释放Objective-C属性的最佳方法是什么？

Question

I'm new to memory-management, and am reading different things about how to best release properties.

If I have:
in .h:

@property(retain) NSString *myStr;

and in .m:

@synthesize myStr = _iVarStr;

Should my dealloc have:

[_iVarStr release];  

or

self.myStr = nil;   

or something else?

Thanks!

Answer 1

Both self.myStr = nil and [myStr release] ultimately do the same thing.

Calling [myStr release] is obvious and just releases it.

Meanwhile, the setter method for myStr looks roughly like this:

- (void)setMyStr:(NSString *)newMyStr
{
    [newMyStr retain];
    [myStr release];
    myStr = newMyStr;
}

So when we do self.myStr = nil , we're first retaining a nil object, which does nothing. Then we release the old variable, which is what we want. Finally, we set the pointer to nil.

What's the difference? The latter sets the pointer to nil. This is better because if we (accidentally) send a message to the released object, we crash if the pointer isn't nil (EXC_BAD_ACCESS). Now honestly, since you're in -dealloc, the object is being destroyed anyways, so it wouldn't really matter what you use.

Answer 2

Your dealloc should be this:

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

Although setting the property to nil is possible, I worry about unintended side effects or KVO actions triggered by the change that may not realize the object is currently being deallocated.

Answer 3

I recommend you use self.ivar=nil (the code ivar=nil previously I wrote was wrong) way in dealloc method. Because, if the ivar's property change from retain to assign (or from assign to retain), you don't have to change your code.

Answer 4

When a property is set to retain then

self.ivar = nil;

will properly manage the memory allocation. For other property types check the at the official documentation page. It also has a bunch of sample code so you can understand what happens "under the hood" for all the options.