Re: Working with C++ and Lua on the same datastructure

[Matthew Comi <matthew.comi@...>]

Carsten Fuchs wrote:
> In order words, my question rephrased:
> How can I have a complex C++ datastructure, and work with it *both* in 
> C++ *and* in Lua code??
Hi,

I'm not sure this answers your question in its entirety, but certainly 
should provide you with some ideas.

My approach to exposing a C++ object to Lua was to create a table with 
functions on it that mirror those on the C++ object being exposed.

So, for the example of "WindowT" you may want to create a "LuaWindow" 
table with keys such as setX, setY and whatever else you would like to 
have exposed to Lua. You would want to have a separate table for each 
instance of WindowT. In order to have this table know about your C++ 
object, you can use lightuserdata. For example, assuming that a 
LuaWindow is on the top of the stack and you wish to 'new' it's C++ 
counterpart:

   WindowT* your_window = new WindowT;

   lua_pushstring("object");
   lua_pushlightuserdata(your_window);

Now, when you call setX or setY on that table, the first thing the 
function would do is get the table on the bottom of the stack and cast 
it's "object" key back to a WindowT. Then, supposing that setX took a 
float as a parameter, you would cast the /second/ element on the stack 
from the bottom to a float and forward that result to your C++ object.

   lua_pushstring(L, "object");
   lua_gettable(L, 1);
   WindowT* window = static_cast<WindowT*>(lua_touserdata(L, -1));
   lua_pop(L, 1);

   window->setX(lua_tonumber(L, 2));
   return 0;

As for writing scripts that get triggered by events, perhaps you could 
pass the appropriate LuaWindow to the script? What is unique about your 
problem though, is the hierarchy of WindowT's. This shouldn't cause to 
much of a problem. I've haven't addressed this myself, as all of my 
objects are constructed in Lua. That way, it is perfectly possible for 
me to do this:

   main = LuaWindow:new()
   child = LuaWindow:new()

   main:attachChild(child)

Not sure if you want to do it this way however.

Matt Comi.

Big Bucket Amusement
http://members.iinet.net.au/~mattcomi/mt/bigbucket/
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