RE: Lunar, C++, and premature object destruction

["Sean Riley" <sean@...>]

Another possible strategy is to store an ID rather than a pointer in the
userdata object. That way you can lookup the ID to see if the object
still exists, instead of dereferencing a pointer that may have been
free'ed.

> -----Original Message-----
> From: lua-bounces@bazar2.conectiva.com.br [mailto:lua-
> bounces@bazar2.conectiva.com.br] On Behalf Of Watusimoto
> Sent: Monday, July 06, 2009 10:06 AM
> To: Lua list
> Subject: Re: Lunar, C++, and premature object destruction
> 
> Sean,
> 
> Thanks for your response.  I may have found a solution just last
night.
> You are right that with Luna, the C++ code is unaware of any
references
> that Lua has to an object, so when C++ is done with it, it gets
deleted,
> creating the problem I reported.  My solution was to rearchitect my
> Ship/LuaShip relationship, such that there is no longer a one-to-one
> relationship.  When Lua wants a LuaShip, I create a new one, and let
Lua
> control its lifespan.  I used a reference-counting pointer on LuaShip
to
> refer back to the Ship, so that if the Ship is killed and deleted, the
> LuaShip pointer to it is set to NULL, and I can detect and handle that
> situation when dealing with the LuaShip object.  That let me remove
the
> pointer from Ship to LuaShip that led to the problems in my previous
> design.
> 
> The only drawback to this solution is that I end up creating a lot of
> LuaShip objects (the Lua script may ask for several such references
each
> game cycle), whereas in my previous scheme, there was only one for
each
> Ship.  But it does work -- Lua can hold or delete the LuaShip as it
> likes, and C++ doesn't care; it will just create another when it needs
> one.
> 
> The only lingering problem I'm having is that while mucking around
with
> this, I found that some objects aren't being fully destroyed, leading
to
> a memory leak, but I think this is purely a C++ problem, and not
related
> to any of the above.
> 
> Chris
> 
> 
> 
> Sean Riley wrote:
> > Looking briefly at the Lunar code, I don't see anything that tries
to
> > handle the case of the object being deleted from the C++ side.  The
only
> > cleanup appears to be when the object's associated userdata is
garbage
> > collected from the lua side.
> >
> > Maybe you could always delete the ship from the lua code? This would
> > require a wrapper function that calls into lua to nil out the
reference
> > to the shup in script, causing it to be garbage collected.
> >
> >
> >> -----Original Message-----
> >> From: lua-bounces@bazar2.conectiva.com.br [mailto:lua-
> >> bounces@bazar2.conectiva.com.br] On Behalf Of Watusimoto
> >> Sent: Thursday, July 02, 2009 1:04 AM
> >> To: lua@bazar2.conectiva.com.br
> >> Subject: Lunar, C++, and premature object destruction
> >>
> >> I'm working on a game that uses Lua to control robot players,
written
> >>
> > in
> >
> >> C++, and bound with Lunar, and am having problems controlling the
> >> lifespan of objects created in C++ and referenced in Lua.  (It's at
> >> http://bitfighter.org, if you are curious, and the linked wiki
> >>
> > contains
> >
> >> examples of some robot scripts.)
> >>
> >> My primary object is a Ship.  The lifetime of the Ship object is
> >> completely controlled by C++.  I also have a proxy object, called
> >> LuaShip, that contains a pointer to the Ship, and handles all the
> >> requests for information about the Ship from the Lua script.  The
goal
> >> is that when the Ship is destroyed, the pointer on the LuaShip will
be
> >> set to NULL, and any future requests to the LuaShip can be
> >>
> > intelligently
> >
> >> handled (probably by generating an error of some sort).  The
LuaShip
> >>
> > is
> >
> >> exposed to Lua via Lunar if the robot script requests it (using a
> >> getShip() method or somesuch).
> >>
> >> The Ship object also contains a pointer to its LuaShip proxy.  My
> >> problem is that when the Ship object is destroyed, C++ seems to
think
> >> that all pointers to the LuaShip are gone, and it is OK to destroy
the
> >> LuaShip as well.  It does not seem to recognize when a Lua instance
is
> >> also referring to the LuaShip.  The result is that the LuaShip is
> >> destroyed, and the Lua script is now left holding a reference to
> >> nothing.  When the script acts on it (thinking it's still valid),
the
> >> app crashes.
> >>
> >> My questions are 1) Is this an appropriate architecture for
managing
> >>
> > Lua
> >
> >> instances that can have persistent references to C++ objects (Ship
in
> >> this case) that can be deleted without warning, and 2) are there
any
> >> good examples of code that uses Lunar in this manner that I can
> >>
> > examine
> >
> >> to see where my problem might be?
> >>
> >> Thank you,
> >>
> >> Chris
> >>
> >
> >