How retain count of NSMutableArray type variable is increased in iOS

Question

Here is the following program about increasing d retain count value of a NSMutableArray type variable in interface part.

@property (nonatomic, retain) NSMutableArray *dataArray;

And implementation Part

NSLog(@"%d",[self.dataArray retainCount]);
self.dataArray = [[NSMutableArray alloc] init];
NSLog(@"%d",[self.dataArray retainCount]);

Now on first line its showing retain count value is O inside NSLog, however when we allocate the array it's increasing retain count to 2. I am not getting this thing clear, that why retain count is increased to 2 instead of 1. Please help me understanding this.

Answer 1

You should use _object only if you are overriding getter/setter methods. "_object =" just assigns while "self.object =" calls its setter method. You must use _object to prevent and managing retain count using setter methods. Check the difference between by writing code:

@property (nonatomic, retain) NSMutableArray *arr;


//=============Accessing with _object==========
 NSLog(@"Retain Count before Allocation => %lu", (unsigned long)_arr.retainCount);

    _arr = [NSMutableArray arrayWithArray:@[@"arrobj1", @"arrObj3"]];
    NSLog(@"Retain Count before Allocation => %lu", (unsigned long)_arr.retainCount);



    _arr = [[NSMutableArray alloc] init];
    NSLog(@"Retain Count After alloc+init => %lu", (unsigned long)_arr.retainCount);
//============================================
Output is :
Retain Count before Allocation => 0
Retain Count before Allocation => 1
Retain Count After alloc+init => 1




###### And also with accessing objects with self.object#######
//=============Accessing with _object==========

    NSLog(@"Retain Count before Allocation => %lu", (unsigned long)self.arr.retainCount);

    self.arr = [NSMutableArray arrayWithArray:@[@"arrobj1", @"arrObj3"]];
    NSLog(@"Retain Count before Allocation => %lu", (unsigned long)self.arr.retainCount);



    self.arr = [[NSMutableArray alloc] init];
    NSLog(@"Retain Count After alloc+init => %lu", (unsigned long)self.arr.retainCount);


//============================================
Output is: 
Retain Count before Allocation => 0
Retain Count before Allocation => 2
Retain Count After alloc+init => 2

I hope this will helps you.

Answer 2

This might possibly help:

http://whentouseretaincount.com

In short you should never rely on the value returned by retain count.

Answer 3

The alloc init creates a retained instance +1. Then the retained property adds a retain on the assignment +2. To get a retain count of one, a) use ARC and a strong property, it will insert a release for you b) create an unretained instance:

self.dataArray = [NSMutableArray array];

or c) autorelease explicitly:

self.dataArray = [[[NSMutableArray alloc] init] autorelease];

Answer 4

Since the setter of the property is also getting called

self.dataArray = [[NSMutableArray alloc] init];

The retain count is getting pushed up to 2.

you can try this as well

NSMutableArray   *temp=[[NSMutableArray alloc]init];
self.dataArray = temp;
[temp release];

The release called on temp balances out the extra retain.