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(lèi)型的值都是引用傳遞,通過(guò)引用計(jì)數(shù)來(lái)判斷是否收掉對(duì)象,而弱引用(weak reference)會(huì)被垃圾回收器忽略.weak table就是它的元素是弱引用的,一個(gè)元素(鍵值對(duì))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(gè)特性是通過(guò)弱表的metatable的__mode的值來(lái)設(shè)置的,特別有意思的是,當(dāng)弱表中一個(gè)鍵值對(duì),它的鍵或值關(guān)聯(lián)(引用/指向)的那個(gè)對(duì)象被垃圾回收器回收的時(shí)候,這個(gè)鍵值對(duì)會(huì)從弱表中被自動(dòng)刪除掉.這是個(gè)很重要的特點(diǎn).
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(guò)userdata綁定c++對(duì)象到腳本中
fulluserdata能夠設(shè)置metatable,也就能模擬出對(duì)象的效果出來(lái),對(duì)一個(gè)C++的類(lèi)的對(duì)象實(shí)例來(lái)說(shuō),push到腳本中,一般是創(chuàng)建了一個(gè)userdata,文章中用弱表避免了同一個(gè)對(duì)象實(shí)例(指針) push到腳本中,多次創(chuàng)建userdata的問(wèn)題.
換句話來(lái)說(shuō),如果C++對(duì)象的生存周期是靠lua的垃圾回收來(lái)控制的話(userdata被回收時(shí),調(diào)用元表的__gc方法,__gc方法中析構(gòu)c++對(duì)象),一個(gè)C++對(duì)象只能有一個(gè)唯一的userdata. 在userdata的metatable中創(chuàng)建一個(gè)值是弱引用的弱表,用C++對(duì)象指針做鍵,每次push c++對(duì)象的時(shí)候,就去用指針值查弱表,如果有,就push那個(gè)userdata,沒(méi)有就創(chuàng)建, 同時(shí),當(dāng)userdata是被弱引用的,當(dāng)被垃圾回收掉的時(shí)候,弱表中它所在的鍵值對(duì)自動(dòng)被銷(xiāo)毀了.