Reading and writing C struct from embedded lua

Question

I'd like to embed lua to allow scripting in my C++ application. In particular, I have two structs which I'd like to pass as arguments to a given lua function. One will be read-only, the other will be read/write. Highly simplified examples of these structs follow:

struct inData
{
    int x;
    int y;
    //many other fields follow
};

struct outData
{
    int a;
    double b;
    //other fields follow
};

Both of these structs are created in the C++ code and will be processed there both before and after calling the lua functions. How can I pass these structs to a lua function such that the function can do things like this:

if(inData.x > 5) then outData.a = 1 end

and have the outData instance actually retain the changes after returning from the lua function?

Answer 1

You can use a userdata and put a outdata* in it. You can set custom functions on them, including a metatable. I'm not going to go through it because it's SO MUCH boilerplate/glue code. Basically, you make the outdata on the heap, you make the light userdata point to it (via an incredibly evil cast), then you set a custom metatable and use __index to make the data accesses happen with a custom function.

Basically, it is doable but boring as shit. Also dangerous because Lua doesn't understand any C (or C++) types and you're dealing with void*s all the time.

Answer 2

boring + boilerplate + multi-language = SWIG.

http://www.swig.org/Doc1.3/Lua.html#Lua_nn13

Answer 3

You could also create a Lua table that reflects this same data structure for passing information between C and Lua.