Objective C: class category not recognized when compiled in a static library

Question

Created Jan ’24

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I have a class category declared and compiled in a mac os static lib:

#import <Foundation/Foundation.h>

@interface NSNumber(MyExtension)
-(NSString *)CallMe;
@end

then this staticLib is added to a mac console app in XCode and used:

#import <Foundation/Foundation.h>
#import "MacStaticLib.h"

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        NSNumber *one = [NSNumber numberWithInt:12];
        NSLog(@"%@",[one CallMe]);        
    }
    return 0;
}

Compiles fine but fails in runtime, unrecognized selector 'CallMe' send to instance... what's wrong or what I missed?

Thanks!!

Answer 1

jlilest OP

Jan ’24

Is the header file with the extension public and included in the framework umbrella header?

Answer 2

wirex OP

Jan ’24

Yes, the extension is public and in the header file, I'm sorry, I don't know what's the framework umbrella header. Another thing I noticed now is if I add -ObjC on 'Other linker flags' then the category is recognized and works fine, but honestly don't know why doesn't work without the flag.

Thanks in advance.

Answer 3

DTS Engineer OP

Apple

Jan ’24

Another thing I noticed now is if I add -ObjC on 'Other linker flags' then the category is recognized and works fine

Yep. And there are a variety of similar flags next to that one in the in the ld man page.

but honestly don't know why doesn't work without the flag.

This is basically a disconnect between the dynamic nature of Objective-C and the static nature of the linker. A static library is an archive containing multiple .o files. By default the static linker only integrates a .o into the final binary if it’s referenced by other code. This acts like a primitive form of dead stripping.

However, Objective-C code is often referenced dynamically, which means that the static linker can’t tell whether the code is used or not. The -ObjC option causes the linker to integrate all of the .o files in the static library, regardless of whether it thinks they’re used or not.

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