A weak table is a table whose elements are weak references. A weak reference is ignored by the garbage collector. In other words, if the only references to an object are weak references, then the garbage collector will collect this object.
A weak table can have weak keys, weak values, or both. A table with weak keys allows the collection of its keys, but prevents the collection of its values. A table with both weak keys and weak values allows the collection of both keys and values. In any case, if either the key or the value is collected, the whole pair is removed from the table. The weakness of a table is controlled by the __mode field of its metatable. If the __mode field is a string containing the character 'k', the keys in the table are weak. If __mode contains 'v', the values in the table are weak.
After you use a table as a metatable, you should not change the value of its __mode field. Otherwise, the weak behavior of the tables controlled by this metatable is undefined.
在lua中,像table,userdata,function這些類型的值都是引用傳遞,通過引用計數來判斷是否收掉對象,而弱引用(weak reference)會被垃圾回收器忽略.weak table就是它的元素是弱引用的,一個元素(鍵值對)可能鍵是弱引用,也可能值是弱引用的,也可能都是弱引用, 這個特性是通過弱表的metatable的__mode的值來設置的,特別有意思的是,當弱表中一個鍵值對,它的鍵或值關聯(引用/指向)的那個對象被垃圾回收器回收的時候,這個鍵值對會從弱表中被自動刪除掉.這是個很重要的特點.
那么弱表到底有什么用呢? 在lua的wiki中有一篇使用userdata的例子 ,其中就很巧妙的用到了弱表,原文地址
http://lua-users.org/wiki/CppBindingWithLunar
這篇文章介紹了如何通過userdata綁定c++對象到腳本中
fulluserdata能夠設置metatable,也就能模擬出對象的效果出來,對一個C++的類的對象實例來說,push到腳本中,一般是創建了一個userdata,文章中用弱表避免了同一個對象實例(指針) push到腳本中,多次創建userdata的問題.
換句話來說,如果C++對象的生存周期是靠lua的垃圾回收來控制的話(userdata被回收時,調用元表的__gc方法,__gc方法中析構c++對象),一個C++對象只能有一個唯一的userdata. 在userdata的metatable中創建一個值是弱引用的弱表,用C++對象指針做鍵,每次push c++對象的時候,就去用指針值查弱表,如果有,就push那個userdata,沒有就創建, 同時,當userdata是被弱引用的,當被垃圾回收掉的時候,弱表中它所在的鍵值對自動被銷毀了.