NSHashTable with weak references

Question

I want to to use a NSHashTable for keeping weak references to the contained objects. Regarding other customizable behaviors(including equality check), I want the exact same behavior as NSSet(so practically I want to have a NSSet with weak references). Can you give me an example on how to initialize such a hash table?

Would following suffice: [NSHashTable hashTableWithOptions:NSPointerFunctionsWeakMemory]

Also will NSHashTable with weak references automatically remove de-allocated objects?

Answer 1

Yes, you can use NSPointerFunctionsWeakMemory. Facebook KVOController also use NSHashTable with that option, see KVOController

- (instancetype)init
{
  self = [super init];
  if (nil != self) {
    NSHashTable *infos = [NSHashTable alloc];
#ifdef __IPHONE_OS_VERSION_MIN_REQUIRED
    _infos = [infos initWithOptions:NSPointerFunctionsWeakMemory|NSPointerFunctionsObjectPointerPersonality capacity:0];
#elif defined(__MAC_OS_X_VERSION_MIN_REQUIRED)
    if ([NSHashTable respondsToSelector:@selector(weakObjectsHashTable)]) {
      _infos = [infos initWithOptions:NSPointerFunctionsWeakMemory|NSPointerFunctionsObjectPointerPersonality capacity:0];
    } else {
      // silence deprecated warnings
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wdeprecated-declarations"
      _infos = [infos initWithOptions:NSPointerFunctionsZeroingWeakMemory|NSPointerFunctionsObjectPointerPersonality capacity:0];
#pragma clang diagnostic pop
    }

#endif
    _lock = OS_SPINLOCK_INIT;
  }
  return self;
}

Also, for more convenient way, you can use weakObjectsHashTable

Returns a new hash table for storing weak references to its contents.

Return Value A new hash table that uses the options NSHashTableZeroingWeakMemory and NSPointerFunctionsObjectPersonality and has an initial capacity of 0.

The document is a little bit old, but it is true. See NSHipster NSHash​Table & NSMap​Table

NSHashTableZeroingWeakMemory: This option has been deprecated. Instead use the NSHashTableWeakMemory option

Note also that

NSHashTableWeakMemory Equal to NSPointerFunctionsWeakMemory