Question

This is my simple piece of code on iOS, using ARC:

@interface Person : NSObject {
    NSObject *objStrong;
    NSObject *objWeek;
}
@property(strong) NSObject *objStrong;
//getting error at this line
@property(weak) NSObject *objWeek; //Existing ivar 'objWeek' for _week property 'objWeek' must be _week 

@end

@implementation Person
@synthesize objStrong;
@synthesize objWeek;

@end

When I try to compile, the compiler complains about an existing ivar ‘objWeek’ for _weak property ‘objWeek’. Why isn’t this code compiling correctly?

Answer 1

It’s complaining because the backing variable, NSObject *objWeek is declared as __strong (all otherwise unannotated Objective C pointers to retainable objects are __strong). Change the backing variable to be __weak NSObject *objWeek, and the compiler will like you again.

Edit: As requested, the ARC documentation from LLVM’s clang:

4.4.1. Objects

If an object is declared with retainable object owner type, but
without an explicit ownership qualifier, its type is implicitly
adjusted to have __strong qualification.

As a special case, if the object’s base type is Class (possibly
protocol-qualified), the type is adjusted to have __unsafe_unretained
qualification instead.

Link: http://clang.llvm.org/docs/AutomaticReferenceCounting.html#ownership.inference