I have a requirement to create some NSDecimalNumber objects objects as part of my application (as I require the precision of calculation they offer) but I note that in calculations they return NSDecimalNumber objects which are, presumably, autoreleased.
My question is really whether this is potentially problematic in an iPhone application where I may carry out lots of calculations.
The question is not just relating to NSDecimalNumber specifically but to the sometimes unavoidable creation of autoreleased objects in the course of developing an iPhone application.
Any detailed answers on this point would be gratefully received.
Yes, creating lots of autoreleased instances on the iPhone can create memory problems, particularly in a tight loop, which is why I tend to avoid them when I can. You can create your own autorelease pools to manage this, but they also add some performance overhead and additional code that you have to keep track of.
It is for this reason that when I do high-precision calculations, I tend to use the NSDecimal C struct instead of NSDecimalNumbers. In fact, I performed some benchmarks on this and found a significant performance increase when going with the C struct (copied from my answer here):
NSDecimal
Additions per second: 3355476.75
Subtractions per second: 3866671.27
Multiplications per second: 3458770.51
Divisions per second: 276242.32
NSDecimalNumber
Additions per second: 676901.32
Subtractions per second: 671474.6
Multiplications per second: 720310.63
Divisions per second: 190249.33
As you can see, there is almost a fivefold increase in calculation speed between the NSDecimal paths and NSDecimalNumber ones. The biggest difference between the NSDecimal and NSDecimalNumber calculations was the memory allocation of the NSDecimalNumber instances. Therefore, you should avoid allocating temporary autoreleased instances wherever you can.