Scripting language with class instance support

I have come across the need to use a scripting engine for my C++ game, but after experimenting with many languages since the last few days, nothing has truly stood out as the obvious choice for a language and/or binding library.

I would like have the ability to

1. call methods of a specific Game instance from within the scripting language,


2. call arbitrary functions in the scripts from C++, and


3. test for function existence in the scripts using strings.

Lua looks nice, but I'm unsure of its ability to do 1 and 3. Python and Boost::Python are great, but the same applies. Regardless, here's an example of an ideal implementation.

C++

class Game
{

    ...
    void printNumber(int arg);
};

int main( )
{
    Game *game = new Game();

    // Make method available to script
    functionRegister("printNumber", game->printNumber);


    // Test and call function in script
    if ( functionExists("greet") )
        functionCall("greet", "Hello world");
}

Script pseudocode

function greet( greeting )
    print( greeting )
    game.printNumber( 42 )

end

The output may be

Hello world
42

Is there a language and library that will pull this off?

Answer

I'm fairly sure Lua can do everything you need relatively simply. I use Lua and C++ in my game. I looked at various wrappers like LuaBind, or using a generator like Swig, but I decided I didn't want any of that stuff and I wrote my own wrapper which I ended up making open source in case other people found it useful.

Using my little library you can do stuff like this in C++:

// Wrap a C++ function
static int Widget_AddChild(lua_State* L)
{
    // Typesafe way to extract userdata from Lua
    Widget* parent = luaW_check(L, 1);
    Widget* child = luaW_check(L, 2);
    lua_pushboolean(L, parent->AddChild(child));
}

static luaL_reg Widget_Metatable[] =

{
    // Register the function you wrote above
    { "AddChild", Widget_AddChild },

    // Widgets also have some getter and setter functions, 
    // Using these templates you can automatically generate 
    // wrapper functions for them
    { "GetStyle", luaU_get },
    { "SetStyle", luaU_set },
    { "Style", luaU_getset },


    { NULL, NULL }
};

int luaopen_Widget(lua_State* L)
{
    luaW_register(L, "Widget", NULL, Widget_Metatable);
    return 1;
}

In lua you could now do something like this:

local w = Widget.new()
w.foo = 10 -- Foo is stored on w, it's not accessible to other instances
-- You can also add values accessible to all Widgets
function Widget.metatable:newfunction() 
    print(self:GetWidth()) 
end
w:newfunction()

With all that, you can fairly easily call C++ function from Lua (and vice versa, but I didn't illustrate that here). Once you know how to interact with the Lua API, writing a function to 'safely' call (i.e. call a function only if it exists) a Lua function should be easy as well.

Edit:

If you want to check for the existence of a function before calling it, like I said, just check that the value is a function before calling it.

lua_getglobal(L, "myFunc"); // You can get your function from anywhere, this is just for example
if (lua_isfunction(L, -1)) // Check the top of the stack, make sure it's a function
{
    lua_call(L, 0, 0);
}