Monday project uses Lua

Submitted by Trezker on Fri, 10/03/2008 - 13:05.

The Monday project has been in development for some time now and it's going rather well in my subjective opinion. Despite having a lead programmer that plays Regnum Online and does other things all the time. It doesn't develop fast, but it never stands still either.

Today I wish to write down my experience with integrating Lua in a C++ project. This is something that I have tried a few times before and I always failed. But this time I broke through the difficulties and so far it all works beautifully.

One thing I can't recall having any trouble with is calling C++ functions from Lua. This was very simple to achieve. Though I did some unsafe things which I will explain later.

The first problem I ran in to was that I didn't seem to be able to call functions that resided in my scripts. It took me a whole day of frustrating manual reading and googling to finally find out that you had to execute a script in order to make it work.

This was because a function in a Lua script has to be assigned to a handle, and this doesn't happen until you execute the script. So if you don't execute, you will be calling handles that have not yet been assigned a function and therefore still just evaluate to nothing and Lua says "lua attempt to call a nil value". This is apparently a frequent problem among new Lua users, I think it should be added to a FAQ somewhere...

Alright, first problem solved and I got communication going between C++ and Lua. Though I could see that a number of very similar sequences of function calls had to be made everytime C++ needed to call a function in lua. So I made a Lua_wrapper class with a few simple functions.

First a function Call(function_handle, reference) which calls the given function with the caller's id as the only argument. I figured this would be a popular thing to do.

Second, a function Prepare(function_handle, reference) which instead of calling the function returns the lua_State* so you can add more arguments. This function is completed with another function Call(args) that takes how many extra arguments you've added.

Ok, so communication both ways is done and works well. It's now also easy to add more functions that use Lua. But I had a safety issue so I will explain that now. In order to use object instances from Lua I chose to give out raw pointers for the scripts to use as references. This can go horribly wrong as the script can pass any value and thus make awful stuff happen. So I needed to make sure the pointers I get are valid.

First, I had added the Game pointer as a global in the Lua state, so i wanted to get rid of that. I did this by adding a static map in Lua_wrapper that maps a lua_State* to a Game*. Then a function that returns the Game* belonging to the state. Very simple.

Then I had to make sure Entity pointers are valid. So I added a function Assert_entity(entity) in the Entity_manager class that simply makes sure the entity is valid. However it does that is irrelevant, as long as it works and is fast enough. Right now it jsut does a std::find in its own list. As of this moment I see several flaws in this which will break the game completely if they are left unattended... But at least it ensures that you can't do anything to entities that don't exist.

Well, that's as much as I'm writing about the subject this time. I could go into detail about some things in another post or so but that'd be hard work.

Trezker's blog
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Monday project uses Lua