Question

I’m working on a little simulation project which uses Lua to drive the behavior of individual units (ants) and using Luabind to glue the C++ and Lua sides together. Each individual ant (there are different types, derived from the base class Ant) has a Run() function, which calls the appropriate script; the script then carries out whatever actions need to be taken, calling the exposed class functions and possibly free functions. I’ve gotten the Run function (in C++) to successfully execute the matching Run function in the Lua script (which just prints some text at the moment).

void AntQueen::Run()
{
    lua->GetObject("QueenRun")(GetID());
}

lua is just a manager class which retrieves the function from the script. The above is calling the following in a Lua file:

function QueenRun(ID)
    print("The Queen is running!")
    print(ID)
end

And Luabind registration looks like this for the AntQueen class:

void Register(lua_State *luaState)
{
    using namespace luabind;
    module(luaState)
    [
        class_<AntQueen, Ant>("AntQueen")
        .def("Eat", &AntQueen::Eat)
        .def("ExtractLarvae", &AntQueen::ExtractLarvae)
        .def("GetMaxLarvaeProduced", &AntQueen::GetMaxLarvaeProduced)
        .def("GetNumAvailLarvae", &AntQueen::GetNumAvailLarvae)
    ];
}

The way it’s set up now, ants are created, removed, and found through a simple factory/manager. Each ant can be retrieved by calling static Ant* AntFactory::GetAntByID(const int ID) which just finds the ant in a hash map and returns a pointer to the ant. What I’m trying to do is get Lua to be able to do something like the following:

function QueenRun(ID)
    ant = GetAntByID(ID)
    larvae = ant:GetNumAvailLarvae()
    print(larvae)
    ant:Eat()
end

The above is just a made up example, but hopefully it shows what I’m trying to achieve. I don’t want Lua to garbage collect the objects, because they are managed already on the C++ side. While testing everything out, any attempt to do the following:

ant = GetAntByID(ID)

in Lua resulted in abort() being called and the program crashing and burning.

R6010
-abort() has been called

I just seem to be missing something with how everything gets shuttled back and forth (this is my first foray into gluing Lua and C++ together beyond toy programs). I’m pretty sure passing a plain pointer isn’t the way to do it; lightuserdata seems to be what I’m looking for, but it also has a bunch of restrictions.

So to sum up: What is going on here that causes abort to be called and how can I use Luabind/the Lua C API to get a pointer to a C++ object passed to Lua and call member functions on that pointer as if it were an object (without letting Lua garbage collect it)?

Answer 1

The solution to this problem seemed to be tied to the AntFactory class/member functions being static. As soon as I switched from registering and using this:

//C++

static int AntFactory::CreateAnt(foo, bar)
{}

//Lua

factory:CreateAnt(foo, bar)

to an instantiated object and regular member functions like this:

//C++

int AntFactory::CreateAnt(foo, bar)
{}

//Lua

factory:CreateAnt(foo, bar)

everything worked with no problems at all (after also pushing the factory to the global table). I think the reason for this is that trying to call static member functions on a non-instantiated C++ object fails due to Lua (or Luabind?) not being able to have an object to query for calls.