Disadvantage of using NSMutableArray vs NSArray?

Question

I'm using an array to store cached objects loaded from a database in my iPhone app, and was wondering: are there any significant disadvantages to using NSMutableArray that I should know of?

edit: I know that NSMutableArray can be modified, but I'm looking for specific reasons (performance, etc..) why one would use NSArray instead. I assume there would be a performance difference, but I have no idea whether it's significant or not.

Answer 1

If you're loading objects from a database and you know exactly how many objects you have, you would likely get the best performance from NSMutableArrays arrayWithCapacity: method, and adding objects to it until full, so it allocates all the memory at once if it can.

Behind the scenes, they're secretly the same thing - NSArray and NSMutableArray are both implemented with CFArrays via toll free bridging (a CFMutableArrayRef and a CFArrayRef are typedef's of the same thing, __CFArray *) *

NSArray and NSMutableArray should have the same performance/complexity (access time being O(lg N) at worst and O(1) at best) and the only difference being how much memory the two objects would use - NSArray has a fixed limit, while NSMutableArray can use up as much space as you have free.

The comments in CFArray.h have much more detail about this.

*: As Catfish_Man points out below, this isn't true anymore.