Question

I can’t get rid of compiler warning when I define property inside protocol. Strange thing is that I have two properties defined, and I only get warnings for the second one (which is object type, while the first property is value type).

Here is screenshot:

Can anyone tell me how to get rid of this warning, and why it is generated? The code is working normally, it is just this warning that annoys me 🙂

Answer 1

Your issue is that the compiler cannot find an implementation for the properties you defined in the protocol.

For this reason, it is not recommended to add properties to a protocol, instead, you would define just a simple method to access the property, and one to set it. That would give you the proper error messages, and while you couldn’t use dot-notation, it keeps the warnings in the right place.

Alternatively, you could do something like this (not recommended, but for educational reasons):

#import <objc/runtime.h>

@protocol myProto

@property (assign) int myProperty;

@end

@implementation NSObject(myProto)

-(int) myProperty
{
    return [objc_getAssociatedObject(self, "myProperty") intValue];
}

-(void) setMyProperty:(int) myProperty
{
    objc_setAssociatedObject(self, "myProperty", [NSNumber numberWithInt:myProperty], OBJC_ASSOCIATION_RETAIN);
}

@end

@interface MyObj : NSObject<myProto> 

@end

@implementation MyObj

@dynamic myProperty;

@end

int main(int argc, char *argv[])
{
    @autoreleasepool
    {
        MyObj *myObj = [MyObj new];

        myObj.myProperty = 10;

        NSLog(@"%i", myObj.myProperty);
    }
}