Re: __methcall

[Rici Lake <lua@...>]

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

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

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.