NSMutableArray, alloc or capacity?

Question

Is there any reason to go with either of these, other than one returning an autoreleased object and the other needing a manual release?

NSMutableDictionary *drink = [[NSMutableDictionary alloc] init];
// do things ...
[drink release];

OR

NSMutableDictionary *drink = [NSMutableDictionary dictionaryWithCapacity:10];

gary

Answer 1

These are convenience methods that are useful for reducing the amount of code you have to write, and are there to help in certain circumstances.

The object you get will have no real differences other than that the +dictionaryWithCapacity: and +dictionary methods will be autoreleased, and the one returned by -init won't.

As an aside, if you know how many items will be put into the dictionary, +dictionaryWithCapacity: and -initWithCapacity: will provide a slight boost in efficiency since the dictionary will be able to allocate the right amount of memory right off the bat.

Answer 2

The first one will init a NSMutableArray with the default size. So internally the NSMutableArray will realloc space as needed when you're adding new key:value to it. Indeed, you have to release it yourself, no autorelease is called by init.

The second one will pre-alloc 10 slot in your object. It's better in this regards if you know in advance how many items you'll have in your dictionary. This method will be autoreleased. You can also call initWithCapacity that won't be autoreleased like init but with a preallocated size.

As a convention all methods beginning with init must be allocated with alloc and release by your code explicitly and static method like dictionary* are autoreleased.

Answer 3

the only reason I can think of to choose one method over the other is for performance reason... and only in an extreme case.

example case, adding a million objects to NSMutableArray

with [[NSMutableArray alloc] init], it'll have to realloc a million times, each time an object is added.

with [[NSMutableArray alloc] initWithCapacity:1000000], the array will be allocated with enough memory for a million objects at the get-go, so you don't incur the added overhead that the first method has.

so if you know how many items you want to put into the array beforehand, go for an initial capacity.

if you don't, then go for the default alloc+init.

here's a nice article that may give more insight: http://cocoawithlove.com/2008/08/nsarray-or-nsset-nsdictionary-or.html