Instance variables with same name in different classes causing compile error

Question

I have two objc .m classes, we'll call them controller1.m and controller2.m. In both controller1.m and controller2.m I have instances of UIAlertController, something like this:

controller1.m

@implementation controller1
    UIAlertController *alertController;
//more code
@end

controller2.m

@implementaion controller2
    UIAlertController *alertController;
//more code
@end

When I try to build the project, I get the error

linker command failed with exit code 1 (use -v to see invocation)

And the error log looks like this

duplicate symbol _alertController in:
/Users/.../Xcode/DerivedData/AppName-cdtbqibmmbsosrdeyqughefeazaa/Build/Intermediates/AppName.build/Debug-iphonesimulator/AppName.build/Objects-normal/x86_64/controller1.o
/Users/.../Xcode/DerivedData/AppName-cdtbqibmmbsosrdeyqughefeazaa/Build/Intermediates/AppName.build/Debug-iphonesimulator/AppName.build/Objects-normal/x86_64/controller2.o

I changed the names of the instance variables for now and it worked... Why do I have to do this? Does Xcode realize that these variables are being used in the same way (due to their name), and that I should have them declared globally, somehow?

@danh Ah, okay. But why is that? [Forgive me, I'm pretty new with objective-c] — Mahmud Ahmad
– Mahmud Ahmad, Commented Aug 6, 2016 at 0:15

rmaddy · Accepted Answer · 2016-08-06 00:15:24Z

5

Your problem is that those variables are not instance variables. They are global variables.

To make them instance variables they need to be in the curly braces of the @implementation block as follows:

@implementation controller1 {
    UIAlertController *alertController;
}

//more code

@end

answered Aug 6, 2016 at 0:15

rmaddy

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3 Comments

Mahmud Ahmad Over a year ago

Ah okay, thanks. And is there a difference between doing it the way you have it versus the way @Danh recommended, by making it a property of the class?

rmaddy Over a year ago

My way fixes your declaration of the instance variable. Dan's way replaces the instance variable with a property. There are plenty of debates about whether you should use private properties or instance variables. Either approach works.

Mahmud Ahmad Over a year ago

Okay, thanks. It looks like I have to read up on the difference between instance variables and properties. So far it looks like they are accessed differently and thats about it

danh · Accepted Answer · 2016-08-06 00:15:31Z

1

As you have it, the code declares a global with the same name in each module. To make the pointers private to their respective classes, use properties...

// controller1.m

@interface controller1 ()

@property(strong,nonatomic) UIAlertController *alertController;
// any other property private to controller1

@end

@implementation controller1

// only methods here

@end

answered Aug 6, 2016 at 0:15

danh

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5 Comments

rmaddy Over a year ago

You do not need to change them to properties to fix them properly.

Mahmud Ahmad Over a year ago

Thanks! Is there a difference in what you are recommending versus what @rmaddy recommended/why did you recommend that? I accepted rmaddy's answer because his answers appeared slightly before yours. I do see that you commented on my question before him though

danh Over a year ago

sure. the difference is that I'm suggesting a little more modern declaration that, under the covers, creates a getter and a setter method for you. So you can say self.alertControlleror self.alertController = // blah.

Mahmud Ahmad Over a year ago

Hmm thats interesting. So I'm guessing that @properties take up more memory (not that it matters for the small scale apps I build), and should be used when I have setting/getting them in mind? Whereas I should make an ivar if I don't need that stuff for it?

danh Over a year ago

They are the same data-wise. The object gets two more methods that I would argue are a net benefit.

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