Returning an NSMutableArray from a method...do I release it prior? - iPhone SDK

Question

I have been very confused on how to handle the releasing of an NSMutableArray when I need to return it from a method. I am not even sure if I am suppose to release it or not.

I have the code:

-(NSArray *)methodToCall {

        NSMutableArray *mutArray = [[NSMutableArray alloc] initWithCapacity:10];

        //Fill the array with stuff here

        [mutArray release];  //Am I suppose to have this to keep the memory down?
        return mutArray;
}

My question is whether or not I am suppose to have the [mutArray release]; in the code or not. Could someone explain this? I am at a loss and I want to keep my code as clean and leak free as possible.

Answer 1

The caller of your method is going to expect a valid NSArray, but which it doesn't own.

If you release it the way you're doing in your question, you're returning an invalid object (because you've both allocated and released it.) So that's not what you want.

You should "autorelease" the array before returning it. Then you'll be allocating it, but relinquishing ownership (without forcibly releasing it) before returning it. It will be valid until the end of the current event loop when the autorelease pool gets cleaned up, and your caller can retain it or let it go as appropriate:

-(NSArray *)methodToCall {

    NSMutableArray *mutArray = [[NSMutableArray alloc] initWithCapacity:10];

    //Fill the array with stuff here

    return [mutArray autorelease];
}

Your other alternative, since you don't need to keep the reference around either, is to use one of the "autoreleasing" convenience methods to create it in the first place:

-(NSArray *)methodToCall {

    // We're not doing an explicit alloc/init here, so...
    NSMutableArray *mutArray = [NSMutableArray arrayWithCapacity:10];

    // ...no autorelease necessary.
    return mutArray;
}

Answer 2

Short answer - No.

As it is now, you are allocating an array and then releasing (freeing) it before the return. So the when you try accessing the return object from your calling method you're going to get a crash.

My suggestion would be to use autorelease or to have the calling method or class ivar be responsible for this array object if it is used often.

An example of the autorelease:

NSMutableArray *mutArray = [[[NSMutableArray alloc] initWithCapacity:10] autorelease];

I also suggest reading the Memory Management from the Developer Documents.