NSMapTable weakToStrongObjectsMapTable

maptable.m

#import <Foundation/Foundation.h>

#if ! __has_feature(objc_arc)
#error This file must be compiled with ARC.
#endif

@interface Number : NSObject
@property (nonatomic, readonly) NSInteger integer;
+ (id)numberWithInteger:(NSInteger)integer;
- (id)initWithInteger:(NSInteger)integer;
@end

@implementation Number
+ (id)numberWithInteger:(NSInteger)integer {
  return [[Number alloc] initWithInteger:integer];
}

- (id)initWithInteger:(NSInteger)integer {
  if (self == [super init]) {
    _integer = integer;
  }
  return self;
}

- (void)dealloc {
  NSLog(@"dealloc number %i", (int) _integer);
}
@end
 
int main(int argc, char *argv[]) {
  @autoreleasepool {
    NSMapTable *mapTable = [NSMapTable weakToStrongObjectsMapTable];
  
    NSArray *keys = nil;
  
    @autoreleasepool {
      keys = [[NSArray alloc] initWithObjects:[Number numberWithInteger:1], [Number numberWithInteger:2], [Number numberWithInteger:3], nil];
      for (id key in keys) {
        [mapTable setObject:[NSNull null] forKey:key];
      }       
      NSLog(@"%@", mapTable);
      keys = nil;
    }
    
    [mapTable setObject:[NSNull null] forKey:@(4)];
    
    NSLog(@"%@", mapTable);
  }
}
 
// Output
//
// 2014-01-07 09:52:22.269 Untitled[31334:507] NSMapTable {
// [1] <Number: 0x7f9ddbc07c50> -> <null>
// [2] <Number: 0x7f9ddbc083a0> -> <null>
// [3] <Number: 0x7f9ddbc0b020> -> <null>
// }
// 2014-01-07 09:52:22.271 Untitled[31334:507] dealloc number 1
// 2014-01-07 09:52:22.271 Untitled[31334:507] dealloc number 2
// 2014-01-07 09:52:22.272 Untitled[31334:507] dealloc number 3
// 2014-01-07 09:52:22.272 Untitled[31334:507] NSMapTable {
// [1] 4 -> <null>
// }

it could be that the Integer objects are optimised to keep common numbers (i.e. lower numbers) around as global constants for a while. I know java does this, so that Integer (the object) does not need to be expensively re-instantiated every time you put a 4 into an array or dictionary.