Reusing NSMutableArray

Question

I'm getting some leaks (obvserved by Instruments) when trying to reuse an existing NSMutableArray (in order to save memory).

Basically I'm creating an NSMutableArray, filling it with objects (UIImages) and passing it onto another object which retains it. However, I now need to use an NSMutableArray again. I figured I would release all its objects, empty it, and everything would be fine, but Instruments reports a CALayer leaked object (??) from that very method which looks something as follows:

NSString *fileName;
NSMutableArray *arrayOfImages = [[NSMutableArray alloc] init];

// fill the array with images
for(int i = 0; i <= 7; i++) {
    fileName = [NSString stringWithFormat:@"myImage_%d.png", i];
    [arrayOfImages addObject:[UIImage imageNamed:fileName]];
}

// create a button with the array
aButton = [[CustomButtonClass buttonWithType:UIButtonTypeCustom] 
                   initWithFrame:someFrame
                   imageArray:arrayOfImages];

// release its objects
for(int i = 0; i < [arrayOfImages count]; i++) {
    [[arrayOfImages objectAtIndex:i] release];
}
// empty array
[arrayOfImages removeAllObjects];

// fill it with other images
for(int i = 0; i <= 7; i++) {
    fileName = [NSString stringWithFormat:@"myOtherImage_%d.png", i];
    [arrayOfImages addObject:[UIImage imageNamed:fileName]];
}
// create another button with other images (same array)
aSecondButton = [[CustomButtonClass buttonWithType:UIButtonTypeCustom] 
                   initWithFrame:someFrame
                   imageArray:arrayOfImages];

[arrayOfImages release];

For the sake of clarity, my button init method looks as follows:

- (id)initWithFrame:(CGRect)frame 
      images:(NSArray *)imageArray
{
    if(self = [super initWithFrame:frame]) {
        myImageArray = [[NSArray arrayWithArray:imageArray] retain];
    }
return self;
}

I know I could just create a new NSMutableArray and be over with this issue but it annoys me not to be able to just reuse the old array. What could be the problem?

Answer 1

You don't need this part:

// release its objects
for(int i = 0; i < [arrayOfImages count]; i++) {
    [[arrayOfImages objectAtIndex:i] release];
}

This is against the ownership rule. You didn't retain the images at

[arrayOfImages addObject:[UIImage imageNamed:fileName]];

so it's not your responsibility to release them. It's NSMutableArray that retains them when -addObject is called, and so it's NSMutableArray's responsibility to release them when -removeObject or others of that ilk is called. The leak you found might be because the system got confused by this over-releasing...

I would also recommend to perform "Build and Analyze" in XCode.

Answer 2

I'm getting some leaks (obvserved by Instruments) when trying to reuse an existing NSMutableArray (in order to save memory).

An array takes a really small amount of memory; 4 bytes per pointer stored (on a 32 bit system) + a tiny bit of overhead. Reusing an array to attempt to save memory is a waste of time in all but the most extraordinary circumstances.

// release its objects
for(int i = 0; i < [arrayOfImages count]; i++) {
    [[arrayOfImages objectAtIndex:i] release];
}
// empty array
[arrayOfImages removeAllObjects];

You didn't retain the objects and, thus, you shouldn't be releasing them! That your app didn't crash after the above indicates that you are likely over-retaining the objects somewhere else.

I know I could just create a new NSMutableArray and be over with this issue but it annoys me not to be able to just reuse the old array. What could be the problem?

There isn't anything in that code that springs out as a memory leak. Just the opposite; you are over-releasing objects.

And the above indicates that you really need to revisit the memory management guidelines as re-using an array versus releasing the array and creating a new one really doesn't have anything to do with this problem.

Answer 3

The Leaks instrument tells you where the object that leaked was first allocated. The fact one of the images was leaked means that you used the image somewhere else, and did not release it there - Leaks cannot tell you where since it cannot show you code that does not exist.

Indeed as others have pointed out this is somewhat surprising since the code as-is is over-releasing objects and should have crashed. But the fact it did not crash is a good sign somewhere else you are using the images from the array and over-retaining them.