Question

I think I am missing something about property attributes.
First, I can’t understand the difference between retain and assign.

If I use assign, does the property increase the retain counter by 1 to the setter and also to the getter, and do I need to use release to both of them?

And how does this work with readwrite or copy? From the view of a retain count.

I am trying to understand when i need to use release after working with a property (setter and getter)

@property (readwrite,assign) int iVar; 

What does assign do here?

What is the difference between:

@property (readwrite,assign) int iVar;

and

@property (readwrite,retain) int iVar;

and

@property (readwrite) int iVar;

Many thanks…

Answer 1

what is the different between : @property (readwrite,assign) int iVar; to @property (readwrite,retain) int iVar; to @property (readwrite) int iVar;

The setter for @property (readwrite,assign) sometype aProperty; is semantically equivalent to

-(void) setAProperty: (sometype) newValue
{
    ivar = newValue;
}

The above is more or less what you will get if you put

@asynthesize aProperty = ivar;

in your implementation.

The setter for @property (readwrite,retain) sometype aProperty; is semantically equivalent to

-(void) setAProperty: (sometype) newValue
{
    [newValue retain];
    [ivar release];
    ivar = newValue;
}

Clearly, it makes no sense to retain or release an int, so sometype must be either id or SomeObjectiveCClass*

The setter for @property (readwrite,copy) sometype aProperty; is semantically equivalent to

-(void) setAProperty: (sometype) newValue
{
    sometype aCopy = [newValue copy];
    [ivar release];
    ivar = aCopy;
}

In this case, not only must sometype be an objective C class but it must respond to -copyWithZone: (or equivalently, implement NSCopying).

If you omit retain or assign or copy, the default is assign.

By the way, I have simplified the above by not considering the locking that occurs because the properties don’t also specify nonatomic.