Differences between strong and weak in Objective-C

Question

I m new to Obj-C  so my first question is   What are the differences between strong and weak in  property declarations of pointers to objects   Also  what does nonatomic mean

User · Answer

strong and weak  these keywords revolves around Object Ownership in Objective-C  What is object ownership    Pointer variables imply ownership of the objects that they point to    When a method  or function  has a local variable that points to an object  that variable is said to own the object being pointed to  When an object has an instance variable that points to another object  the object with the pointer is said to own the object being pointed to    Anytime a pointer variable points to an object  that object has an owner and will stay alive  This is known as a strong reference   A variable can optionally not take ownership of an object that it points to  A variable that does not take ownership of an object is known as a weak reference   Have a look for a detailed explanation here Demystifying  property and attributes

User · Answer

A dummy answer  -  I think explanation is given in above answer  so i am just gonna tell you where to use STRONG and where to use WEAK    Use of Weak  - 1  Delegates 2  Outlets 3  Subviews 4  Controls  etc   Use of Strong  -  Remaining everywhere which is not included in WEAK

User · Answer

To understand Strong and Weak reference consider below example  suppose we have method named as displayLocalVariable    - void displayLocalVariable          UIView  myView     UIView alloc  init        NSLog   myView tag is    ld   myView tag         In above method scope of myView variable is limited to displayLocalVariable method  once the method gets finished myView variable which is holding the UIView object will get deallocated from the memory   Now what if we want to hold the myView variable throughout our view controller s life cycle  For this we can create the property named as usernameView which will have Strong reference to the variable myView see  property nonatomic strong  UIView  usernameView  and self usernameView   myView  in below code   as below    interface LoginViewController      property nonatomic strong  UIView  usernameView   property nonatomic weak  UIView  dummyNameView   -  void displayLocalVariable    end   implementation LoginViewController  -  void viewDidLoad        super viewDidLoad       - void viewWillAppear  BOOL animated         self displayLocalVariable      -  void displayLocalVariable      UIView  myView     UIView alloc  init      NSLog   myView tag is    ld   myView tag      self usernameView   myView     -  void didReceiveMemoryWarning        super didReceiveMemoryWarning        end   Now in above code you can see myView has been assigned to self usernameView and self usernameView is having a strong reference as we declared in interface using  property  to myView  Hence myView will not get deallocated from memory till self usernameView is alive    Weak reference   Now consider assigning myName to dummyNameView which is a Weak reference  self dummyNameView   myView  Unlike Strong reference Weak will hold the myView only till there is Strong reference to myView  See below code to understand Weak reference   - void displayLocalVariable          UIView  myView     UIView alloc  init        NSLog   myView tag is    ld   myView tag        self dummyNameView   myView        In above code there is Weak reference to myView i e  self dummyNameView is having Weak reference to myView  but there is no Strong reference to myView  hence self dummyNameView will not be able to hold the myView value   Now again consider the below code   - void displayLocalVariable                  UIView  myView     UIView alloc  init            NSLog   myView tag is    ld   myView tag            self usernameView   myView           self dummyNameView   myView             In above code self usernameView has a Strong reference to myView  hence self dummyNameView will now have a value of myView even after method ends since myView has a Strong reference associated with it   Now whenever we make a Strong reference to a variable it s retain count get increased by one and the variable will not get deallocated till it s retain count reaches to 0   Hope this helps

User · Answer

strong is the default  An object remains    alive    as long as there is a strong pointer to it   weak specifies a reference that does not keep the referenced object alive  A weak reference is set to nil when there are no strong references to the object

User · Answer

A strong reference  which you will use in most cases  means that you want to  own  the object you are referencing with this property variable  The compiler will take care that any object that you assign to this property will not be destroyed as long as you point to it with a strong reference  Only once you set the property to nil will the object get destroyed  unless one or more other objects also hold a strong reference to it    In contrast  with a weak reference you signify that you don t want to have control over the object s lifetime  The object you are referencing weakly only lives on because at least one other object holds a strong reference to it  Once that is no longer the case  the object gets destroyed and your weak property will automatically get set to nil  The most frequent use cases of weak references in iOS are    delegate properties  which are often referenced weakly to avoid retain cycles  and subviews controls of a view controller s main view because those views are already strongly held by the main view    atomic vs  nonatomic refers to the thread safety of the getter and setter methods that the compiler synthesizes for the property  atomic  the default  tells the compiler to make the accessor methods thread-safe  by adding a lock before an ivar is accessed  and nonatomic does the opposite  The advantage of nonatomic is slightly higher performance  On iOS  Apple uses nonatomic for almost all their properties so the general advice is for you to do the same

User · Answer

Strong  Basically Used With Properties we used to get or send data from into another classes  Weak  Usually all outlets  connections are of Weak type from Interface   Nonatomic  Such type of properties are used in conditions when we don t want to share our outlet or object into different simultaneous Threads  In other words  Nonatomic instance make our properties to deal with one thread at a time  Hopefully it helpful for you

User · Answer

It may be helpful to think about strong and weak references in terms of balloons   A balloon will not fly away as long as at least one person is holding on to a string attached to it  The number of people holding strings is the retain count  When no one is holding on to a string  the ballon will fly away  dealloc   Many people can have strings to that same balloon  You can get set properties and call methods on the referenced object with both strong and weak references   A strong reference is like holding on to a string to that balloon  As long as you are holding on to a string attached to the balloon  it will not fly away   A weak reference is like looking at the balloon  You can see it  access it s properties  call it s methods  but you have no string to that balloon  If everyone holding onto the string lets go  the balloon flies away  and you cannot access it anymore

User · Answer

Here  Apple Documentation has explained the difference between weak and strong property using various examples    https   developer apple com library ios documentation cocoa conceptual ProgrammingWithObjectiveC EncapsulatingData EncapsulatingData html   apple ref doc uid TP40011210-CH5-SW3   Here  In this blog author has collected all the properties in same place  It will help to compare properties characteristics    http   rdcworld-iphone blogspot in 2012 12 variable-property-attributes-or html

User · Answer

strong  assigns the incoming value to it  it will retain the incoming value and release the existing value of the instance variable  weak  will assign the incoming value to it without retaining it   So the basic difference is the retaining of the new variable   Generaly you want to retain it but there are situations where you don t want to have it otherwise you will get a retain cycle and can not free the memory the objects  Eg  obj1 retains obj2 and obj2 retains obj1  To solve this kind of situation you use weak references

[objective-c] Differences between strong and weak in Objective-C

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