What is the right choice between NSDecimal, NSDecimalNumber, CFNumber?

Question

I've read a lot about NSDecimal, NSNumber, NSNumberDecimal, CFNumber... and it begins to be a kind of jungle to me. Basically, I'm trying to create a simple model class that will handle simple computations, like this one: ``` #import <Foundation/Foundation.h> @interface Test : NSObject { float rate; float amount; int duration; } - (float)capitalizedAmount; @end @implementation Test - (float)capitalizedAmount { return (amount*pow((1.0+rate),duration)); } @end ``` I want to access these methods and setters with their names as strings, since I plan to have a lot of other classes like this one, and I am only keeping a list of field to do key value coding. ``` // This is just the desired behavior // This evidently won't work with the previous class definition Test *obj = [[Test alloc] init]; [NSNumber numberWithInt:10] ... float r; r = [obj performSelector:NSSelectorFromString(@"capitalizedAmount")]; ``` I understand that this is not possible, that `performSelector:` will return an object,…

Accepted Answer

If you are dealing with financial computations, you really should use base-10 arithmetic to avoid the rounding errors that can occur with the standard base-2 floating point types. So it's either NSDecimal or NSDecimalNumber. And since you're writing object-oriented code, NSDecimalNumber is the right choice for you.

To answer your questions: only testing of your code can reveal whether the memory overhead and performance loss are acceptable to you. I haven't really worked much with NSDecimalNumber but I'd wager that Apple's implementation is quite efficient and will be more than adequate for most people's needs.

Unfortunately, you won't be able to avoid the likes of `decimalNumberByAdding:` since Objective-C does not support operator overloading like C++ does. I agree that it makes your code somewhat less elegant.

One comment on the code you posted: `r = [obj performSelector:NSSelectorFromString(@"capitalizedAmount")];` is rather unelegant. Either

```
r = [obj performSelector:@selector(capitalizedAmount)];
```

or even the simple

```
r = [obj capitalizedAmount];
```

would be better unless you require the `NSSelectorFromString` syntax for some other reason.

What is the right choice between NSDecimal, NSDecimalNumber, CFNumber?

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