objc_weak_atomic.mm

#import <Foundation/Foundation.h>

OBJC_EXPORT id objc_storeWeak(id *location, id newObj);
OBJC_EXPORT id objc_loadWeak(id *location);
OBJC_EXPORT void objc_destroyWeak(id *location);
OBJC_EXPORT void objc_setProperty_atomic(id _Nullable self, SEL _Nonnull _cmd, id _Nullable newValue, ptrdiff_t offset); // objc_setProperty, objc_setProperty_atomic, objc_setProperty_nonatomic, objc_setProperty_nonatomic_copy
OBJC_EXPORT id _Nullable objc_getProperty(id _Nullable self, SEL _Nonnull _cmd, ptrdiff_t offset, BOOL atomic);

@interface MyObject : NSObject {
@private id _weak_property;
@private id _atomic_property;
}
@property (assign, nonatomic) id weak_property;
@property (retain, nonatomic) id atomic_property;
@end

@implementation MyObject

- (void)dealloc {
    objc_destroyWeak(&_weak_property);
    [_atomic_property release];
    [super dealloc];
}

- (void)setWeak_property:(id)property {
    objc_storeWeak(&_weak_property, property);
}

- (id)weak_property {
    return objc_loadWeak(&_weak_property);
}

- (id)atomic_property {
    return objc_getProperty(self, 
                            @selector(atomic_property),
                            reinterpret_cast<uintptr_t>(self) - reinterpret_cast<uintptr_t>(&_atomic_property),
                            YES);
}

- (void)setAtomic_property:(id)atomic_property {
//    [_atomic_property release];
//    _atomic_property = [atomic_property retain];
    objc_setProperty_atomic(self,
                            @selector(setAtomic_property:),
                            atomic_property,
                            reinterpret_cast<uintptr_t>(self) - reinterpret_cast<uintptr_t>(&_atomic_property));
}

@end

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        MyObject *object = [MyObject new];
        
        //
        
        NSObject *weak_property = [NSObject new];
        object.weak_property = weak_property;
        
        @autoreleasepool {
            NSLog(@"%@", object.weak_property);
        }
        
        [weak_property release];
        
        @autoreleasepool {
            NSLog(@"%@", object.weak_property);
        }
        
        //
        
        dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);
        
        dispatch_async(dispatch_get_global_queue(QOS_CLASS_DEFAULT, 0), ^{
            for (int i = 0; i < 10000; i++) {
                @autoreleasepool {
                    NSObject *atomic_property = [NSObject new];
                    object.atomic_property = atomic_property;
                    [atomic_property release];
                    
                    [object atomic_property];
                }
            }
            
            dispatch_semaphore_signal(semaphore);
        });
        dispatch_async(dispatch_get_global_queue(QOS_CLASS_DEFAULT, 0), ^{
            for (int i = 0; i < 10000; i++) {
                @autoreleasepool {
                    NSObject *atomic_property = [NSObject new];
                    object.atomic_property = atomic_property;
                    [atomic_property release];
                    
                    [object atomic_property];
                }
            }
            
            dispatch_semaphore_signal(semaphore);
        });
        dispatch_async(dispatch_get_global_queue(QOS_CLASS_DEFAULT, 0), ^{
            @autoreleasepool {
                NSObject *atomic_property = [NSObject new];
                object.atomic_property = atomic_property;
                [atomic_property release];
                
                [object atomic_property];
            }
            
            dispatch_semaphore_signal(semaphore);
        });
        
        dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
        dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
        dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
        dispatch_release(semaphore);
        
        [object release];
    }
    
    return EXIT_SUCCESS;
}