Undefined symbols: "_OBJC_CLASS_$ error

Question

I've been looking through countless posts about this error:

Undefined symbols:
"_OBJC_CLASS_$_BoxView", referenced from:
  objc-class-ref-to-BoxView in ViewMovingViewController.o
ld: symbol(s) not found
collect2: ld returned 1 exit status

BoxView is a subclass of UIView, and the UIKit framework has been included. BoxView.h has been imported in the ViewController.

The ViewController contains this code:

-(void) addBoxViewAtLocation:(CGPoint)point {
    CGRect rect;  
    rect.origin.x = point.x;  
    rect.origin.y = point.y;  
    rect.size.width = 80;  
    rect.size.width = 40;  
    BoxView *newView = [[BoxView alloc] initWithFrame:rect];  
    newView.backgroundColor = [UIColor yellowColor];  
    [mainView addSubview:newView];  
}  

And BoxView contains this code:

- (id)initWithFrame:(CGRect)frame {     
    self = [super initWithFrame:frame];  
    if (self) {  
        // no further initialization  
    }  
    return self; 
}  

This is the line that's causing the error, from the code above:

BoxView *newView = [[BoxView alloc] initWithFrame:rect];

When I change BoxView to UIView in that line, the error goes away. Does anyone know what I need to change here? I have looked through many posts about this, but most answers say it's link related, but I've tried ticking and unticking certain boxes with no success. I'm wondering if the error is in my code? Any suggestions would be appreciated!

Solution 1

In general, this will occur when the code for BoxView is not being compiled into your target correctly.

You need to ensure that the target you're building has its corresponding box checked for your BoxView.m implementation file. Your question suggests that you've tried this, but here's a screenshot (from Xcode 4) just for clarity's sake.

A 'Clean and Build' never hurts, either.

Solution 2

I just want to add that Ben Mosher's answer is totally right. But there's another way to include the files to build in the Target settings.

Solution 3

Added Scenario

If your project has module dependencies(framework), rebuild them before building your main project.

Comments

[cms]

Thank you, definitely tried the clean build but haven't come across a tick box for BoxView.m .. Where would I find that tickbox? I'm looking through the Build settings but don't see it.. I'm working with an older version of X-Code, X-Code 3, so I don't think my version conforms to the screenshot you sent

[cms]

Oh actually, I just found it. Don't know if its the case in Xcode 4 but in Xcode 3 you can literally drag and drop the .m file into the target

[wcochran]

Sometimes when I add files to projects they are not compiled when the project is built. In Xcode 4, I explicitly add them under "Build Phases" -> "Compile Sources". Why these are not set (sometimes) by default, I don't know.

[Ben Mosher]

@cms If this did solve (or help with) your problem, it would be great if you could accept the answer. :-)

[Patrice Cote]

I can't tell how you made my day with this answer. Thanks a lot for sharing the knowledge.

[Sjakelien]

Thanks a lot! I don't want to claim that I now understand all the ins and outs of this problem, but hey, I can compile my app!

[e.ozmen]

You saved my day!! Thanks

[ecoe]

works in Xcode 7 very similarly but you access Linked Frameworks and Libraries from the General tab for a given selected target.

[Julius Naeumann]

I got this error because I defined a class in a header file, without any @implementation. Adding an empty @implementation into the .m solved my issue.