Re: @property problem

FROM : Bill Bumgarner
DATE : Sun Feb 17 21:17:51 2008

On Feb 17, 2008, at 12:03 PM, Sherm Pendley wrote:
> Why would the parent's size be hard-coded? You can get the size of 
> the parent at run time, by dereferencing a couple of levels into 
> isa. All the compiler would have to do is emit code that walks down 
> isa to get the parent's size, and add it to self to establish a base 
> address. The only hard-coded offsets would then be relative to that 
> base, for the current class' own ivars.
>
> Changing the size of the parent class wouldn't break such a scheme, 
> so far as ordinary usage goes. People playing games with sizeof() or 
> @defs deserve whatever breakage they get. :-)
>
> Doesn't the current (I refuse to call it "legacy" quite yet...) 32-
> bit compiler emit code like that when ivars are accessed?


To dive a little deeper, consider the instance variables for NSView, 
including  inherited ivars.  Well, a subset, really:

@interface NSObject <NSObject> {
    Class    isa;
}

@interface NSResponder : NSObject <NSCoding>
{
    id _nextResponder;
}

@interface NSView : NSResponder
{
    NSRect              _frame;
    NSRect              _bounds;
    id                  _superview;
    id            _subviews;
    NSWindow            *_window;
}

Within the legacy runtime ;), the memory for NSView looks just like a 
C struct with this layout:

struct NSViewLikeStruct {
    Class    isa;
    id _nextResponder;
    NSRect              _frame;
    NSRect              _bounds;
    id                  _superview;
    id            _subviews;
    NSWindow            *_window;
};

So, in the implementation of an NSView, an access to an instance 
variable -- say _subviews -- is compiled down to something equivalent 
to:

((struct NSViewLikeStruct *) aView)->_subviews

Now, if an instance variable is added to NSObject or NSResponder and 
the subclasses are not compiled, the above code will have the wrong 
offset.

This is fragile, but allows instances to be allocated as a single 
chunk of memory while also reducing the amount of metadata and/or 
indirection required to get at any given instance variable's slot.

To fix the issue, it has to be done partially at runtime and requires 
a different layout for the instances...

b.bum