Re: Is it possible for a subclass to set the ivar of its base class?
Patrick,
On 24. 9. 2016, at 17:11, Patrick J. Collins
wrote:
> I am trying to change the behavior of a library by subclassing one of the
> files, but am finding this is not working because I can't access (change) one
> of the internal instance variables. ... Is there a way I can get access to
> the _wut ivar in the base class
Note please that this kind of design is conceptually wrong and extremely
error-prone, as Manoah did point out: *never* do this, unless *very* necessary
and no cleaner way out.
If indeed *extremely* necessary, and if you *really* have no cleaner way out,
you can do that, exploiting ObjC runtime, essentially like this:
===
43 /tmp>
#import
@interface Foo:NSObject
-(void)setFoo:(NSInteger)foo;
-(void)showFoo;
@end
@interface Bar:Foo // could be a category just as well, etc.
-(void)dirtyHackSetFooDirectly:(NSInteger)foo;
@end
int main(int ac,char **av) {
id foo=[Bar new]; // if we used a category, subclass would not be needed
[foo dirtyHackSetFooDirectly:666];
[foo showFoo];
return 0;
}
@implementation Foo {
NSInteger _foo;
}
-(void)setFoo:(NSInteger)foo {
_foo=foo;
NSLog(@"unwanted side-effect");
}
-(void)showFoo {
NSLog(@"foo is %ld",(long)_foo);
}
@end
@implementation Bar
-(void)dirtyHackSetFooDirectly:(NSInteger)foo {
object_setInstanceVariable(self,"_foo",(void*)foo);
}
@end
44 /tmp> cc -Wall -framework Foundation q.m && ./a.out
2016-10-16 17:53:21.363 a.out[57747:707] foo is 666
45 /tmp>
===
All the best,
OC
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Re: Is it possible for a subclass to set the ivar of its base class?
On Sep 24, 2016, at 08:11:00, Patrick J. Collins wrote:
I am trying to change the behavior of a library by subclassing one
of the
files, but am finding this is not working because I can't access
(change) one
of the internal instance variables.
To illustrate my problem:
Rofl *rofl = [[Rofl alloc] init];
[rofl otherSetWut:123];
[rofl speak];
@implementation Lol {
int _wut;
}
-(void)setWut:(int)wut {
_wut = wut;
// side effects I do not want.
}
-(void)speak {
NSLog(@"so wut up? %i", _wut);
}
@end
@implementation Rofl {
int _wut;
}
-(void)otherSetWut:(int)wut {
_wut = wut;
}
@end
When I run this, I get 0 instead of 123. Is there a way I can get
access to the _wut ivar in the base class so that I will
How can I make it so that I will get the desired 123 output?
Thanks!
Patrick J. Collins
http://collinatorstudios.com
For one thing, I would avoid knowingly shadowing an iVar, as I don't
see a benefit, and indeed, as I see it, it is preventing you from
reaching the one the superclass is using.
More importantly, if the superclass is indeed providing unwanted side-
effects, then there is likely a design issue with that class -- in so
far as the setter is doing too much.
They should be using a separate method for actual operations.
Further, bypassing the setter, when the rest of the superclass uses it
(which is likely, though not necessarily true) will cause the super-
class's code to behave improperly,
so before using the runtime to directly modify it, check the bass
class code to make sure it is not internally using the setter.
Manoah F. Adams
mhfad...@federaladamsfamily.com
federaladamsfamily.com/manoah
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Re: Is it possible for a subclass to set the ivar of its base class?
> You can’t add a method (setOtherWut) to the base class in the subproject? > That would be easier than mucking with the run-time. Yeah I can.. I just was trying to avoid modifying someone else's code... If that's the way to do it then I guess that's what I will have to do. That just seems highly unfortunate. Patrick J. Collins http://collinatorstudios.com ___ Do not post admin requests to the list. They will be ignored. Objc-language mailing list (Objc-language@lists.apple.com) Help/Unsubscribe/Update your Subscription: https://lists.apple.com/mailman/options/objc-language/archive%40mail-archive.com This email sent to arch...@mail-archive.com
Re: Is it possible for a subclass to set the ivar of its base class?
