NSMutableArray arrayWithCapacity与initWithCapacity

Question

I'm an iPhone/Objective-C newbie with an extensive Java background.

I'm learning more about memory management in objective-c and I'm reading Apple's documentation on Memory Management: http://developer.apple.com/iphone/library/documentation/Cocoa/Conceptual/MemoryMgmt/MemoryMgmt.html

In the Object Ownership Policy section, it says that you own any object you create via a method that begins with alloc, new or contains copy. Ownership implies that you need to explicitly release the object when you are done with it.

So I'm looking at the NSMutableArray documentation: http://developer.apple.com/mac/library/documentation/Cocoa/Reference/Foundation/Classes/NSMutableArray_Class/Reference/Reference.html

There are two methods that pretty much do the same thing...they both create an array with some initial capacity. One is a class method and the other is an instance method.

+ (id)arrayWithCapacity:(NSUInteger)numItems;
- (id)initWithCapacity:(NSUInteger)numItems;

Now being the lazy Java developer I am, why would I ever choose the instance method over the class method knowing that at some point in time I have to clean up after myself?

I guess I may be missing a fundamental point here...is it simply a matter of determining when the object gets released? autorelease in the class method vs. release in the instance method? I suppose that on a platform with very limited resources (iPhone) I should refrain from using the class method and release the object as soon as I'm done with it?

Thanks!

Answer 1

You will usually choose based on whether or not you are going to own the object for more than the life of the current method (eg, assign it to some static or directly to an ivar). In that case, you can use the alloc/init method since you know you want to own it already. If you plan to only use it for the scope of the current method, or you are assigning it to something managed like a property, then you would probably use the convenience method.

When you know that you are going to own an object that you are creating, the alloc/init call is always more efficient than the convenience/retain way since the latter is required to basically alloc/init/autorelease the object and then you retain it when it is returned.

You might also use the direct alloc/init methods when you are allocating in a loop and don't need/want to deal with an autorelease pool.

Answer 2

arrayWithCapacity: already has the autorelease applied to it.

initWithCapacity: is explicitly retained and you will need to release it yourself. Since you call it usually as [[A alloc] init...], this will trigger a lightbulb "I need to manage the memory for this," other similar magic words besides "alloc" being "new" and "copy" as you read in the memory management guide. But from your question, looks like you understand the principles of it well.

You are correct that you should keep your memory footprint managed and low, but this does not mean that you need to always do explicit init/release. As Nick says, one use case to use autorelease factory methods is when you are passing them around as parameters.

Another example is that when you add something to a collection like NSDictionary or NSArray, that "something" can be created with the autorelease factory method, since the collection "takes over" retaining it. (Things are retained when added to collection, and released when removed.)

You may argue that

Blah *blah = [Blah blahWithSomething];
[myMutableArray addObject:blah];

is just cleaner to type than

Blah *blah = [[Blah alloc] initWithSomething];
[myMutableArray addObject:blah];
[blah release];

In the first case, you do not need to worry about a release call at all. The downside is that when you do this many times in the same runloop, memory footprint of first case is greater IF it is a temporary/throwaway array object that goes away at the end of the runloop. But if it's a small object not done in a loop and retained for longer time as is a common case, their footprints are the same.

Answer 3

In this case I try to stick to the following rules to avoid memory related ( == "very nasty") bugs:

• if you are passing that array as a parameter, then you can use the factory method no problem, since it's the responsibility of the accepting function to retain/release it
• if you want to keep working with the object, use the initialization method and release the object at the end