How to properly manage id instance variable assign then retain

I have a class that has a property:

@pr开发者_JAVA百科operty (nonatomic, assign) id customDatePicker

Based upon user choice I will need a UIDatePicker or a UIPicker so I typecast the ivar and retain it and subsequently dealloc it. Is this the correct way to manage the property's memory?

// init snippet
if (useUIDatePicker) {
  customDatePicker = (UIDatePicker *)[[[UIDatePicker alloc] initWithFrame:frame] retain];
} else {
  customDatePicker = (UIPickerView *)[[[UIPickerView alloc] initWithFrame:frame] retain];
}

- (void)dealloc {
  [customDatePicker release];
  [super dealloc];
}

No.

When you declare the property as assign, you should not be retaining the object. The assign option is used for non-object variables and for situations where having a retained property would create a cycle, with both objects retaining each other. Declaring a property as assign means you will not be managing the memory of the object; you should neither call retain nor release on it.

You are also over-retaining the picker object. Retaining an object creates a claim on the object; you don't want it to disappear until you say you are done with it. You relinquish a claim, allowing an object to be deleted, by calling release. When you call alloc, that creates the same kind of claim as calling retain. So this line:

[[[UIDatePicker alloc] initWithFrame:frame] retain];

creates two claims, one for alloc and one for retain. Later, you only call release once, which means that you will always still have one claim on this object, and it will have turned into a memory leak.

What you should do is:

@property (nonatomic, retain) id customDatePicker


if (useUIDatePicker) {
    customDatePicker = [[UIDatePicker alloc] initWithFrame:frame];
} else {
    customDatePicker = [[UIPickerView alloc] initWithFrame:frame];
}

Now you have only one claim on the picker because you used alloc.

(You don't need to cast the assignment; when you are assign to a generic pointer you can use any kind of object.)

Also take a look at the Apple Memory Management docs.