Re: self = [super init] debate.

FROM : Jens Alfke
DATE : Sun Feb 10 22:44:32 2008

The real killer argument for reassigning 'self' is that the call to 
[super init] might fail, returning nil. If this happens, the 
superclass init method has released/dealloced the object (to avoid a 
leak) so your 'self' is now a dangling pointer to freed memory. In 
this case you must immediately return nil instead of doing any more 
setup.

And -init methods do return nil for reasons other than out-of-memory. 
For example, -[NSString initWithUTF8String:] returns NIL if the 
parameter isn't valid UTF-8, and -[NSDictionary 
initWithContentsOfFile:] returns nil if the file isn't found or isn't 
a plist.

So the following is buggy, and will crash or corrupt memory if the 
superclass's initializer ever fails:

-(id) initWithFoo: (int)foo {
   [super initWithFoo: foo];
   _bar = 1;
   return self;
}

This is a real problem. I have run into actual bugs that were caused 
by writing an initializer this way.

The following change is no help:

-(id) initWithFoo: (int)foo {
   [super initWithFoo: foo];
   if( self )
       _bar = 1;
   return self;
}

The "if(self)" check is pointless because 'self' is a local variable; 
the superclass init method won't affect it. The only way to find out 
if the superclass init returned nil is to actually check its return 
value. The following would work:

-(id) initWithFoo: (int)foo {
   if( [super initWithFoo: foo] )
       _bar = 1;
   return self;
}

If the superclass init fails, this will correctly avoid accessing 
'self', and instead return nil. What this doesn't support, however, is 
the previously-described case where the superclass initializer 
succeeds but returns a different object. To support that, you actually 
have to assign the return value of 'super init' to 'self'.

-(id) initWithFoo: (int)foo {
   self = [super initWithFoo: foo];
   if( self )
       _bar = 1;
   return self;
}

It's possible to coalesce the first two lines into "if( self = [super 
initWithFoo: foo] )", but I avoid this because putting assignments 
inside 'if' or 'while' tests is a dangerous idiom ... it's very easy 
to type "==" instead of "=" and end up with a difficult-to-detect bug.

—Jens_______________________________________________

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