CODEWITHSUNDEEP
iphoneobjective-c
Jonathan.
Jonathan.

Reputation: 55534

Why put [super init] inside of an if statement since whether or not the return is nil we return it anyway?

On the CS193p course they says that in an init method there should be an if statement to check if the [super init] works:

if (self = [super init]) {
    self.someProperty = parameter;    
}
return self;

I don't understand why this is done, as if the [super init] returns nil, the method itself will also return nil, no matter the outcome of the if statement?

EDIT: The question is; why put self = [super init] inside an if statement. (Not: Why have self = [super init] at all)

Upvotes: 3

Views: 708

Answers (6)

Lucas
Lucas

Reputation: 11

Apple explains this particular concept in detail in there Objective-c programming guide. (Scroll down to the section "Handling Initialization Failure")

However Apple example code often keeps the self assignment outside the if statement

self = [super init];
if (self) {
    //init stuff...
}
return self;

And if you are using the LLVM 2.0 compiler, it will give you a warning and tell you to wrap your statement in parentheses, too. Like this:

if ((self = [super init])) {
    //init stuff...
}
return self;

Upvotes: 1

Chris Reid
Chris Reid

Reputation: 488

self = [[super alloc] init]; creates a base class and the runtime will choke when you try to invoke subclass-only methods later on.

self = [super init]; is a common sight in Objective-C when the base class needs to initialize variable values or instantiate composite members, otherwise they are created with Nil or (0) values.

Upvotes: -1

filipe
filipe

Reputation: 3380

It doesn't have to be in an if statement.
you can very well have it like

self = [super init];
if ( self ) {
    self.someProperty = parameter;    
}
return self;

the if statement is there to check that self was initialized properly and it is safe to do self.someProperty = parameter

Upvotes: 0

Max
Max

Reputation: 16709

That's make sense because in some cases [super init] can return nil, and in this case if you try to access some ivar you'll get crash.

Example:

-(id) init {
    self = [super init];
    some_ivar = [[NSObject alloc] init]; //compiler treats this as self->some_ivar
    //if self == nil you'll get EXC_BAD_ACCESS

    return self;
}

Upvotes: 4

Omar
Omar

Reputation: 1574

You need to call [super init] so that the superclass can do any one-time initialization that it needs to do. The init method returns a pointer representing the object that was initialized.

Assigning the result of [super init] back to self is a standard Objective-C convention. It's done in case the superclass, as part of its initialization work, returns a different object than the one originally created. NSString's, for example, do this

So it's self = [super init]

I have never seen [[super alloc] init], i've always used [super init], and as far as my knowledge, that's the convention

Upvotes: 0

Ben Mosher
Ben Mosher

Reputation: 13381

It should be if(self = [super init]){.... init is an instance method and thus can only be called on an object that has already been alloc'd.

Upvotes: 0

Related Questions