Re: partially define a table in C++, and partially in script

[Shea Martin <shea08@...>]

Doug Rogers wrote:
> Shea Martin wrote:
>
> > Sorry to be dense, but I am still not following.
> > lua_pushstring( vm, "Event:log" );
> > lua_pushcclosure( vm, l_event_log, 0 );
> > lua_settable( vm, -3 );
> > lua_pushstring( vm, "Event:name" );
> > lua_pushstring( vm, eventname );
> > lua_settable( vm, -3 );
>
> The code above is the same as saying
>
> mt["Event:log"] = l_event_log
> mt["Event:name"] = event_name
>
> Note that luaL_newmetatable() places the table's name in the registry,
> not in the global table, so 'mt' above is really just the anonymous
> table that's left on the top of the stack.
>
> These are understandable newbie mistakes. Just (re)read the sections of
> PiL that contain C examples - chapters 25 and 26 in particular. Also,
> you may want to (re)read the section on methods (the ':' syntactical
> sugar), chapter 16. Note that ':' only works for function calls.
>
> What you really want is a function with key "log", and a string with key
> "name" to hold the name field. I suspect that what you *really* want is
> the 'name' field to be present in an instance of an event, not in the
> class (table "Event" plus its metatable).
>
> // Untested...
> // Create global table to hold "Event".
> lua_newtable( vm);
>
> // Create metatable.
> luaL_newmetatable( vm, "Event.mt");
> lua_pushstring( vm, "log" );
> lua_pushcclosure( vm, l_event_log, 0 );  // Need a closure?
> lua_settable( vm, -3 );
>
> lua_pushvalue( vm, -2);    // Get copy of table Event to top.
> lua_setfield( vm, -2, "__index");  // mt.__index = Event
>
> lua_setmetatable( vm, -2);  // Attach metatable.
>
> lua_pushcfunction( vm, l_event_new);  // Sample below.
> lua_setfield( vm, -2, "new");
> lua_setglobal( vm, "Event");   // Put main table in module.
>
> Most likely you will also need some sort of mechanism for creating
> objects of that type. I called that 'new' above. In that function you
> will use luaL_getmetatable() to retrieve the metatable by name from the
> registry, then you'll attach that to a new table, possibly like this
> (untested):
>
> int l_event_new(lua_State* L, const char* event_name)
> {
>   lua_newtable(L);                    // local obj = {}
>   luaL_getmetatable(L, "Event.mt");   // setmetatable(obj, mt)
>   lua_setmetatable(L, -2);
>   lua_pushstring(L, event_name);     // obj.name = event_name
>   lua_settable(L, -2, "name");
>   return 1;                           // return obj
> }
>
> -- In Lua:
> ev = Event.new('attack')
> ev:log()
>
> Those are some untested hints. You may want to invest the time (it's not
> too long) to understand what is happening in the Lua-only version of
> this, then see how it is implemented using the C API. It may be that
> 'log' should be a global function rather than an object method.
>
> Good luck!
>
> Doug

I want to pass an Event object to a function called 'start', which is 
defined in the script.  I used your example for defining Event.  Instead 
of registereing l_event_new, I just create the new table on the stack, 
and do a pcall( vm, 1, 1, 0 ).  It seems that everything works except my 
member function 'log'.  Index 'log' is nil, ie, not defined.



void DefinePlayerEvent( lua_State* vm )
{	
	lua_newtable( vm );

	lua_pushstring( vm, "log" );
	lua_pushcclosure( vm, l_event_log, 0 );
	lua_settable( vm, -3 );

	lua_pushvalue( vm, -2 );  //get Event from global space
	lua_setfield( vm, -2, "__index" ); // mt.__index = Event

	lua_setmetatable( vm, -2 ); //attach metatable

	//lua_setglobal( vm, "Event" );
}

void PutPlayerEventOnLuaStack( lua_State* vm, const char* eventname, int 
player_id, float x, float y )
{
	lua_newtable( vm );
	luaL_getmetatable( vm, "Event.mt" );
	lua_setmetatable( vm, -2 );

	
	lua_pushinteger( vm, player_id );
	lua_setfield( vm, -2, "player_id" );

	lua_pushnumber( vm, x );
	lua_setfield( vm, -2, "x" );
	
	lua_pushnumber( vm, y );
	lua_setfield( vm, -2, "y" );

	lua_pushstring( vm, eventname );
	lua_setfield( vm, -2, "name" );
}

Still reading, not sure what I am missing here.

~S