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>
// }

Shouldn't the 2nd log be an empty map table?

@mrwalker it should be, but first you need to compile the code with ARC.

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.