当两个对象互相引用时，就构成了循环引用，其中每个对象的引用计数值都被赋1。
A circular reference is formed when two objects reference each other, giving each object a reference count of 1.
当两个对象互相引用时，就构成了循环引用，其中每个对象的引用计数值都被赋1。
A circular reference is formed when two objects reference each other, giving each object a reference count of 1.
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