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On 4-Nov-04, at 1:15 PM, Daniel Silverstone wrote:

  local _gmt = getmetatable
  local _smt = setmetatable
  function getmetatable(foo)
    if type(foo) == "userdata" then
      error("Cannot fetch a metatable from a userdata")
    return _gmt(foo)
  function setmetatable(foo,mt)
    if type(foo) == "userdata" then
      error("Cannot set a metatable on a userdata")
    return _smt(foo,mt)

Oh yeah? Watch this:

function type(foo) return "Hey, I'm an object" end
meta = getmetatable(someUserdata)


Of course, in real life I would change getmetatable in the startup C code. It is not necessary to change setmetatable, by the way, but it is interesting to note that the Lua basic library prevents Lua code from setting a userdata's metatable, but not from getting a userdata's metatable.

However, if you cannot get a userdata's metatable, setting the metatable is not actually dangerous, in the sense that it might disable the object's functionality, but it cannot be used to forge a self argument to a metamethod. So there is actually an argument for allowing setmetatable and forbidding getmetatable.

I know there are ways of doing it (I use the closure technique a lot); but it is worth thinking about whether there are ways that the language can just guarantee it for you. Maybe not. Maybe so.