someta3.lua

-- READ ONLY FORM

local mt = {}        -- The real metatable
local mt_proxy = {}  -- The object returned by getmetatable()

setmetatable(mt_proxy,{
  __index = function(self,k) return mt[k] end,  -- Allow the proxy to read fields. use a function so it's opaque (user cannot access real metatable)
  __newindex = function() end,  -- But writing is a nop (__newindex must exist, otherwise we may accidentally barr access to __index fields)
  __metatable = false,  -- Do not allow user code to overwrite the behavior of the proxy
})
mt.__metatable = mt_proxy  -- Make getmetatable() return the proxy

test = {}
setmetatable(test,mt)

mt.__newindex = print  -- insert into real metatable

assert(getmetatable(test).__newindex ~= nil)  -- we can access the field
getmetatable(test).__newindex = function(...)
  print('overruled',...)
end  -- this should be discarded because this form is read-only
test[1]=1  -- should invoke the standard print, not 'overruled'
assert(not pcall(setmetatable,test,nil))  -- setting metatable should error