self = nil; (OBJECTIVE C) vs this = null (JAVA)

Discussion in 'Mac Programming' started by akabeer1983, Jan 20, 2009.

  1. akabeer1983 macrumors newbie

    Jan 7, 2009
    in java, 'this' is a keyword that refers to the object on which the method containing the keyword 'this' is invoked.when we assign any value to 'this'
    this = null;
    it shows compile time error: CANNOT ASSIGN VALU TO FINAL VARIABLE 'this'

    However, in Objective C, if i assign

    self = nil;

    it accepts.
    Can anyone explain what happens as a result of this.


    int main()
    id OBJ_Alpha;
    OBJ_Alpha = [[Alpha alloc]init];
    [OBJ_Alpha SomeMethod];
    [OBJ_Alpha PrintMethod]; //is this line equivalent to [nil PrintMethod]?

    /*__________________class Alpha_____________________*/

    @implementation Alpha

    self = nil;

    - (void) PrintMethod
    printf("I am alive!!!");

    /*__________________________Example ends_____________*/

    Will the PrintMethod work?
  2. Eraserhead macrumors G4


    Nov 3, 2005
    I Imagine it will clear the pointer without freeing the memory of the object it is pointing at. That could be occassionally useful but I can't think of a case now. Generally you should use dealloc...
  3. robbieduncan Moderator emeritus


    Jul 24, 2002
    The only time I've seen self=nil used is if something fails in your init method and you want to abort object creation. If you use it anywhere else I'd imagine very bad things will happen to your object and most likely your program.
  4. MacRumors Guy macrumors member

    Sep 17, 2008
    It's not "abort object creation", the object is already created just not initalized. The self pointer in objc is fundamentally different from that of java.
  5. robbieduncan Moderator emeritus


    Jul 24, 2002
    OK, sure I agree with what you are saying: the memory space for the object has been allocated in the alloc call and the basic structure set up in the chained calls via [super init] up to NSObject.
  6. hazmatzak macrumors regular

    Apr 29, 2008
    self and this are both object references. In Java, this is more special, in that the compiler won't allow you to assign to it, so you don't get yourself into trouble. But if you could, the same rule applies to all references: you're changing that reference, not affecting other references nor the object.

    void foo(id someReference) {
      id a, b;
      a = someReference;
      b = a;
      a = nil;
      if ( b == someReference ) printf("Not nil\n");
    self is special because it is initialized at the start of a method to point to the object, and it is used to reference instance variables. But otherwise, it is just another local variable.

    So when you assign nil in someMethod, it has no side effects, because you don't do anything after. (If you were to call that method again or any other method, self will be reinitialized.) And it certainly does not affect OBJ_Alpha in main, which still points to the object. So printMethod does get called.

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