Objective-C Static Class Level variables

Question

I have a class Film  each of which stores a unique ID  In C   Java etc I can define a static int currentID and each time i set the ID i can increase the currentID and the change occurs at the class level not object level  Can this be done in Objective-C  I ve found it very hard to find an answer for this

User · Answer

In your  m file  declare a file global variable   static int currentID   1    then in your init routine  refernce that   -  id  init       self    super init       if  self    nil             myID   currentID       not thread safe           return self      or if it needs to change at some other time  eg in your openConnection method   then increment it there   Remember it is not thread safe as is  you ll need to do syncronization  or better yet  use an atomic add  if there may be any threading issues

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Issue Description   You want your ClassA to have a ClassB class variable  You are using Objective-C as programming language  Objective-C does not support class variables as C   does   One Alternative  Simulate a class variable behavior using Objective-C features  Declare Define an static variable within the classA m so it will be only accessible for the classA methods  and everything you put inside classA m    Overwrite the NSObject initialize class method to initialize just once the static variable with an instance of ClassB   You will be wondering  why should I overwrite the NSObject initialize method  Apple documentation about this method has the answer   quot The runtime sends initialize to each class in a program exactly one time just before the class  or any class that inherits from it  is sent its first message from within the program   Thus the method may never be invoked if the class is not used   quot    Feel free to use the static variable within any ClassA class instance method    Code sample  file  classA m static ClassB  classVariableName   nil    implementation ClassA          void  initialize       if    classVariableName          classVariableName     ClassB alloc  init        void  classMethodName        classVariableName doSomething       - void  instanceMethodName        classVariableName doSomething             end  References   Class variables explained comparing Objective-C and C   approaches

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Strictly speaking not an answer to the question  but in my experience likely to be useful when looking for class variables   A class method can often play many of the roles a class variable would in other languages  e g  changed configuration during tests     interface MyCls  NSObject    NSString  theNameThing  -  void doTheThing   end  implementation    NSString  theNameThing   return   Something general     -  void doTheThing      SomeResource changeSomething  self class theNameThing       end   interface MySpecialCase  MyCls  end  implementation    NSString  theNameThing   return   Something specific      end   Now  an object of class MyCls calls Resource changeSomething  with the string   Something general  upon a call to doTheThing   but an object derived from MySpecialCase with the string   Something specific

User · Answer

Another possibility would be to have a little NSNumber subclass singleton

User · Answer

On your  m file  you can declare a variable as static   static ClassName  variableName   nil    Then you can initialize it on your   void initialize method   Please note that this is a plain C static variable and is not static in the sense Java or C  consider it  but will yield similar results

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As pgb said  there are no  class variables   only  instance variables   The objective-c way of doing class variables is a static global variable inside the  m file of the class  The  static  ensures that the variable can not be used outside of that file  i e  it can t be extern

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As of Xcode 8  you can define class properties in Obj-C  This has been added to interoperate with Swift s static properties      Objective-C now supports class properties  which interoperate with Swift type properties  They are declared as   property  class  NSString  someStringProperty   They are never synthesized   23891898    Here is an example   interface YourClass   NSObject   property  class  nonatomic  assign  NSInteger currentId    end   implementation YourClass  static NSInteger  currentId   0      NSInteger currentId       return  currentId        void setCurrentId  NSInteger newValue        currentId   newValue      end   Then you can access it like this   YourClass currentId   1  val   YourClass currentId    Here is a very interesting explanatory post I used as a reference to edit this old answer     2011 Answer   don t use this  it s terrible   If you really really don t want to declare a global variable  there another option  maybe not very orthodox  -   but works    You can declare a  get amp set  method like this  with an static variable inside      NSString  testHolder  NSString   test       static NSString  test       if  test    nil            if test    nil               test release           test     test retain                 if test    nil             test     Initialize the var here if you need to        return test      So  if you need to get the value  just call   NSString  testVal    MyClass testHolder nil    And then  when you want to set it    MyClass testHolder testVal    In the case you want to be able to set this pseudo-static-var to nil  you can declare testHolder as this      NSString  testHolderSet  BOOL shouldSet newValue  NSString   test       static NSString  test       if shouldSet            if test    nil               test release           test     test retain              return test      And two handy methods      NSString  test       return  MyClass testHolderSet NO newValue nil         void setTest  NSString   test        MyClass testHolderSet YES newValue  test       Hope it helps  Good luck

User · Answer

Here would be an option     int getId      static int id        Do anything you need to update the ID here     return id      Note that this method will be the only method to access id  so you will have to update it somehow in this code

User · Answer

u can rename the class as classA mm and add C   features in it

[objective-c] Objective-C Static Class Level variables

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