关于弱引用：
https://www.lua.org/pil/17.html
官方的解释很好理解，弱引用就是不会阻挡GC的引用
那为什么需要这样的功能呢？
Nevertheless, sometimes even the smarter collector needs your help. No garbage collector allows you to forget all worries about memory management.
Similarly, any object stored in a global variable is not garbage for Lua, even if your program will never use it again.
a = {} b = {} setmetatable(a, b) b.__mode = "k" -- now `a' has weak keys key = {} -- creates first key a[key] = 1 -- 在这一步，key指向了新的内存位置，原来的key不会再继续保持占用了 key = {} -- creates second key a[key] = 2 collectgarbage() -- forces a garbage collection cycle for k, v in pairs(a) do print(v) end