while ((key = [enumerator nextObject]))
NSLog(@"%@ : %@", key, [dict objectForKey:key]);
set = [NSSet setWithObjects:@"c", @"b", @"e", @"d", nil];
NSString * value = [dict objectForKey:set];
NSLog(@"set: %@ : key: %@", set, value);
Outputs:
2009-12-08 15:42:17.885 x[4989] (d, e, b, c) : 2
2009-12-08 15:42:17.887 x[4989] (d, a, b, c) : 1
2009-12-08 15:42:17.887 x[4989] set: (d, e, b, c) : key: 2
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NSDictionaries 会复制它们的键。
不可变集可能会通过返回保留的自身来响应复制，从而使“复制”实际上是免费的。
可变集将通过返回其自身的副本来响应复制，这就是为什么使用可变对象作为键通常是一个坏主意（在改变它之后你将无法找到原始对象，因为它不再比较等于字典中的键）。
NSDictionaries do copy their keys.
An immutable set will probably respond to
copyby returning itself retained, making the “copy” practically free.A mutable set will respond to
copyby returning a copy of itself, which is why using mutable objects as keys is generally a bad idea (you won't be able to find the original after mutating it because it no longer compares equal to the key in the dictionary).哦。是的。性能存在很大的缺点。碰巧
-[NSSet hash]被实现为[set count]。这意味着，如果您的所有集合都有 2 个对象，那么它们都具有相同的哈希值，并且集合的性能将非常差。Ooh. Yes. There is a big performance downside. It happens that
-[NSSet hash]is implemented as[set count]. That means that if all your sets have 2 objects, say, then they all have the same hash, and the collection will perform very poorly.