On Mar 4, 2013, at 6:23 PM, Ignacio Enriquez <nach...@gmail.com> wrote:
> I am trying to understand the correct usage and caveats, of
> object_setInstanceValue function from objc/objc-runtime.h.
>
> Recently I found this code:
>
> object_setInstanceVariable(foo, "_isRevealed", (void*)YES);
>
> where foo is:
>
> @interface Foo : NSObject{
> double _doubled;
> NSObject *obj;
> BOOL _booled;
> BOOL _isRevealed;
> BOOL _isBar;
> }
> ...
> - (NSString *)description {
> return [NSString stringWithFormat:@"%p %f, %@, %d %d %d", self, _doubled,
> _obj, _booled, _isRevealed, _isBar];
> }
> @end
>
> An old post in this list (Re: ivars and fundamental types -
> http://lists.apple.com/archives/cocoa-dev/2010/Oct/msg00402.html) suggests
> its brother object_getInstanceValue function should not be used for
> fundamental types, only objects. "It happens to work for pointer-size
> non-object values -- Greg Parker". I am trying this in the iOS simulator
> and even though BOOL (size=1) and pointers (size=4) are not the same size
> it seems to work.
It doesn't work. object_setInstanceVariable() will write a 4-byte value to your
1-byte ivar. That may set your ivar correctly, but it will also trample on
other nearby ivars.
Example:
foo = [Foo new];
foo->_booled = YES;
foo->_isRevealed = YES;
foo->_isBar = YES;
NSLog(@"before %@", foo);
object_setInstanceVariable(foo, "_isRevealed", (void*)NO);
NSLog(@"after %@", foo);
2013-03-04 18:32:46.832 a.out[77319:303] before 0x7ae13ff0 0.000000,
(null), 1 1 1
2013-03-04 18:32:46.834 a.out[77319:303] after 0x7ae13ff0 0.000000,
(null), 1 0 0
Note that object_setInstanceVariable() incorrectly set two of your ivars to NO.
--
Greg Parker gpar...@apple.com Runtime Wrangler
_______________________________________________
Cocoa-dev mailing list (Cocoa-dev@lists.apple.com)
Please do not post admin requests or moderator comments to the list.
Contact the moderators at cocoa-dev-admins(at)lists.apple.com
Help/Unsubscribe/Update your Subscription:
https://lists.apple.com/mailman/options/cocoa-dev/archive%40mail-archive.com
This email sent to arch...@mail-archive.com
