[cfe-dev] Objc ivar visibility question

[Douglas Gregor]

On May 4, 2010, at 8:02 AM, Jean-Daniel Dupas wrote:

> Hello,
> 
> I have a question about ivar visibility in Objective-C.
> If you try to compile the following sample code with GCC, it refuses to compile the method -bar because _parent is a private field of Foo and you try to access it from a Bar variable.
> Actually clang behaves the same.
> 
> private.m:26:27: error: instance variable '_parent' is private
>  while (parent && parent->_parent)
>                           ^
> private.m:27:21: error: instance variable '_parent' is private
>    parent = parent->_parent;
> 
> 
> Is there a reason why GCC and clang prevent access to a Foo private ivar from a Foo method implementation ?
> I know that I can simply workaround this issues with a couple of casts, but by doing this I have to feeling I'm fighting against the compiler.
> Shouldn't clang be smarter and allow that ? 

I think Clang is doing the right thing here. You're accessing a private ivar through a class of a different type than the class in which the method is being defined, so it's treated like an access to that ivar with no special privileges.  That seems like a reasonable model for access control.

	- Doug

> ================================================
> @class Bar;
> @interface Foo {
> @private
>   Bar *_parent;
> }
> - (Foo *)foo;
> - (Bar *)bar;
> @end
> 
> @interface Bar : Foo {}
> @end
> 
> @implementation Foo 
> 
> - (Foo *)foo {
>  Foo *parent = _parent;
>  while (parent && parent->_parent)
>    parent = parent->_parent;
>  return parent;
> }
> 
> - (Bar *)bar {
>  Bar *parent = _parent;
>  while (parent && parent->_parent)
>    parent = parent->_parent;
>  return parent;
> }
> 
> @end
> ==================================================
> 
> 
> -- Jean-Daniel
> 
> 
> 
> 
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