Iphone – execute the NSDecimalCompact() function on every NSDecimal I create

Performance-wise, a static method in a static class compile to almost the same thing as a function.

Any real performance hits you'd incur would be in object instantiation, which you said you'd want to avoid, so that should not be an issue.

As far as preference or readability, there is a trend to use static methods more than necessary because people are viewing Obj-C is an "OO-only" language, like Java or C#. In that paradigm, (almost) everything must belong to a class, so class methods are the norm. In fact, they may even call them functions. The two terms are interchangeable there. However, this is purely convention. Convention may even be too strong of a word. There is absolutely nothing wrong with using functions in their place and it is probably more appropriate if there are no class members (even static ones) that are needed to assist in the processing of those methods/functions.

Although the other answer on this question links to a correct explanation of the concept of significant digits in general, NSNumberFormatter's {uses|minimum|maximum}SignificantDigits properties have nothing to do with precision of calculations.

The significant digits are the group of digits in a number from the first nonzero digit to the last nonzero digit, inclusive, usually unless trailing zeroes are fractional. Restricting output to a specific number of significant digits is useful if a relative (percentage) error is known or desired.

First of all, the minimumSignificantDigits and maximumSignificantDigits have no effect unless usesSignificantDigits is set to YES. If this is the case, their effect is probably most easily explained using examples.

Let's take the numbers a = 123.4567, b = 1.23, and c = 0.00123:

Assuming minimumSignificantDigits = 0, 1 or 2:

If maximumSignificantDigits = 3, then a will be formatted as "123", b as "1.23", and c as "0.00123".

If maximumSignificantDigits = 4, then a will be formatted as "123​.5", b as "1.23" and c as "0.00123".

If maximumSignificantDigits = 2, then a will be formatted as "12​0", b as "1.2" and c as "0.0012".

Assuming minimumSignificantDigits = 4:

If maximumSignificantDigits = 4, then a will be formatted as "123.​5", b as "1.23​0", and c as "0.00123​0".

Note: The 4 → 5 conversions occur due to the round-to-nearest mode, as the digit following the 4 in a is 5.