Type-testing with protected metatables

[Mark Hamburg <mhamburg@...>]

If I have Lua code implementing a type via a table and I choose to protect
the metatable by defining the __metatable entry, is there a good way to have
the code that defines the type do tests on objects to see if they match the
type?

My solution was to define a C routine: checkmetatable( obj, meta ) that
returns true if and only if the metatable for obj is raw-equal to meta. This
bypasses the protection applied to metatables with respect to Lua.

If this is the way to do it, then I would suggest that this or something
like it be added to the standard library. It doesn't seem like it would
violate security to ask whether an object has a metatable that we already
know about via other means.

Mark

/* Not compiled since I stripped out some stuff special to my environment...
*/

int checkmetatable( lua_State* L ) {
    lua_settop( L, 2 ); /* Error checking for sufficient parameters? */
    lua_pushboolean( L,
        lua_getmetatable( L, 1 )  && lua_rawequal( L, -1, 2 ) );
    return 1;
}