Bartosz Ciechanowski

Exposing NSDictionary

Hash tables are just awesome. To this day I find it fascinating that one can fetch an object corresponding to an arbitrary key in a constant time. Although iOS 6.0 introduced an explicit hash table, it is NSDictionary that’s almost exclusively used for associative storage.

NSDictionary doesn’t make any promise of its internal implementation. It would make little sense for a dictionary to store its data in a completely random fashion. However, this assumption doesn’t answer the key question: does NSDictionary make use of a hash table? This is what I’ve decided to investigate.

Why not to tackle the full featured NSMutableDictionary? A mutable dictionary is, understandably, much more complex and the amount of disassembly I would have had to go through was terrifying. Regular NSDictionary still provided a nontrivial ARM64 deciphering challenge. Despite being immutable, the class has some very interesting implementation details which should make the following ride enjoyable.

This blog post has a companion repo which contains discussed pieces of code. While the entire investigation has been based on the iOS 7.1 SDK targeting a 64-bit device, neither iOS 7.0 nor 32-bit devices impact the findings.

The Class

Plenty of Foundation classes are class clusters and NSDictionary is no exception. For quite a long time NSDictionary used CFDictionary as its default implementation, however, starting with iOS 6.0 things have changed:

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(lldb) po [[NSDictionary new] class]
__NSDictionaryI

Similarly to __NSArrayM, __NSDictionaryI rests within the CoreFoundation framework, in spite of being publicly presented as a part of Foundation. Running the library through class-dump generates the following ivar layout:

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@interface __NSDictionaryI : NSDictionary
{
    NSUIngeter _used:58;
    NSUIngeter _szidx:6;
}

It’s surprisingly short. There doesn’t seem to be any pointer to either keys or objects storage. As we will soon see, __NSDictionary literally keeps its storage to itself.

The Storage

Instance Creation

To understand where __NSDictionaryI keeps its contents, let’s take a quick tour through the instance creation process. There is just one class method that’s responsible for spawning new instances of __NSDictionaryI. According to class-dump, the method has the following signature:

+ (id)__new:(const id *)arg1:(const id *)arg2:(unsigned long long)arg3:(_Bool)arg4:(_Bool)arg5;

It takes five arguments, of which only the first one is named. Seriously, if you were to use it in a @selector statement it would have a form of @selector(__new:::::). The first three arguments are easily inferred by setting a breakpoint on this method and peeking into the contents of x2, x3 and x4 registers which contain the array of keys, array of objects and number of keys (objects) respectively. Notice, that keys and objects arrays are swapped in comparison to the public facing API which takes a form of:

+ (instancetype)dictionaryWithObjects:(const id [])objects forKeys:(const id <NSCopying> [])keys count:(NSUInteger)cnt;

It doesn’t matter whether an argument is defined as const id * or const id [] since arrays decay into pointers when passed as function arguments.

With three arguments covered we’re left with the two unidentified boolean parameters. I’ve done some assembly digging with the following results: the fourth argument governs whether the keys should be copied, and the last one decides whether the arguments should not be retained. We can now rewrite the method with named parameters:

+ (id)__new:(const id *)keys :(const id *)objects :(unsigned long long)count :(_Bool)copyKeys :(_Bool)dontRetain;

Unfortunately, we don’t have explicit access to this private method, so by using the regular means of allocation the last two arguments are always set to YES and NO respectively. It is nonetheless interesting that __NSDictionaryI is capable of a more sophisticated keys and objects control.

Indexed ivars

Skimming through the disassembly of + __new::::: reveals that both malloc and calloc are nowhere to be found. Instead, the method calls into __CFAllocateObject2 passing the __NSDictionaryI class as first argument and requested storage size as a second. Stepping down into the sea of ARM64 shows that the first thing __CFAllocateObject2 does is call into class_createInstance with the exact same arguments.

Fortunately, at this point we have access to the source code of Objective-C runtime which makes further investigation much easier.

