Re: Problem with Lua in c++ and stl

[Lenny Palozzi <domain@...>]

I think I figured this out...
I needed to call an objects method that I instantiated in the host
program from Lua. The class method is required to access private member
data. 

This is what I did:

I defined in my class a static method. Which is
bound/exported to Lua with toLua. In my Lua code I can call
class_name:static_method(). Static members though cannot access private
data, or non-static data. So I made my static method and constructor
similar to Mike's examples below. My static method is callable by Lua, it
now acts as a binding function which calls a non-static method which
accesses the private data.
 
Is there a better way to do what I need?

Thanks for the code samples Mike!

Lenny.

On Wed, 1 Sep 1999, Mike Cuddy wrote:

> 
> > BTW, I think LUA is great, I just wish I can get this to work. Thanks.
> > 
> > My app dumps core somewhere within the stl code. Here is the backtrace:
> > #0  GameMap::AddTileBitmap (this=0x0, image=0x805f608)
> >     at /usr/local/include/g++/stl_vector.h:145
> > 145         if (finish != end_of_storage) {
> > (gdb) bt
> > #0  GameMap::AddTileBitmap (this=0x0, image=0x805f608)
> >     at /usr/local/include/g++/stl_vector.h:145
> 
> Here's your clue:
> > #1  0x8049b83 in GameMap::AddTile (this=0x0) at game_map.cpp:56
>                                     ^^^^^^^^^^
> You cast the member function GameMap::AddTile to function you registered
> with lua.  lua has no way to know it's a "thiscall" (i.e.: attached to
> an object) so it just calls it like a standard C function (which it isn't).
> 
> What I generally do, is make my tables have a "ptr" member, which I 
> set to point to the C++ object I'm working with.  Then in the LUA->C
> binding function, I fetch that member from the table passed to the C
> function and dereference it to call the object member function.
> 
> lua_Object getField(lua_Object t, char *fld)
> {
>     lua_pushobject(t);
>     lua_pushstring(fld);
>     return lua_gettable();
> }
> 
> // push a userdata (void*) object..
> void setField(lua_Object t, char *fld, void *data, int tag = 0)
> {
>     lua_pushobject(t);
>     lua_pushstring(fld);
>     lua_pushusertag(data,type);
>     lus_settable();
> }
> 
> // push a cfunction object..
> void setField(lua_Object t, char *fld, void (*data)())
`> {
>     lua_pushobject(t);
>     lua_pushstring(fld);
>     lua_pushcfunction(data);
>     lus_settable();
> }
> 
> // functions to bind to LUA; make sure they are C callable.
> // (in general, a C++ function with no args has the same linkage as a
> // C function, but that's implementation dependent, so avoid link-time
> // errors, by declaring the function as a C function).
> extern "C" { 
>     static void lua_myClass_AddTile();
> }
> 
> 
> // class which is usable from lua.  each member function that is to be
> // called, should be added as a field ; we might want to create a tag,
> // etc.  We could also make a "luaObject" baseclass which would do 
> // all of the common objects-accessable-to-lua initialization.  
> class myClass {
>     void AddTile();
> 
>     lua_Object t;
>     int ref;
> 
>     // constructor ..
>     myClass() {
> 	t = lua_createTable();	// create new lua object
> 	ref = lua_ref(1);	// lock it in memory.
> 
> 	// add member function
> 	setField(t,"AddTile", lua_myClass_AddTile);
> 
> 	// add pointer to 'this' for lua linkage
> 	setField(t,"ptr", (void*)this);
>     }
> 
>     ~myClass() {
> 	if (ref) lua_unref(ref);
> 	// ... etc ..
>     }
> }
> 
> // it's a pity that cfunction's in lua don't get to carry userdat, then we
> // could pass the object "implicitly" to the C -> C++ binding function.
> // instead of digging it out of a table.
> static void lua_myClass_AddTile() {
>     lua_Object table = luaL_tablearg(1);	// get table from lua
>     MyClass *self = (MyClass*) lua_getuserdata( getField(t,"ptr"));
> 
>     self->addTile();
> }
> 
> 
> --
> Mike Cuddy (mcuddy@FensEnde.com, MC312), Programmer, Daddy, Human.
> Fen's Ende Software, Redwood City, CA, USA, Earth, Sol System, Milky Way.
> I remember asking why ... Let it rain, and protect us from this Cruel Sun.
> 
>        Join CAUCE: The Coalition Against Unsolicited Commercial E-mail.  
>                           <http://www.cauce.org/>
>  
>