RE: set/getuserdata set/gettype tag methods

["Michael Flad" <Michael@...>]

Hi Mike,

your framework looks interesting and i'm sure a take a look
on it, if you release it.

I already have a working implementation, i don't use toLua
because my specification is one binding that works with
different scripting languages so i try to use an as simple
approach as possible, i.e. no extra tool, binding to each
language with the same lines of code, from a clients
point of view of course. Right now if i set/get members from
inside Lua what happens is, that lua calls my tagmethod,
if have to look if it's a known member (use a std::map for
the initial implementation). If not, i map back to lua
using the raw set/get methods so lua needs to do another
lookup. With a set/get tagmethod i could optimize
away my own lookup and make my code smaller and faster.

It's just a suggestion, but i think it would be very
handy to have these additional tag methods.

Kind Regards,
Michael Flad

--
Fantastic Realms Interactive GmbH - Germany


> -----Original Message-----
> From: owner-lua-l@tecgraf.puc-rio.br
> [mailto:owner-lua-l@tecgraf.puc-rio.br]On Behalf Of Michael Cuddy
> Sent: Friday, September 28, 2001 11:54
> To: Multiple recipients of list
> Subject: Re: set/getuserdata set/gettype tag methods
>
>
> > is there any chance to get tag methods for reading
> > and assigning from/to userdata/usertypes?
>
> You could use toLua to do this.  However, this didn't suit my needs
> (too much overhead -- the data is accessed mostly from C++, and I didn't
> want to be asking Lua for the data all the time) so I'm writing
> my own C++ framework (called, unimaginatively "luapp") for exporting
> objects to Lua.  The thing that my framework handles that most of the
> other Lua/C++ frameworks do not is the case where you have classes
> which want to export member objects which themselves are Lua objects:
>
> class Point2d {
>     double x, y;
> }
>
> class Rect2d {
>     Point2d p1, p2;
> }
>
> With my framework you can do (Lua) stuff like:
>
>     r = Rect2d{ p1 = Point{ x=10, y=20 }, p2 = Point{ x=30, y=40 } }
>
> as well as:
>
>     r.p1 = Point{ x=1, y=2 }
>
> (actually, I generally write my constructors for 'simple' objects like
> 'point's and 'color's and 'rect's so that they take a () style
> constructor:
> p = Point(10,20) since nearly everyone knows that means: p =
> Point{x=10,y=20},
> but for more complex objects, the named construction is pretty spiffy ;-)
>
> ... and it doesn't lose any of the objects ..
>
> If you roll your own, watch out for this:
>
>     Rect2d r;
>
>     ...
>
>     // assumes that 'this' has been pulled from Lua stack arg 1 already ..
>     // by a 'thunk' function which handles the gettable() tag
> method callback
>     Rect2d::gettable(lua_State *L)
>     {
> 	const char *fld = luaL_check_string(L,2);
>
> 	if (strcmp(fld,"p1")==0) {
> 	    lua_newuserdatabox(L,&p1,Point::tag);	// XXX -- danger!
> 	} ...
>     }
>
> Watch out for the line marked XXX -- it seems like the obvious thing
> to do, but if you have a GC function for the 'Point' tag, it wil be
> called with the address of p1 -- WHICH IS INSIDE another object...
>
> My framework 'knows' (well, you tell it ;-) about 'member' objects
> and will not free them when they go out of scope.
>
> Another gotcha is that potentially the object which holds 'p1' in
> this case could be GC'ed before the object returned by 'p1'.  This
> would case the pointer inside of the userdata to become 'wild'.
> My framework handles this by holding a locked reference on the
> 'parent' object until all member objects have been gc'ed.
>
> (This stuff is hard-won knowledge .. the interaction of userdata
> and GC is not very clearly documented, and the semantics of user
> data objects changes from 4.0 to 4.1 ;-)
>
> The framework keeps track of it's objects with a C++ STL::map<>,
> and is pretty quick.  Once I get a real application behind it, I'll
> start doing some real benchmarking (it may be faster to keep them
> in a sorted STL::vector<> instead of a STL::map<>).
>
> If anyone is interested in my framework, I'll clean it up and post
> it on my Lua page.
>
> --
> Mike Cuddy (mcuddy@FensEnde.com, MC312), Programmer, Daddy, Human.
> Fen's Ende Software, Redwood City, CA, USA, Earth, Sol System, Milky Way.
>
>        Now I lay me down to sleep / CVS, I pray, my code to keep.
>        If disks crash before I wake: / format, newfs, cvs up, make.
>
>        Join CAUCE: The Coalition Against Unsolicited Commercial E-mail.
>                           <http://www.cauce.org/>
>