Objective-C ARC: strong vs retain and weak vs assign

Question

There are two new memory management attributes for properties introduced by ARC, strong and weak.

Apart from copy, which is ob

Answer 1

As far as I know, strong and retain are synonyms, so they do exactly the same.

Then the weak is almost like assign, but automatically set to nil after the object, it is pointing to, is deallocated.

That means, you can simply replace them.

However, there is one special case I've encountered, where I had to use assign, rather than weak. Let's say we have two properties delegateAssign and delegateWeak. In both is stored our delegate, that is owning us by having the only strong reference. The delegate is deallocating, so our -dealloc method is called too.

// Our delegate is deallocating and there is no other strong ref.
- (void)dealloc {
    [delegateWeak doSomething];
    [delegateAssign doSomething];
}

The delegate is already in deallocation process, but still not fully deallocated. The problem is that weak references to him are already nullified! Property delegateWeak contains nil, but delegateAssign contains valid object (with all properties already released and nullified, but still valid).

// Our delegate is deallocating and there is no other strong ref.
- (void)dealloc {
    [delegateWeak doSomething]; // Does nothing, already nil.
    [delegateAssign doSomething]; // Successful call.
}

It is quite special case, but it reveal us how those weak variables work and when they are nullified.

Answer 2

After reading so many articles Stackoverflow posts and demo applications to check variable property attributes, I decided to put all the attributes information together:

1. atomic //default
2. nonatomic
3. strong=retain //default
4. weak
5. retain
6. assign //default
7. unsafe_unretained
8. copy
9. readonly
10. 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;