The class_createInstance(Class cls, size_t extraBytes) function merely calls into _class_createInstanceFromZone passing nil as a zone, but this is the final step of object allocation. While the function itself has many additional checks for different various circumstances, its gist can be covered with just three lines:

_class_createInstanceFromZone(Class cls, size_t extraBytes, void *zone)
{
    ...
    size_t size = cls->alignedInstanceSize() + extraBytes;
    ...
    id obj = (id)calloc(1, size);
    ...
    return obj;
}

The extraBytes argument couldn’t have been more descriptive. It’s literally the number of extra bytes that inflate the default instance size. As an added bonus, notice that it’s the calloc call that ensures all the ivars are zeroed out when the object gets allocated.

The indexed ivars section is nothing more than an additional space that sits at the end of regular ivars:

Allocating objects

Allocating space on its own doesn’t sound very thrilling so the runtime publishes an accessor:

void *object_getIndexedIvars(id obj)

There is no magic whatsoever in this function, it just returns a pointer to the beginning of indexed ivars section:

Indexed ivars section

There are few cool things about indexed ivars. First of all, each instance can have different amount of extra bytes dedicated to it. This is exactly the feature __NSDictionaryI makes use of.

Secondly, they provide faster access to the storage. It all comes down to being cache-friendly. Generally speaking, jumping to random memory locations (by dereferencing a pointer) can be expensive. Since the object has just been accessed (somebody has called a method on it), it’s very likely that its indexed ivars have landed in cache. By keeping everything that’s needed very close, the object can provide as good performance as possible.

Finally, indexed ivars can be used as a crude defensive measure to make object’s internals invisible to the utilities like class-dump. This is a very basic protection since a dedicated attacker can simply look for object_getIndexedIvars calls in the disassembly or randomly probe the instance past its regular ivars section to see what’s going on.

While powerful, indexed ivars come with two caveats. First of all, class_createInstance can’t be used under ARC, so you’ll have to compile some parts of your class with -fno-objc-arc flag to make it shine. Secondly, the runtime doesn’t keep the indexed ivar size information anywhere. Even though dealloc will clean everything up (as it calls free internally), you should keep the storage size somewhere, assuming you use variable number of extra bytes.

Looking for Key and Fetching Object

Analyzing Assembly

Although at this point we could poke the __NSDictionaryI instances to figure out how they work, the ultimate truth lies within the assembly. Instead of going through the entire wall of ARM64 we will discuss the equivalent Objective-C code instead.

The class itself implements very few methods, but I claim the most important is objectForKey: – this is what we’re going to discuss in more detail. Since I made the assembly analysis anyway, you can read it on a separate page. It’s dense, but the thorough pass should convince you the following code is more or less correct.

The C Code

Unfortunately, I don’t have access to the Apple’s code base, so the reverse-engineered code below is not identical to the original implementation. On the other hand, it seems to be working well and I’ve yet to find an edge case that behaves differently in comparison to the genuine method.

The following code is written from the perspective of __NSDictionaryI class:

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- (id)objectForKey:(id)aKey
{
    NSUInteger sizeIndex = _szidx;
    NSUInteger size = __NSDictionarySizes[sizeIndex];

    id *storage = (id *)object_getIndexedIvars(dict);

    NSUInteger fetchIndex = [aKey hash] % size;

    for (int i = 0; i < size; i++) {
        id fetchedKey = storage[2 * fetchIndex];

        if (fetchedKey == nil) {
            return nil;
        }

        if (fetchedKey == aKey || [fetchedKey isEqual:aKey]) {
            return storage[2 * fetchIndex + 1];
        }

        fetchIndex++;

        if (fetchIndex == size) {
            fetchIndex = 0;
        }
    }

    return nil;
}

When you take a closer look at the C code you might notice something strange about key fetching. It’s always taken from even offsets, while the returned object is at the very next index. This is the dead giveaway of __NSDictionaryI’s internal storage: it keeps keys and objects alternately:

Keys and objects are stored alternately

Update: Joan Lluch provided a very convincing explanation for this layout. The original code could use an array of very simple structs:

struct KeyObjectPair {
    id key;
    id object;
};

The objectForKey: method is very straightforward and I highly encourage you to walk through it in your head. It’s nonetheless worth pointing out a few things. First of all, the _szidx ivar is used as an index into the __NSDictionarySizes array, thus it most likely stands for “size index”.

