[cfe-dev] Objective-C in `goto` label block.

[Hoon Hwangbo]

Thanks for great answers!

On Feb 22, 2013, at 1:02 PM, John McCall <rjmccall at apple.com> wrote:

> On Feb 21, 2013, at 7:24 PM, Hoon Hwangbo <drawtree at me.com> wrote:
>> I am testing `goto` with Objective-C (ARC enabled).
>> And I discovered Objective-C object cannot be exist under `goto` label.
>> 
>> 			{
>> 				AA*	a1;			//	Fine.
>> 			
>> 			LABEL3:
>> 				AA*	a2;			//	Compiler error: "Expected expression".
>> 			}
>> 
>> Anyway it's possible to put Objective-C object in nested block.
>> 
>> 			{
>> 			LABEL1:
>> 				{
>> 					AA*	a1	=	[AA new];
>> 				}
>> 			}
>> 
>> This makes me confused.
>> 
>> (1) Why Objective-C objects cannot be placed directly in label?
>> (2) Why it's possible in nested block?
>> (3) Is it designed or buggy behavior?
>> 
>> Can I know accurate behavior of `goto` and labels under Objective-C context?
> 
> ARC relies on local variables of object type being initialized to nil.
> A goto which jumps into the scope of an uninitialized local variable
> could leave the function in an undefined state, causing it to crash
> when the variable goes out of scope.
> 
> It could be argued that the compiler should implicitly zero-initialize
> such variables when performing a goto, but that's actually quite
> complicated to implement, and so the current rule is simply to ban this
> kind of jump.
> 
> What's going on in your nested block is that you've explicitly limited
> the scope of 'a1' so that "goto LABEL1;" completely skips its scope
> instead of jumping into the middle of it.
> 
> John.