CODEWITHSUNDEEP
iosobjective-c
CodeHelp
CodeHelp

Reputation: 1328

Objective-c: Questions about self = [super init]


I have seen self = [super init] in init methods. I don't understand why. Wouldn't [super init] return the superclass? And if we point self = [super init], are we not getting self = superclass?
Here's an example code fragment

- (id)init 
{
    if (self = [super init]) {
        creationDate = [[NSDate alloc] init];
    }
    return self;
}

Hope someone can clarify this for me. Thank you.

Upvotes: 24

Views: 11214

Answers (8)

Dharmesh Mansata
Dharmesh Mansata

Reputation: 4728

Self = [super init];

According to JAVA, this mean a pointer to instance itself so object can message itself.

Same meainng of Self here in objective C,

According to JAVA, Super mean that allow to access base or parent class

Same meainng of Super here in objective C,

Now init instance to to complete the initialization process.

Upvotes: 2

eliasetm
eliasetm

Reputation: 1657

Every method that you declare has two hidden parameters: self and _cmd.

The following method:

- (id)initWithString:(NSString *)aString;

is converted by the compiler to the following function call:

id initWithString(id self, SEL _cmd, NSString *aString);

see this link for more:

http://www.cocoawithlove.com/2009/04/what-does-it-mean-when-you-assign-super.html

Upvotes: 2

Joride
Joride

Reputation: 3763

From Apple's Documentation:

Because an init... method might return nil or an object other than the one explicitly allocated, it is dangerous to use the instance returned by alloc or allocWithZone: instead of the one returned by the initializer. Consider the following code:

id myObject = [MyClass alloc];
[myObject init];
[myObject doSomething];

The init method in the example above could have returned nil or could have substituted a different object. Because you can send a message to nil without raising an exception, nothing would happen in the former case except (perhaps) a debugging headache. But you should always rely on the initialized instance instead of the “raw” just-allocated one. Therefore, you should nest the allocation message inside the initialization message and test the object returned from the initializer before proceeding.

id myObject = [[MyClass alloc] init];
if ( myObject ) {
    [myObject doSomething];
} else {
    // error recovery... 
}

Upvotes: 1

Mystery
Mystery

Reputation: 552

As you have Question self = [super init] in the if Condition suggest a specific meaning.

First of all [super init] gives the initialization of the superclass of the existing class which is in use currently. Using [super init] gives the super class initialization which shows that object exist of the class.

Now when you use self = [super init] that means you are assigning the class to the self for the further utilization of the same class.

And at the end you put it in if condition as if(self = [super init]) this means you are checking whether the object of the class exist of not to prevent the foul behavior of the application.

I think it is clear now!!!

Upvotes: 6

onmyway133
onmyway133

Reputation: 48085

@MartinR has a very good answer. But do you ever wonder why "[super init] calls the superclass implementation of init with the same (hidden) self argument. (This might be the point that you understood wrongly.)" works in his 3rd point ?

Here is the excerpt from Big Nerd Ranch guide 3rd edition, chapter 2 Objective C that clarifies this point

“How does super work? Usually when you send a message to an object, the search for a method of that name starts in the object’s class. If there is no such method, the search continues in the superclass of the object. The search will continue up the inheritance hierarchy until a suitable method is found. (If it gets to the top of the hierarchy and no method is found, an exception is thrown.)”

“When you send a message to super, you are sending a message to self, but the search for the method skips the object’s class and starts at the superclass.”

This code shows how iOS Runtime performs this task

objc_msgSendSuper(self, @selector(init));

Upvotes: 4

Martin R
Martin R

Reputation: 539725

Assuming that MyClass is a subclass of BaseClass, the following happens when you call

MyClass *mc = [[MyClass alloc] init];
  1. [MyClass alloc] allocates an instance of MyClass.
  2. The init message is sent to this instance to complete the initialization process.
    In that method, self (which is a hidden argument to all Objective-C methods) is the allocated instance from step 1.
  3. [super init] calls the superclass implementation of init with the same (hidden) self argument. (This might be the point that you understood wrongly.)

  4. In the init method of BaseClass, self is still the same instance of MyClass. This superclass init method can now either

    • Do the base initialization of self and return self, or
    • Discard self and allocate/initialize and return a different object.

  5. Back in the init method of MyClass: self = [super init] is now either

    • The MyClass object that was allocated in step 1, or
    • Something different. (That's why one should check and use this return value.)
  6. The initialization is completed (using the self returned by the superclass init).

So, if I understood your question correctly, the main point is that

[super init]

calls the superclass implementation of init with the self argument, which is a MyClass object, not a BaseClass object.

Upvotes: 45

Martin Babacaev
Martin Babacaev

Reputation: 6280

[super init] is the same as [self superclass_variant_of_init]

If you want to send a message to superclass, there is another approach (without using runtime library):

[[self superclass] init];

Upvotes: 1

Fonix
Fonix

Reputation: 11597

I would think of it as, init'ing all the supers variables etc, then you get to init your extended classes variables before it is returned.

Upvotes: 1

Related Questions