为什么用“ self”设置值会导致内存泄漏？

Question

I need to understand why in this code i get a memory leak if i assign value to ws_data variable using self.ws_data and not if i use only ws_data.

self.ws_data is a @property (copy)NSString *, released on dealloc.

dispatch_queue_t ws_queue = dispatch_queue_create("ws check win", NULL);
dispatch_async(ws_queue, ^{
    self.ws_data = [[NSString alloc]initWithContentsOfURL:url]; 
});
dispatch_release(ws_queue);

Thank you!

Answer 1

self.variableName accesses the variable through its accessors. Because your ws_data NSString is set to copy in its property declaration, the string is retained when you set it through that declaration.

Not using self. references the ivar without going through those accessors. It also means the variable isn't copied, so it will vanish when references to it disappear (like at the end of your method).

Odds are you want the copy. You just need to release it when you're done, as you do in your dealloc.

Are you retaining it somewhere else, making this access a second retention?

Answer 2

If I understand things correctly (and I quite possibly don't as I've not be doing iOS development for very long at all) in this instance, as you're using the copy attribute on the property, what you're doing when you use self.ws_data is effectively calling the copy method on an already alloced NSString, which is creating a separate instance of the NSString with a retain count of one.

However, the original NSString (that's being alloced in your above sample) isn't being released at any point, hence the leak.

You could use...

self.ws_data = [[[NSString alloc]initWithContentsOfURL:url] autorelease]; 

...instead, I'd have thought.