strong weak copy

 

use of copy

    If it is mutable, copy is the new obj (reference count is 1);

    Enter the stock market immutable, then retain;

 

strong points to an object, reference count +1

 

assign

 

weak points to an object, but not +1

 

NSString

NSNumber

NSString/NSMutableString 

NSArray/NSMutableArray 

NSDictionary/NSMutableDictionary 

NSSet/NSMutableSet

 

modifiers for variables

__strong    +1

__weak does not add 1, the vacancy is nil

__unsafe_unretained does not add 1, when it is empty, it is a wild pointer and can be ignored

 

apple documentation:

https://developer.apple.com/library/ios/releasenotes/ObjectiveC/RN-TransitioningToARC/Introduction/Introduction.html#//apple_ref/doc/uid/TP40011226-CH1-SW11

 

 

http://www.jianshu.com/p/4a1d1921284b