Secondly, the only method called on the passed key is hash. The reminder of dividing key’s hash value by dictionary’s size is used to calculate the offset into the index ivars section.

If the key at the offset is nil, we simply return nil, the job is done:

When the key slot is empty, nil is returned

However, if the key at the offset is non nil, then the two cases may occur. If the keys are equal, then we return the adjacent object. If they’re not equal then the hash collision occurred and we have to keep looking further. __NSDictionaryI simply keeps looking until either match or nil is found:

Key found after one collision

This kind of searching is known as linear probing. Notice how __NSDictionaryI wraps the fetchIndex around when the storage end is hit. The for loop is there to limit the number of checks – if the storage was full and the loop condition was missing we’d keep looking forever.

__NSDictionarySizes & __NSDictionaryCapacities

We already know __NSDictionarySizes is some kind of array that stores different possible sizes of __NSDictionaryI. We can reason that it’s an array of NSUIntegers and indeed, if we ask Hopper to treat the values as 64-bit unsigned integers it suddenly makes a lot of sense:

                ___NSDictionarySizes:
0x00000000001577a8         dq         0x0000000000000000
0x00000000001577b0         dq         0x0000000000000003
0x00000000001577b8         dq         0x0000000000000007
0x00000000001577c0         dq         0x000000000000000d
0x00000000001577c8         dq         0x0000000000000017
0x00000000001577d0         dq         0x0000000000000029
0x00000000001577d8         dq         0x0000000000000047
0x00000000001577e0         dq         0x000000000000007f
...

In a more familiar decimal form it presents as a beautiful list of 64 primes starting with the following sequence: 0, 3, 7, 13, 23, 41, 71, 127. Notice, that those are not consecutive primes which begs the question: what’s the average ratio of the two neighboring numbers? It’s actually around 1.637 – a very close match to the 1.625 which was the growth factor for NSMutableArray. For details of why primes are used for the storage size this Stack Overflow answer is a good start.

We already know how much storage can __NSDictionaryI have, but how does it know which size index to pick on initialization? The answer lies within the previously mentioned + __new::::: class method. Converting some parts of the assembly back to C renders the following code:

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int szidx;
for (szidx = 0; szidx < 64; szidx++) {
    if (__NSDictionaryCapacities[szidx] >= count) {
        break;
    }
}

if (szidx == 64) {
    goto fail;
}

The method looks linearly through __NSDictionaryCapacities array until count fits into the size. A quick glance in Hopper shows the contents of the array:

                ___NSDictionaryCapacities:
0x00000000001579b0         dq         0x0000000000000000
0x00000000001579b8         dq         0x0000000000000003
0x00000000001579c0         dq         0x0000000000000006
0x00000000001579c8         dq         0x000000000000000b
0x00000000001579d0         dq         0x0000000000000013
0x00000000001579d8         dq         0x0000000000000020
0x00000000001579e0         dq         0x0000000000000034
0x00000000001579e8         dq         0x0000000000000055
...

Converting to base-10 provides 0, 3, 6, 11, 19, 32, 52, 85 and so on. Notice that those are smaller numbers than the primes listed before. If you were to fit 32 key-value pairs into __NSDictionaryI it will allocate space for 41 pairs, conservatively saving quite a few empty slots. This helps reducing the number of hash collisions, keeping the fetching time as close to constant as possible. Apart from trivial case of 3 elements, __NSDictionaryI will never have its storage full, on average filling at most 62% of its space.

As a trivia, the last nonempty value of __NSDictionaryCapacities is 0x11089481C742 which is 18728548943682 in base-10. You’d have to try really hard not to fit in within the pairs count limit, at least on 64-bit architecture.

Non Exported Symbols

If you were to use __NSDictionarySizes in your code by declaring it as an extern array, you’d quickly realize it’s not that easy. The code wouldn’t compile due to a linker error – the __NSDictionarySizes symbol is undefined. Inspecting the CoreFoundation library with nm utility:

nm CoreFoundation | grep ___NSDictionarySizes

…clearly shows the symbols are there (for ARMv7, ARMv7s and ARM64 respectively):

00139c80 s ___NSDictionarySizes
0013ac80 s ___NSDictionarySizes
0000000000156f38 s ___NSDictionarySizes

Unfortunately the nm manual clearly states:

If the symbol is local (non-external), the symbol’s type is instead represented by the corresponding lowercase letter.

