Re: Best way to find out to what userdata is on the stack?

[Graham Wakefield <lists@...>]

Another log in the fire:

I use a C++ binding, in which every bound class has a static const  
char * name member (I also use the char * for reflection purposes).   
The bound class metatable has the const char * (stored as a  
lightuserdata, not a string for efficiency), as a key, and true as a  
value.  This means that I can support multiple inheritance by pushing  
each superclass name to the metatable.

Potentially I could store something other than true as the value,  
perhaps something more useful, but so far I haven't found the need.

The code looks something like this (not complete, but you get the idea):

template <class T>
bool isUsertype(lua_State * L, int idx)
{
	if (lua_isuserdata(L, idx) && lua_getmetatable(L, idx))
	{
		lua_pushlightuserdata(L, (void *)T::name);	// lightuserdata lookup  
seems to be a lot faster than string lookup
		lua_rawget(L, -2);
		if (!lua_isnil(L, -1)) {
			lua_pop(L, 2);
			return true;
		} else {
			printf("doesInherit error: %s\n", T::name);
		}
		lua_pop(L, 1);
	}
	return false;
}

// add an inheritance key to a metatable (assumes metatable is on the  
stack)
template <class T>
void inherit(lua_State * L)
{
	lua_pushlightuserdata(L, (void *)T::name);
	lua_pushboolean(L, true);
	lua_settable(L, -3);
}



// example

class Foo
{
	static const char * name;
};

const char * Foo :: name = "Foo";


// sometime later:

printf("stack top is Foo ? %i\n", isUsertype<Foo>(L, -1) );

On Mar 13, 2007, at 5:41 AM, Mike Pall wrote:

> Hi,
>
> Lothar Scholz wrote:
> > At the moment i'm using an hash table for all my own userdata,
> > lookup the pointer there and if found i get a verification that this
> > userdata is really a "Foobar" struct.
>
> No hash lookups required if you do it right:
>
>   http://lua-users.org/lists/lua-l/2004-07/msg00369.html
>
> Bye,
>      Mike