Objective C strange decimal to long long conversion

Question

NSLog(@"%llu\n\n", ULONG_LONG_MAX);

NSDecimalNumber *decimal = [NSDecimalNumber decimalNumberWithString:@"154550038129946620"];
NSLog(@"%@", decimal);
NSLog(@"%llu\n\n", [decimal unsignedLongLongValue]);

decimal = [NSDecimalNumber decimalNumberWithString:@"154550038129946628"];
NSLog(@"%@", decimal);
NSLog(@"%llu", [decimal unsignedLongLongValue]);

Both value are less than ULONG_LONG_MAX. But, what we see in output is:

2012-01-05 17:41:55.879 test[1276:207] 18446744073709551615

2012-01-05 17:41:55.969 test[1276:207] 154550038129946620
2012-01-05 17:41:56.095 test[1276:207] 154550038129946624

2012-01-05 17:41:56.096 test[1276:207] 154550038129946628
2012-01-05 17:41:56.096 test[1276:207] 154550038129946624

What am I doing wrong? I have no idea how to explain this behaviour.

Answer 1

NSDecimalNumber lacks the unsignedLongLongValue method, so it inherits a default implementation from NSNumber. The only ways to get value of NSDecimalNumber are getting an NSDecimal structure, or a double. The conversion error is introduced in the process of converting NSDecimalNumber to a double.