Re: Using multiple lua_States

[Jamie Webb <j@...>]

On Tue, Aug 03, 2004 at 07:10:37PM -0700, Brenden Rheaume wrote:
> I am beginning to imbed lua into a game project. One of the things
> that I would like to do is, define general
> functions(OnHit,OnDamaged,OnClick) for each object type in the game
> (each enemy, ship type, weapon type, etc), which would be defined in
> a lua script file for each type. What I need to know is how
> expensive would be to use multiple lua_States for each type, loading
> the lua script associated with each type, assumming there are quite
> a few of them (50+).

The memory overhead for each state is a few tens of kilobytes, plus
whatever stuff you have to duplicate in each state in order to provide
your API. The only CPU overhead is whatever duplicate work you have to
do to register your API, etc.

> If it would be really expensive (memory/cpu) to use that many
> lua_States is there another convienent way to acheive the same
> result?

Basically, what you are wanting is to do object-oriented programming
in Lua, i.e. to define a class for each object type with method
implementations for each of your events. There's a fair amount of
information about OOP in general on the lua-users.org wiki.

In this instance, a simple approach is create some tables with names
like BadGuy, MachineGun, whatever. Then your scripts contain code
like this:

function BadGuy:OnDamaged() ... end

function MachineGun:OnClick() ... end

etc.

You just run all the scripts one after the other in the same state.
Now to find the right event handler for a given type, you just look it
up in the right table.

Of course, that approach isn't very 'safe', in that it doesn't enforce
where each event is defined, but it could well be adequate. For the
more complex solution, where you do just define a bunch of functions
in each file and they are all properly isolated, but still in only one
state, search the list archives for 'sandboxing'.

-- Jamie Webb