How to use NSDecimalNumber?

Question

I'm building a app that needs to perform calculations on money.

I wonder how to properly use NSDecimalNumber, especially how to initialize it from integers, floats & doubles?

I only found it easy to use the -decimalNumberWithString: method. The -initWith... methods are discouraged so that only left the ones with mantissa, but never in any of 7 languages I used before did I need that so I don't know what put there...

Answer 1

Do NOT use NSNumber's +numberWith... methods to create NSDecimalNumber objects. They are declared to return NSNumber objects and are not guaranteed to function as NSDecimalNumber instances.

This is explained in this thread by Bill Bumgarner, a developer at Apple. I would encourage you to file a bug against this behavior, referencing bug rdar://6487304.

As an alternative these are all of the appropriate methods to use to create an NSDecimalNumber:

+ (NSDecimalNumber *)decimalNumberWithMantissa:(unsigned long long)mantissa                      exponent:(short)exponent isNegative:(BOOL)flag; + (NSDecimalNumber *)decimalNumberWithDecimal:(NSDecimal)dcm; + (NSDecimalNumber *)decimalNumberWithString:(NSString *)numberValue; + (NSDecimalNumber *)decimalNumberWithString:(NSString *)numberValue locale:(id)locale;  + (NSDecimalNumber *)zero; + (NSDecimalNumber *)one; + (NSDecimalNumber *)minimumDecimalNumber; + (NSDecimalNumber *)maximumDecimalNumber; + (NSDecimalNumber *)notANumber; 

If you simply want an NSDecimalNumber from a float or int constant try something like this:

NSDecimalNumber *dn = [NSDecimalNumber decimalNumberWithDecimal:                              [[NSNumber numberWithFloat:2.75f] decimalValue];