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开发者_JS百科 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)

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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;
}

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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;

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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.

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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

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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

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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.