On Sep 24, 2016, at 12:15 , Patrick J. Collins wrote: > > There's no way to do this without duplicating tons of their code? You can’t add a method (setOtherWut) to the base class in the subproject? That would be easier than mucking with the run-time. ___ Do not post admin requests to the list. They will be ignored. Objc-language mailing list (Objc-language@lists.apple.com) Help/Unsubscribe/Update your Subscription: https://lists.apple.com/mailman/options/objc-language/archive%40mail-archive.com This email sent to arch...@mail-archive.com
Re: Is it possible for a subclass to set the ivar of its base class?
> could just subclass as make my own custom setter. So I can call both Oops sorry-- subclass *AND* make my own custom setter. Patrick J. Collins http://collinatorstudios.com ___ Do not post admin requests to the list. They will be ignored. Objc-language mailing list (Objc-language@lists.apple.com) Help/Unsubscribe/Update your Subscription: https://lists.apple.com/mailman/options/objc-language/archive%40mail-archive.com This email sent to arch...@mail-archive.com
Re: Is it possible for a subclass to set the ivar of its base class?
> How do you know this *is* the implementation in the base class? How do you > know there *is* a private variable called “_wut”, or its type, or what value > it actually contains, or if there are any other > variables that must be set consistently whenever the value of the “wut” > property changes? > > I don’t see how you can safely make your change without seeing the source > code of the base class, and it you have access to the source code you’d be > better off making a custom library. I do have access to the base class-- the library is a sub-project within my project. I just simply want the option to be able to prevent a single side effect that happens in their setter method, so I was hoping I could just subclass as make my own custom setter. So I can call both theirs and mine when appropriate. There's no way to do this without duplicating tons of their code? Patrick J. Collins http://collinatorstudios.com ___ Do not post admin requests to the list. They will be ignored. Objc-language mailing list (Objc-language@lists.apple.com) Help/Unsubscribe/Update your Subscription: https://lists.apple.com/mailman/options/objc-language/archive%40mail-archive.com This email sent to arch...@mail-archive.com
Re: Is it possible for a subclass to set the ivar of its base class?
On Sep 24, 2016, at 08:11 , Patrick J. Collins
wrote:
>
>@implementation Lol {
>int _wut;
>}
>
>-(void)setWut:(int)wut {
>_wut = wut;
>// side effects I do not want.
>}
How do you know this *is* the implementation in the base class? How do you know
there *is* a private variable called “_wut”, or its type, or what value it
actually contains, or if there are any other variables that must be set
consistently whenever the value of the “wut” property changes?
I don’t see how you can safely make your change without seeing the source code
of the base class, and it you have access to the source code you’d be better
off making a custom library.
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Re: Is it possible for a subclass to set the ivar of its base class?
> On Sep 24, 2016, at 8:11 AM, Patrick J. Collins > wrote: > > When I run this, I get 0 instead of 123. Right; you’ve created a new, different ivar in the subclass, which just happens to have the same name. > Is there a way I can get access to the _wut ivar in the base class Yes, by using the power of . This is a super-powerful API that lets you open up and mess with classes like Legos. You can find the ivars and methods, modify variables, even create and remove methods and create new classes. Looks like the function you’ll need is object_setInstanceVariable. Obviously this great power should be used responsibly. I’d only recommend doing this as a last resort, if there’s no way to modify the library yourself. Also, if it's a system library, be careful because the library’s implementation could change in any future OS release and invalidate your hack, breaking your app. —Jens ___ Do not post admin requests to the list. They will be ignored. Objc-language mailing list (Objc-language@lists.apple.com) Help/Unsubscribe/Update your Subscription: https://lists.apple.com/mailman/options/objc-language/archive%40mail-archive.com This email sent to arch...@mail-archive.com