The symbols for __NSDictionarySizes are simply not exported – they’re intended for internal use of the library. I’ve done some research to figure out if it’s possible to link with non-exported symbols, but apparently it is not (please tell me if it is!). We can’t access them. That is to say, we can’t access them easily.

Sneaking in

Here’s an interesting observation: in both iOS 7.0 an 7.1 the kCFAbsoluteTimeIntervalSince1904 constant is laid out directly before __NSDictionarySizes:

                _kCFAbsoluteTimeIntervalSince1904:
0x00000000001577a0         dq         0x41e6ceaf20000000
                ___NSDictionarySizes:
0x00000000001577a8         dq         0x0000000000000000

The best thing about kCFAbsoluteTimeIntervalSince1904 is that it is exported! We’re going to add 8 bytes (size of double) to the address of this constant and reinterpret the result as pointer to NSUInteger:

NSUInteger *Explored__NSDictionarySizes = (NSUInteger *)((char *)&kCFAbsoluteTimeIntervalSince1904 + 8);

Then we can access its values by convenient indexing:

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(lldb) p Explored__NSDictionarySizes[0]
(NSUInteger) $0 = 0
(lldb) p Explored__NSDictionarySizes[1]
(NSUInteger) $1 = 3
(lldb) p Explored__NSDictionarySizes[2]
(NSUInteger) $2 = 7

This hack is very fragile and will most likely break in the future, but this is the test project so it’s perfectly fine.

__NSDictionaryI Characteristics

Now that we’ve discovered the internal structure of __NSDictionaryI we can use this information to figure out why things work they way they work and what unforeseen consequences does the present implementation of __NSDictionaryI introduce.

Printout Code

To make our investigation a little bit easier we will create a helper NSDictionary category method that will print the contents of the instance

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- (NSString *)explored_description
{
    assert([NSStringFromClass([self class]) isEqualToString:@"__NSDictionaryI"]);

    BCExploredDictionary *dict = (BCExploredDictionary *)self;

    NSUInteger count = dict->_used;
    NSUInteger sizeIndex = dict->_szidx;
    NSUInteger size = Explored__NSDictionarySizes[sizeIndex];

    __unsafe_unretained id *storage = (__unsafe_unretained id *)object_getIndexedIvars(dict);

    NSMutableString *description = [NSMutableString stringWithString:@"\n"];

    [description appendFormat:@"Count: %lu\n", (unsigned long)count];
    [description appendFormat:@"Size index: %lu\n", (unsigned long)sizeIndex];
    [description appendFormat:@"Size: %lu\n", (unsigned long)size];

    for (int i = 0; i < size; i++) {
        [description appendFormat:@"[%d] %@ - %@\n", i, [storage[2*i] description], [storage[2*i + 1] description]];
    }

    return description;
}

Order of keys/objects on enumeration is the same as order of keys/objects in storage

Let’s create a simple dictionary containing four values:

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NSDictionary *dict = @{@1 : @"Value 1",
                       @2 : @"Value 2",
                       @3 : @"Value 3",
                       @4 : @"Value 4"};

NSLog(@"%@", [dict explored_description]);

The output of the explored description is:

Count: 4
Size index: 2
Size: 7
[0] (null) - (null)
[1] 3 - Value 3
[2] (null) - (null)
[3] 2 - Value 2
[4] (null) - (null)
[5] 1 - Value 1
[6] 4 - Value 4

With that in mind let’s do a quick enumeration over dictionary:

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[dict enumerateKeysAndObjectsUsingBlock:^(id key, id obj, BOOL *stop) {
    NSLog(@"%@ - %@", key, obj);
}];

And the output:

3 - Value 3
2 - Value 2
1 - Value 1
4 - Value 4

Enumeration seems to simply walk through the storage, ignoring the nil keys and calling the block only for non-empty slots. This is also the case for fast enumeration, keyEnumerator, allKeys and allValues methods. It makes perfect sense. The NSDictionary is not ordered, so it doesn’t really matter in what sequence the keys and values are provided. Using the internal layout is the easiest and probably the fastest option.

