WARNING, OUTDATED ANSWER: this answer is not up to date as per WWDC 2015, for the correct answer refer to the accepted answer (Daniel Hall) above. This answer will stay for record.
Summarized from the developer library:
From a practical perspective, in iOS and OS X outlets should be defined as declared properties. Outlets should generally be weak, except for those from File’s Owner to top-level objects in a nib file (or, in iOS, a storyboard scene) which should be strong. Outlets that you create will therefore typically be weak by default, because:
Outlets that you create to, for example, subviews of a view controller’s view or a window controller’s window, are arbitrary references between objects that do not imply ownership.
The strong outlets are frequently specified by framework classes (for example, UIViewController’s view outlet, or NSWindowController’s window outlet).
@property (weak) IBOutlet MyView *viewContainerSubview;
@property (strong) IBOutlet MyOtherClass *topLevelObject;
After reading so many articles Stackoverflow posts and demo applications to check variable property attributes, I decided to put all the attributes information together:
- atomic //default
- nonatomic
- strong=retain //default
- weak
- retain
- assign //default
- unsafe_unretained
- copy
- readonly
- readwrite //default
Below is the detailed article link where you can find above mentioned all attributes, that will definitely help you.
Many thanks to all the people who give best answers here!!
Variable property attributes or Modifiers in iOS
1.strong (iOS4 = retain )
- it says "keep this in the heap until I don't point to it anymore"
- in other words " I'am the owner, you cannot dealloc this before aim fine with that same as retain"
- You use strong only if you need to retain the object.
- By default all instance variables and local variables are strong pointers.
- We generally use strong for UIViewControllers (UI item's parents)
- strong is used with ARC and it basically helps you , by not having to worry about the retain count of an object. ARC automatically releases it for you when you are done with it.Using the keyword strong means that you own the object.
Example:
@property (strong, nonatomic) ViewController *viewController;
@synthesize viewController;
2.weak
-
- it says "keep this as long as someone else points to it strongly"
- the same thing as assign, no retain or release
- A "weak" reference is a reference that you do not retain.
- We generally use weak for IBOutlets (UIViewController's Childs).This works because the child object only needs to exist as long as the parent object does.
- a weak reference is a reference that does not protect the referenced object from collection by a garbage collector.
- Weak is essentially assign, a unretained property. Except the when the object is deallocated the weak pointer is automatically set to nil
Example :
@property (weak, nonatomic) IBOutlet UIButton *myButton;
@synthesize myButton;
Strong & Weak Explanation, Thanks to BJ Homer:
Imagine our object is a dog, and that the dog wants to run away (be
deallocated).
Strong pointers are like a leash on the dog. As long as you have the
leash attached to the dog, the dog will not run away. If five people
attach their leash to one dog, (five strong pointers to one object),
then the dog will not run away until all five leashes are detached.
Weak pointers, on the other hand, are like little kids pointing at the
dog and saying "Look! A dog!" As long as the dog is still on the
leash, the little kids can still see the dog, and they'll still point
to it. As soon as all the leashes are detached, though, the dog runs
away no matter how many little kids are pointing to it.
As soon as the last strong pointer (leash) no longer points to an
object, the object will be deallocated, and all weak pointers will be
zeroed out.
When we use weak?
The only time you would want to use weak, is if you wanted to avoid retain cycles
(e.g. the parent retains the child and the child retains the parent so neither is ever released).
3.retain = strong
- it is retained, old value is released and it is assigned retain specifies the new value should be sent
- retain on assignment and the old value sent -release
- retain is the same as strong.
- apple says if you write retain it will auto converted/work like strong only.
- methods like "alloc" include an implicit "retain"
Example:
@property (nonatomic, retain) NSString *name;
@synthesize name;
4.assign
- assign is the default and simply performs a variable assignment
- assign is a property attribute that tells the compiler how to synthesize the property's setter implementation
- I would use assign for C primitive properties and weak for weak references to Objective-C objects.
Example:
@property (nonatomic, assign) NSString *address;
@synthesize address;
Best Answer
Most outlets for subviews don't need to be strong references because, after all, they're subviews loaded as part of a view hierarchy. As long as the top-level view exists, and as long as you don't go removing the subviews from their parents, the subviews in the view hierarchy will be retained by their parents for the lifetime of the top-level view.
In the days before ARC, some people were happy to rely on the view hierarchy to retain their views for them and so set their outlet properties to
assign. Others didn't like the idea that a hiccup in the view hierarchy could leave them with a handful of dangling pointers, and so set their properties toretain. ARC gives us zeroing weak references, so that your outlets will be set tonilif the objects they point to are deallocated, and that makes using weak references for outlets seem a lot safer. On the other hand, if you want to maintain a reference to a view even if the view hierarchy that contains is is deallocated, you should set that reference tostrong.Since your view controller is responsible for the view hierarchy it manages (i.e. "owns"), it should have a strong reference to the top-level view. You don't need to worry too much about this, as the
viewproperty of any UIViewController-derived view controller is set toretain(i.e.strong).