設(shè)置元表后，填充__metatable字段，元表就無法被訪問，依然可以被調(diào)用。

Set.mt = {}

Set.mt.__add = Set.union

function Set.new(t)

local set = {}

setmetatable(set,Set.mt)

for _,l in ipairs(t) do

set[l] = true

end

return set

end

Set.mt.__metatable = "not your businesss"

s1 = Set.new{}

print(getmetatable(s1))? ?-- 顯示 not your business

setmetatable(s1,{})? ?-- cannot change a protected metatable