assignment of property and allocation leads to retain count of 2

Question

Hi,

I had a look at instruments and I saw that the alloc increased the retain count by 1. So far everything clear. But the assignment of the class to my property also increased the retain count to 2.

self.myProperty = [[MyClass alloc] init]

Vs.

MyClass *myCreatedVariable = [[MyClass alloc] init];
self.myProperty = myCreatedVariable

To decrease the retain count to zero I released myCreatedVariable right after my calls. The myProperty instance variable is released in the dealloc method. Am I right that a property only is released in the dealloc method?

Now to my question:

Is the allocation and the assignment to a property always creating a retain count of 2? So don't use

self.myProperty = [[MyClass alloc] init]

because the retain count is never getting zero? Or is this only the case if I'm allocating a class?

Cheers

Answer 1

Your property is most probably declared as retaining or copying:

@property (retain) MyClass myProperty;

or

@property (copy) MyClass myProperty;

This calls your setter that does what its attributes say: retain! Copy will also retain.

Although it worked here, you shouldn't try to get useful information out of the retainCount property.

I cannot recommend the Memory Management Programming Guide highly enough, well worth a first, second and third read. :-)

Answer 2

Creating objects using the init function returns a retained instance by default. ( See the Memory Management Programming Guide)

If the property is defined with the 'retain' attribute, then your object is retained one more time.

So the right way to do is

MyClass *myCreatedVariable = [[MyClass alloc] init];
self.myProperty = myCreatedVariable;
[myCreatedVariable release];

By the way this is good to know also when you using Arrays. Once an object created with the alloc and init functions is added into an array, it is retained by the array, so you can release your instance after you add it in the array.

In both case, retainCount is then 1, as expected.

if your property is defined with the 'copy' attribute, you can release the object as well, and even kill it, since it has been fully copied and retained once. ( I think there is something there if you use garbage collection instead of managed memory... To check.. )

Finally if your property is set with the 'assign' attribute, only the object's adress is copied, so you should not release your original object in this case.

It is however not recommanded to use the 'assign' attribute, since you may set property with objects that you did not create yourself, and which could be released anytime, letting your property pointing in the fields...

Finally, don't forget that static creators in Cocoa do not return retained objects. ( This is a convention, exceptions may exist... )

example:

NSArray* myArray = [NSArray array];
self.myProperty = myArray;

In this case, do not release myArray, it is already done in the creator function. Assigning it to the property will retain it.( with retain or copy attribute).

Hope it will help,

Cheers

Answer 3

Hi,

Sorry ? Noooo. I'm afraid programming is trying to know things we don't know everyday !

Static creators are convenience function, to ease common objects allocations. A lot of classes in the cocoa framework have this kind of functions. Arrays, Dictionary, Paths, ...

Let's take your class as an example, and suppose you often have to create objects of this class. You may write a function in your 'myClass' implementation like:

+(MyClass*)myClass
{
  MyClass *myNewInstance = [[myNewInstance alloc] init];
  return [myNewInstance autorelease];
}

Then you can rewrite your original example as:

..
self.myProperty = [MyClass myClass];
..

Straight! Or you could write a method like

-(void)myFunction
{
  MyClass* myTempObject = [MyClass myClass];
  if (myTempObject) {
    // do something with your temporary object
  }
  // Simply exit, object will be released later on.
}

It is much shorter ( we should handle the case where object creation failed ) ..

Note that this is all conventions, you can basically do has you like and create retained objects, or use a different name for the creator. But it is safer to follow the framework rule, it then becomes a reflex when you code. See methods like [NSDictionary dictionary], [NSArray array], [NSArray arrayWithObjects:] ,...

Cheers