Question

What is the difference beetween :

// 1
-(id) init {
    self = [super init];
    if (self) {
        ... do init ....
    }
    return self;
}

// 2 - I guess it's exactly the same as previous
-(id) init {
    if (self = [super init]) {
        ... do init ....
    }
    return self;
}

// 3 - is this one a really bad idea and if yes, Why ?
-(id) init {
    self = [super init];
    if (!self) return nil;

    ... do init ....

    return self;
}

// 4 - I think this one sounds OK, no ? But what is returned... nil ?
-(id) init {
    self = [super init];
    if (!self) return self;

    ... do init ....

    return self;
}

EDIT : Added thanks to Peter M.

// 5 - I really like the readability of this one
-(id) init {
    if(!(self = [super init])) return self;  // or nil

    ... do init ....

    return self;
}

Answer 1

1. The common one used by Apple
2. Does the same but the compiler does not know if you didn’t mean == instead of =. You can block the warning by using another (...) around the expression, but it’s not easy to read.
3. This one would be preferred by any good coding standards (usually not as a one-liner but with bracers). When your initialization code is long, this one significantly increase readibility.
4. The same as 3 but you lose some readibility. Returning nil makes the code easier to understand (maybe this is only my opinion, someone else can say 4 is better than 3).

In summary: use 1 or 3. When initialization code is long, you should use 3 to avoid having most of method code in one if block. Apple uses only 1 but don’t follow it blindly. Apple has no documented coding standards and sometimes what they recommend is very questionable.

You can use 4 instead of 3 but don’t mix 3 & 4 in your code.