Re: PiL2, ch 28.3: Adding Lua methods to an array?

[Carsten Fuchs <CarstenFuchs@...>]

Hi Daniel, hi all,

the approach that I mentioned in my initial post actually works!

I just set the metatable of the "a" table to the same metatable that was 
originally only intended to be used with the userdata.
That is, the metatable of "a" and the metatable of "a.userdata" are the 
same, namely the one that was originally only intended for the userdata 
as described in chapter 28.3.

Daniel Collins wrote:
> The approach I took to extending the functionality of userdata with lua
> code is quite simple, but not totally transparent. I just aggregate the
> userdata into a table. This is similar to the approach you mention,
> except I didn't try to make it work like inheritance via metatables
> since I cant see how it can work.

Sounds like our approaches are very similar, but I did not explicitly 
think about inheritance issues so far...

> -- create a userdata object
> local ud = system.createUD()
>
> -- create the enclosing table
> local wrapper = { userdata = ud }
> setmetatable(wrapper, getmetatable(ud))

Yes, looks very much like my approach, too, except for the fact that I 
do all this in the C++ code.

> -- Now, if ud has a method "methodOne", the following call succeeds
> since methodOne is found in the metatable of wrapper
> wrapper:methodOne()

Yes!   :-)

> but the first parameter to the C function that implements methodOne is
> not the userdata, but the table. So the function fails.

Well, I just rewrote my C functions to expect the "a" or "wrapper" table 
as the first argument, and then look up the userdata field in this table 
in order to get what used to be in the first argument of the C function.

> It would work for userdata functions that don't expect the userdata as
> the first parameter, but that is not the usual situation. The only way I
> can get the above code to work is with:
>
> wrapper.methodOne(wrapper.userdata)
>
> Which defeats the entire purpose of setting the userdata metatable into
> the table in the first place.

See above - just look up the userdata field in the C function.

> I am still a Lua newb though, so there may well be tried and tested ways
> of extending userdata with additional lua functions. If there are, then
> I am really interested in using them.

The only other problem that I encountered with the approach outlined so 
far was that I had trouble with calling the "a:OnMouseClick()" or 
"wrapper:someMethod()" functions from C code, because

    lua_getglobal(LuaState, "a");
    // if (!lua_istable(LuaState, -1)) ...
    lua_pushstring(LuaState, "OnMouseEnter");
    lua_gettable(LuaState, -2);

yielded a message saying "PANIC: unprotected error in call to Lua API 
(loop in gettable)" and my program was hard aborted. I don't understand 
exactly why it loops (infinite recursion related to metatables?) here, 
but using lua_rawget() instead of lua_gettable() solved the problem.

(If somebody knows why lua_rawget() works while lua_gettable() doesn't, 
please let me know!)

Best regards,
Carsten



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Carsten Fuchs    http://www.Ca3D-Engine.de/c_Carsten.php