Objective-C declared property attributes nonatomic copy strong weak

Question

Can someone explain to me in detail when I must use each attribute: nonatomic, copy, strong, weak, and so on, for a declared property, and explain what each does? Some sort of example would be great also. I am using ARC.

This question is related to objective-c automatic-ref-counting declared-property

User · Answer

nonatomic property means  synthesized methods are not going to be generated threadsafe -- but this is much faster than the atomic property since extra checks are eliminated  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  weak ownership means that you don t own it and it just keeps track of the object till the object it was assigned to stays   as soon as the second object is released it loses is value  For eg  obj a objectB  is used and a has weak property   than its value will only be valid till objectB remains in memory  copy property is very well explained here strong weak retain copy assign are mutually exclusive so you can t use them on one single object    read the  quot Declared Properties  quot  section hoping this helps you out a bit

User · Answer

Great answers  One thing that I would like to clarify deeper is nonatomic atomic  The user should understand that this property -  atomicity  spreads only on the attribute s reference and not on it s contents  I e  atomic will guarantee the user atomicity for reading setting the pointer and only the pointer to the attribute  For example    interface MyClass  NSObject  property  atomic  strong  NSDictionary  dict        In this case it is guaranteed that the pointer to the dict will be read set in the atomic manner by different threads  BUT the dict itself  the dictionary dict pointing to  is still thread unsafe  i e  all read add operations to the dictionary are still thread unsafe   If you need thread safe collection you either have bad architecture  more often  OR real requirement  more rare   If it is  real requirement  - you should either find good amp tested thread safe collection component OR be prepared for trials and tribulations writing your own one  It latter case look at  lock-free    wait-free  paradigms  Looks like rocket-science at a first glance  but could help you achieving fantastic performance in comparison to  usual locking

User · Answer

Nonatomic Nonatomic will not generate threadsafe routines thru  synthesize accessors  atomic will generate threadsafe accessors so atomic variables are threadsafe  can be accessed from multiple threads without botching of data  Copy copy is required when the object is mutable  Use this if you need the value of the object as it is at this moment  and you don t want that value to reflect any changes made by other owners of the object  You will need to release the object when you are finished with it because you are retaining the copy  Assign Assign is somewhat the opposite to copy  When calling the getter of an assign property  it returns a reference to the actual data  Typically you use this attribute when you have a property of primitive type  float  int  BOOL     Retain retain is required when the attribute is a pointer to a reference counted object that was allocated on the heap  Allocation should look something like  NSObject  obj     NSObject alloc  init      ref counted var  The setter generated by  synthesize will add a reference count to the object when it is copied so the underlying object is not autodestroyed if the original copy goes out of scope  You will need to release the object when you are finished with it   propertys using retain will increase the reference count and occupy memory in the autorelease pool  Strong strong is a replacement for the retain attribute  as part of Objective-C Automated Reference Counting  ARC   In non-ARC code it s just a synonym for retain  This is a good website to learn about strong and weak for iOS 5  http   www raywenderlich com 5677 beginning-arc-in-ios-5-part-1 Weak weak is similar to strong except that it won t increase the reference count by 1  It does not become an owner of that object but just holds a reference to it  If the object s reference count drops to 0  even though you may still be pointing to it here  it will be deallocated from memory  The above link contain both Good information regarding Weak and Strong

User · Answer

This link has the break down  http   clang llvm org docs AutomaticReferenceCounting html ownership spelling property     assign implies   unsafe unretained ownership       copy implies   strong ownership  as well as the usual behavior of copy   semantics on the setter       retain implies   strong ownership       strong implies   strong ownership       unsafe unretained implies   unsafe unretained ownership       weak implies   weak ownership

[objective-c] Objective-C declared @property attributes (nonatomic, copy, strong, weak)

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