weak or strong for IBOutlet and other [duplicate]

Question

I have switched my project to ARC, and I don't understand if I have to use strong or weak for IBOutlets. Xcode do this: in interface builder, if a create a UILabel for example and I connect it with assistant editor to my ViewController, it create this:

@property (nonatomic, strong) UILabel *aLabel;

It uses the strong, instead I read a tutorial on RayWenderlich website that say this:

But for these two particular properties I have other plans. Instead of strong, we will declare them as weak.

@property (nonatomic, weak) IBOutlet UITableView *tableView;
@property (nonatomic, weak) IBOutlet UISearchBar *searchBar;

Weak is the recommended relationship for all outlet properties. These view objects are already part of the view controller’s view hierarchy and don’t need to be retained elsewhere. The big advantage of declaring your outlets weak is that it saves you time writing the viewDidUnload method.

Currently our viewDidUnload looks like this:

- (void)viewDidUnload
{
    [super viewDidUnload];
    self.tableView = nil;
    self.searchBar = nil;
    soundEffect = nil;
}

You can now simplify it to the following:

- (void)viewDidUnload
{
    [super viewDidUnload];
    soundEffect = nil;
}

So use weak, instead of the strong, and remove the set to nil in the videDidUnload, instead Xcode use the strong, and use the self... = nil in the viewDidUnload.

My question is: when do I have to use strong, and when weak? I want also use for deployment target iOS 4, so when do I have to use the unsafe_unretain? Anyone can help to explain me well with a small tutorial, when use strong, weak and unsafe_unretain with ARC?

Gleich

Answer 1

A rule of thumb

When a parent has a reference to a child object, you should use a strong reference. When a child has a reference to its parent object, you should use a weak reference or a unsafe_unretained one (if the former is not available). A typical scenario is when you deal with delegates. For example, a UITableViewDelegate doesn't retain a controller class that contains a table view.

Here a simple schema to present the main concepts.

Suppose first A,B and C are strong references. In particular, C has a strong ref to its parent. When obj1 is released (somewhere), the A reference doesn't exist anymore but you have a leak since there is a cycle between obj1 and obj2. Speaking in terms of retain counts (only for explain purposes), obj1 has a retain count of 2 (obj2 has a strong reference to it), while obj2 has a retain count of 1. If obj1 is released, its retain count is now 1 and its dealloc method is not called. obj1 and obj2 still remain in memory but no one has a reference to them: Leak.

On the contary, if only A and B are strong refs and C is qualified as weak all is ok. You have no leaks. In fact, when obj1 is released, it also releases obj2. Speaking in terms of retain counts, obj1 has a retain count of 1, obj2 has a retain count of 1. If obj1 is released, its retain count is now 0 and its dealloc method is called. obj1 and obj2 are removed from memory.

A simple suggestion: Start to think in terms of object graph when you deal with ARC.

About your first question, both solutions are valid when you deal with XIBs. In general weak references are used when you deal with memory cycles. Concerning XIBs files, if you use strong you need to set nil in viewDidUnload since if you don't do it, in memory low conditions, you could cause unexpected leaks. You don't release them in dealloc because ARC will do it for you. weak instead doesn't need that treatment since, when the target object is destroyed, those values are set as nil automatically. No dangling pointers anymore.

If you are interested in, I really suggest you to read friday-qa-2012-04-13-nib-memory-management by Mike Ash.

About your second question, if you need to support iOS 4, instead of weak you have to use unsafe_unretained.

Within SO there are a lot of questions/answers. Here the main ones:

How do I replace weak references when using ARC and targeting iOS 4.0?

What kind of leaks does automatic reference counting in Objective-C not prevent or minimize?

using ARC, lifetime qualifier assign and unsafe_unretained

strong / weak / retain / unsafe_unretained / assign

Update

As per shaunlim's comment, starting from iOS 6 viewDidUnload method is deprecated. Here I really suggest to see Rob's answer: iOS 6 - viewDidUnload migrate to didReceiveMemoryWarning?.

Isadora

you say that, for iboutlet if i use strong i have to use nil in viewdidunload, and for low memory i don't have leaks, instead with weak i don't have to use nil in videwdidunlaod, and for low memory warning it's better the strong solution or the weak? – Gleich

It's the same. Using weak allows you to save time on writing code (two lines in your case) by hand. But, Xcode does it for you. My personal opinion. I like to use strong. – Isadora

Just came across this and wanted to point out that viewDidUnload has since been deprecated. This is still an incredibly informative answer though! – Formularize

@Formularize Thank you very much! I will update the answer based on your comment. Thanks you. – Isadora

Answer 2

You can use weak for objects which are connected via IBOutlets to objects in IB because in this case the objects will be there as long as the superview is there. This is because the superview has a strong pointer to its subviews.

If the pointer you are defining is the only pointer to an object you should declare it as strong.

If you are a registered developer I strongly recommend that you have a look into the videos from the WWDC11 and WWDC12. Another good resource is the iOS development podcast from Stanford.

Sweven

Are the WWDC12 videos available yet? – Recipience

Yes, they are! Really fast this year. – Sweven

ok, but i don't understand, my question cover also unsafe_unretain, but for IBOutlet i'm explain well, why apple use strong?...instead of using weak?...so i have to follow apple? or follow the raywenderlich tutorial where i write above a snippet that use weak? – Gleich

My guess is that strong is safer in a sense, because the outlets are retained 'once more' besides being retained by the superview as children. Imagine you programatically add/remove some of the subviews on and off, at runtime. By having a strong reference, the removed subviews persist (are not deallocated). In exchange, you have to set the strong references to nil in viewDidUnload. – Pictogram

On the other hand, if you setup your nib in IB and it stays that way forever (no removing/re-adding subviews at runtime), then weak should be enough (or 'unsafe_unretained, in pre-iOS 5) – Pictogram