If you mess up, __NSDictionaryI may return something for nil key!

Let’s consider an example. Imagine we’re building a simple 3D strategy game set in space. The entire universe is split into cube-like sectors that imaginary factions can fight over. A sector can be referenced by its i, j and k indexes. We shouldn’t use a 3D array to store the sectors info – the game space is huge and most of it is empty, so we would waste memory storing nil pointers. Instead, we’re going to use a sparse storage in a form of NSDictionary with a custom key class that will make it super easy to query if there is something at a given location.

Here’s the interface for key, a BC3DIndex class:

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@interface BC3DIndex : NSObject <NSCopying>

@property (nonatomic, readonly) NSUInteger i, j, k; // you actually can do that

- (instancetype)initWithI:(NSUInteger)i j:(NSUInteger)j k:(NSUInteger)k;

@end

And its equally trivial implementation:

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@implementation BC3DIndex

- (instancetype)initWithI:(NSUInteger)i j:(NSUInteger)j k:(NSUInteger)k
{
    self = [super init];
    if (self) {
        _i = i;
        _j = j;
        _k = k;
    }
    return self;
}

- (BOOL)isEqual:(BC3DIndex *)other
{
    return other.i == _i && other.j == _j && other.k == _k;
}

- (NSUInteger)hash
{
    return _i ^ _j ^ _k;
}

- (id)copyWithZone:(NSZone *)zone
{
    return self; // we're immutable so it's OK
}

@end

Notice how we’re being a good subclassing citizen: we implemented both isEqual: and hash methods and made sure that if two 3D-indexes are equal then their hash values are equal as well. The object equality requirements are fulfilled.

Here’s a trivia: what will the following code print?

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NSDictionary *indexes = @{[[BC3DIndex alloc] initWithI:2 j:8 k:5] : @"A black hole!",
                          [[BC3DIndex alloc] initWithI:0 j:0 k:0] : @"Asteroids!",
                          [[BC3DIndex alloc] initWithI:4 j:3 k:4] : @"A planet!"};

NSLog(@"%@", [indexes objectForKey:nil]);

It should be (null) right? Nope:

Asteroids!

To investigate this further let’s grab a dictionary’s description:

Count: 3
Size index: 1
Size: 3
[0] <BC3DIndex: 0x17803d340> - A black hole!
[1] <BC3DIndex: 0x17803d360> - Asteroids!
[2] <BC3DIndex: 0x17803d380> - A planet!

It turns out __NSDictionaryI doesn’t check if the key passed into objectForKey: is nil (and I’d argue this is a good design decision). Calling hash method on nil returns 0, which causes the class to compare key at index 0 with nil. This is important: it is the stored key that executes the isEqual: method, not the passed in key.

The first comparison fails, since i index for “A black hole!” is 2 whereas for nil it’s zero. The keys are not equal which causes the dictionary to keep looking, hitting another stored key: the one for “Asteroids!”. This key has all three i, j, and k properties equal to 0 which is also what nil will return when asked for its properties (by the means of nil check inside objc_msgSend).

This is the crux of the problem. The isEqual: implementation of BC3DIndex may, under some conditions, return YES for nil comparison. As you can see, this is a very dangerous behavior that can mess things up easily. Always ensure that your object is not equal to nil.

A Helper Key Class

For the next two tests we’re going to craft a special key class that will have a configurable hash value and will print stuff to the console when executing hash and isEqual: method.

