[Objective-C] strong/weak 屬性——狗項圈的比喻

轉自：

http://stackoverflow.com/questions/8927727/objective-c-arc-strong-vs-retain-and-weak-vs-assign

http://rdcworld-iphone.blogspot.in/2012/12/variable-property-attributes-or.html

BJ Homer 對 strong/weak reference 給出了非常形象的比喻，很到位，摘抄如下：

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.

對於 strong/weak/retain/assign 的解釋，摘抄如下：

atomic //default nonatomic strong = retain weak = assign 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;