How do I fix a memory leak?

Question

After doing a long profile test, I found that in one of my ".m" file memory leak occurs in the viewdidload section. i checked and the xcode highlighted the part where i have initialized picker arrays with values. my program uses pickers for user input. and i have 3 5 different views in my program. the first is a disclaimer ,the second is a menu where the user can choose the type of calculation he/she wants to do. each calculation requires certain inputs which the user enters from a picker. for eg. one of the view has 5 inputs which are handled by 5 different uipickers with seperate arrays for holding the values. these arrays are initialized with the values in the viewdidload method of that view. here is what i found after running the test:

...................................................................................................

This is my first time developing an app and i'm kinda confused about what to do. Any help would be appreciated.

Answer 1

Objects in objective c have a retain count. If this retain count is greater that 0 when the object goes out of scope (when you stop using it), it leaks.

The following things increase the retain count

• [[alloc] init]
• new
• copy
• [retain]
• adding an object to an array
• adding an object as a child (e.g. views)
• There are likely more, but you don't appear to use any others in your code

The following decrease the retain count

• [release]
• removing an object from an array
• if you dealloc an array, all of its objects are released

You should go through your code and ensure each of the retains or additions to an array are matched with a corresponding release. (You can release member variables in the dealloc method).

EDIT: Jeremy made a valid point that my answer doesn't

Once you add an object to an array, it takes ownership and will release the object when it is done with it. All you need to do is make sure you release anything you own according to the memory management rules

There are also autorelease objects, have a look at this example;

-(init){
    ...
    stagePickerArray = [[NSMutableArray alloc] init];
    for (int i = 0; i < 3; i++)
    {
        //this string is autoreleased, you don't have call release on it.
        //methods with the format [CLASS CLASSwithsomething] tend to be autorelease
        NSString *s = [NSString stringWithFormat:@"%d", i);
        [stagePickerArray addObject:s];
    }
    ...
 }

I think the only thing you are missing is a call to release in your dealloc method

-(void) dealloc
{
    [stagepickerarray release];  //Do this for each of your arrays
    [super dealloc];
}

Answer 2

The leaks tool will only tell you where yo allocated the objects that it thinks leaks. So, it's telling you, for instance, that

NSString* answer = [NSString stringWithFormat: ...

allocates an object that is never deallocated. Now, -stringWithFormat: gives you an object that you do not own and you don't seem to retain it anywhere. Therefore, you do not need to release it, so it can't be leaking by itself.

That means something else that you do own must be retaining it and you never release that something else. The prime suspect would appear to be stagePickerArray. Check that you are releasing stagePickerArray somewhere. If it's local to -viewDidLoad it must be released or autoreleased before the end of that method. If it's an instance variable, it must be released in the class's -dealloc method.

Answer 3

In Objective-C you need to take care of the retain count of allocated memory. If you don't need it -> release it.

Whenever you alloc an object, it will return an object with retain count = 1.

By using retain, the retain count gets incremented, by using release, the retain count gets decremented. Whenever the retain count is equals 0, the object will be destroyed.

So whenever you want to use the object somewhere else you need to retain it. So you make sure that the object is not deleted after the other 'person' (or whatever it used ;)) called release.

This was a very very very short description. Check the following guide Memory Management Guide for iOS.

(Also you want to read something about ARC - Automatic Retain Counting - which is new in iOS5! ios5 best practice release retain