What happens if we don't check for "if (self)" in init methods?

Question

I started to look into the code done by our senior, I found the init method always have code -(id)init method. They used the code with the following ways. The code below is used for all viewControllers.

    self = [super initWithNibName:@"ViewController" bundle:[NSBundle mainBundle]];
    return self;

What is the use of the if(self) and self in this part?

    //And in some viewcontroller contains.
    self = [super initWithNibName:@"ViewController" bundle:[NSBundle mainBundle]];
    if (self) {
        //Do some stuff
    }
    return self;

Not doing `self` check is just wrong. There is no explanation, IMO. — Amar, Mar 24 '14 at 11:56
@Amar,self = [super initWithNibName:@"ViewController" bundle:[NSBundle mainBundle]]; return self; — iniyan, Mar 24 '14 at 11:58
@iniyaniOS That is wrong. There should always be a `if(self)` check. — Amar, Mar 24 '14 at 12:00
Amar, nope, there is no need for a self check in above code as there's nothing that uses self to cause problems in his example. Most of the answers have a better explanation. — Thomas Tempelmann, Mar 24 '14 at 12:04
@ThomasTempelmann I understand there is no need to check in this particular case, usage of the word `wrong` was towards coding habits. — Amar, Mar 24 '14 at 12:12

score 3 · Accepted Answer · answered Mar 24 '14 at 11:49

When you access instance variables in your methods the code is equivalent to addressing them via self pointer:

- (void)method {
  iVar = 1;
  // actually the following line will be executed:
  self->iVar = 1;
}

So if for some reason self pointer is nil and you access your ivars in init method, your application will crash as you will try to derefenrence null pointer. e.g. the following simple example will crash:

@implementation TestObj {
  int a;
}

- (id) init {
  self = [super init];
  self = nil;

  a = 1; // Crash
  return self;
}
@end

Also if your [super init] method returned nil for any reason, that might be indicator that something went wrong and you should not proceed with object initialization and any other required work with it.

score 2 · Answer 2 · answered Mar 24 '14 at 11:45

2

The code regarding the check if self exists :

if (self) {
   //Do some stuff
}
return self;

Is only necessary, if you want to set initial property values. This is why there is the comment

// Do some stuff

If you are not going to set any initial values to variables or properties, no check for self is necessary and it is okay to return self immediately.

answered Mar 24 '14 at 11:45

Volker

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If you're not initializing anything, don't override `init`. – jlehr Mar 24 '14 at 12:27

score 2 · Answer 3 · answered Mar 24 '14 at 11:46

if (self) is checking that the call to super actually returns an object after initializing. If something goes wrong it makes no sense to continue, and it would also generate runtime errors.

Keep in mind that the init method is supposed to initialize an object of the type you are creating a class for, and return that object. Your init method overrides the super method, and that is why you call super, so that the code from Apple is run first, and then you do some stuff with the object returned from the init method on i.e. UIViewController.

Then, of course, YOUR init method must also return that object (self).

score 1 · Answer 4 · answered Mar 24 '14 at 11:46

There's always the possibility that the [super initWith...] method failed, thus returning a self that is nil because it's not been created.

In this eventuality, it's a good practice to test if self has been correctly initialized, before operating on it.

The if (self) is actually a "shortcut" to if (self != nil), as any valid object will have a non-null memory address, and thus the if test will succedd with anything different than 0 (null).

score -1 · Answer 5 · edited Mar 24 '14 at 12:33

-1

if(self) is checking to make sure that whatever 'self' is exists before it actually starts doing things to it.

self is an object, so if it doesnt exist then self will be undefined and if(self) will evaluate to false, skipping the code block.

edited Mar 24 '14 at 12:33

Kumar KL

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answered Mar 24 '14 at 11:45

celem

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That's incorrect. The super call will either return the object or nil, nothing else (otherwise your app is seriously broken anyway). And invoking messages on nil objects is also allowed (they do nothing). See the other answers that almost all got this right. – Thomas Tempelmann Mar 24 '14 at 12:02
The "super init" method can and often will return a _different_ object than the one passed in. You mustn't make any assumptions about the result. – gnasher729 Mar 24 '14 at 12:35
Yeah, but it's not "undefined" as celem claims. Also, the question was "what happens if", and this answer does not answer this at all. It rather answers the question "what does checking self do?". I downvoted the answer for that and added an explanation for it, though I see now I didn't do that precisely enough. – Thomas Tempelmann Mar 25 '14 at 13:06

What happens if we don't check for "if (self)" in init methods?

5 Answers5