弱引用 。
A weak reference is a holder for a reference to an object, called the referent.
弱引用是对一个对象(称为referent)的引用的持有者。
Returns the current generation number of the target of a specified weak reference.
返回指定弱引用的目标的当前代数。
Returns the current generation number of the target of a specified weak reference.
返回指定弱引用的目标的当前代数。
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