gistfile1.m

@protocol ChildProtocol;

@protocol ParentProtocol <NSObject>
@property (nonatomic, strong, readonly) id<ChildProtocol> child;
@end

@protocol ChildProtocol <NSObject>
@property (nonatomic, weak, readonly) id<ParentProtocol> parent;
@end

@class Child;

@interface Parent : NSObject <ParentProtocol>
@property (nonatomic, strong) Child* child;
@end

@interface Child : NSObject <ChildProtocol>
@property (nonatomic, weak) Parent* parent;
@end