解决ARC的循环引用问题

@interface BNRItem : NSObject

@property (nonatomic, strong) BNRItem *containedItem;
@property (nonatomic, strong) BNRItem *container;

...

@end

#import "BNRItem.h"

@implementation BNRItem

...

- (void)setContainedItem:(BNRItem *)containedItem
{
    _containedItem = containedItem;
    self.containedItem.container = self;
}

@end

int main(int argc, const char * argv[])
{

    @autoreleasepool {

        // Create a mutable array object, store its address in items variable
        NSMutableArray *items = [[NSMutableArray alloc] init];

        BNRItem *backpack = [[BNRItem alloc] initWithItemName:@"Backpack"];
        [items addObject:backpack];

        BNRItem *calculator = [[BNRItem alloc] initWithItemName:@"Calculator"];
        [items addObject:calculator];

        backpack.containedItem = calculator;

        backpack = nil;
        calculator = nil;

        for (BNRItem *item in items) {
            NSLog(@"%@", item);
        }

        // Destroy the mutable array object
        NSLog(@"Setting items to nil...");
        items = nil;

    }
    return 0;
}