Here’s the interface:

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@interface BCNastyKey : NSObject <NSCopying>

@property (nonatomic, readonly) NSUInteger hashValue;

+ (instancetype)keyWithHashValue:(NSUInteger)hashValue;

@end

And the implementation:

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@implementation BCNastyKey

+ (instancetype)keyWithHashValue:(NSUInteger)hashValue
{
    return [[BCNastyKey alloc] initWithHashValue:hashValue];
}

- (instancetype)initWithHashValue:(NSUInteger)hashValue
{
    self = [super init];
    if (self) {
        _hashValue = hashValue;
    }
    return self;
}

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

- (NSUInteger)hash
{
    NSLog(@"Key %@ is asked for its hash", [self description]);

    return _hashValue;
}

- (BOOL)isEqual:(BCNastyKey *)object
{
    NSLog(@"Key %@ equality test with %@: %@", [self description], [object description], object == self ? @"YES" : @"NO");

    return object == self;
}

- (NSString *)description
{
    return [NSString stringWithFormat:@"(&:%p #:%lu)", self, (unsigned long)_hashValue];
}

@end

This key is awful: we’re only equal to self, but we’re returning an arbitrary hash. Notice that this doesn’t break the equality contract.

isEqual doesn’t have to be called to match the key

Let’s create a key and a dictionary:

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BCNastyKey *key = [BCNastyKey keyWithHashValue:3];
NSDictionary *dict = @{key : @"Hello there!"};

The following call:

[dict objectForKey:key];

Prints this to the console:

Key (&:0x17800e240 #:3) is asked for its hash

As you can see, the isEqual: method has not been called. This is very cool! Since the vast majority of keys out there are NSString literals, they share the same address in the entire application. Even if the key is a very long literal string, the __NSDictionaryI won’t run the potentially time consuming isEqual: method unless it absolutely has to. And since 64-bit architectures introduced tagged pointers, some instances of NSNumber, NSDate and, apparently, NSIndexPath benefit from this optimization as well.

Worst case performance is linear

Let’s create a very simple test case:

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BCNastyKey *targetKey = [BCNastyKey keyWithHashValue:36];

NSDictionary *b = @{[BCNastyKey keyWithHashValue:1] : @1,
                    [BCNastyKey keyWithHashValue:8] : @2,
                    [BCNastyKey keyWithHashValue:15] : @3,
                    [BCNastyKey keyWithHashValue:22] : @4,
                    [BCNastyKey keyWithHashValue:29] : @5,
                    targetKey : @6
                    };

A single killer line:

NSLog(@"Result: %@", [[b objectForKey:targetKey] description]);

Reveals the disaster:

Key (&:0x170017640 #:36) is asked for its hash
Key (&:0x170017670 #:1) equality test with (&:0x170017640 #:36): NO
Key (&:0x170017660 #:8) equality test with (&:0x170017640 #:36): NO
Key (&:0x170017680 #:15) equality test with (&:0x170017640 #:36): NO
Key (&:0x1700176e0 #:22) equality test with (&:0x170017640 #:36): NO
Key (&:0x170017760 #:29) equality test with (&:0x170017640 #:36): NO
Result: 6

This is extremely pathological case – every single key in the dictionary has ben equality tested. Even though each hash was different, it still collided with every other key, because the keys’ hashes were congruent modulo 7, which turned out to be the storage size of the dictionary.

As mentioned before, notice that the last isEqual: test is missing. The __NSDictionaryI simply compared the pointers and figured out it must be the same key.

Should you care for this linear time fetching? Absolutely not. I’m not that into probabilistic analysis of hash distribution, but you’d have to be extremely lucky for all your hashes to be modulo congruent to dictionary’s size. Some collisions here and there will always happen, this is the nature of hash tables, but you will probably never run into the linear time issue. That is, unless you mess up your hash function.

Final Words

I’m fascinated how simple the __NSDictionaryI turned out to be. Needless to say, the class certainly serves its purpose and there’s no need to make things excessively complex. For me, the most beautiful aspect of the implementation is the key-object-key-object layout. This is a brilliant idea.

If you were to take one tip from this article then I’d go with watching out for your hash and isEqual: methods. Granted, one rarely writes custom key classes to be used in a dictionary, but those rules apply to NSSet as well.

I’m aware that at some point in time NSDictionary will change and my findings will become obsolete. Internalizing the current implementation details may become a burden in the future, when the memorized assumptions will no longer apply. However, right here and right now it’s just so much fun to see how things work and hopefully you share my excitement.

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