Re: Lua addition proposal: Userdata as function environment

[Rici Lake <lua@...>]

On 4-Oct-06, at 3:02 PM, Jerome Vuarand wrote:

> While trying to answer some recent mails on the list I did some
> experiment with environments and noticed that it is not possible to set
> anything else than a table as a function environment.
>
> I think it would be nice to be able to set any value as the 
> environment,
> especially a userdata. The value in question should have __index and
> __newindex metamethods. If it don't variable reads would return nil and
> variable writes would be ignored.
>
> An example of use would be a configuration mechanism similar to one 
> that
> has been discussed recently. Here a C++/Lua hypothetical example:

Why not just use an empty table whose __index and __newindex methods 
use the userdata? In effect, they could be the same __index and 
__newindex metamethods you would use with the userdata.

The following hardly changes your example at all:

(Note: I'm using lightuserdata here, something I would normally never 
do, but you don't seem to be attaching a __gc metamethod to your 
userdata so I'm assuming that you are prepared to manage them as 
resources without assistance from the Lua garbage collector).

So: here we create the proxy environment table and its metatable,
attaching the configuration object as an upvalue to both getter and
setter:

static int pushobject(lua_State* L, Object* object)
{
    // Put object address as a userdata
    lua_pushlightuserdata(L, (void *)object*);
    // Create the environment table
    lua_newtable(L);
    // Create the metatable
    lua_newtable(L);
    lua_pushvalue(L, -3); /* Copy of l.u.d */
    lua_pushcclosure(L, config_setvar, 1);
    lua_setfield(L, -2, "__newindex");
    lua_pushvalue(L, -3); /* Copy of l.u.d */
    lua_pushcclosure(L, config_getchild, 1);
    lua_setfield(L, -2, "__index");
    lua_setmetatable(L, -2);
    return 1;
}

Now, a slight modification to the getter and setter is all it takes:

// Get a child, will be called as __index
static int config_getchild(lua_State* L)
{
    // Small change to next line ********
    Object** pconfig = lua_touserdata(L, lua_upvalueindex(1));
    Object* child = (*pconfig)->getchild(lua_tostring(L, 2));
    if (child)
        return pushobject(L, child);
    else
        return 0;
}

The same change needs to be made to the first line of config_setvar.