Scripting language with class instance support

Question

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 1

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<Widget>(L, 1);
    Widget* child = luaW_check<Widget>(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<Widget, Style, &Widget::GetStyle> },
    { "SetStyle", luaU_set<Widget, Style, &Widget::SetStyle> },
    { "Style", luaU_getset<Widget, Style, &Widget::GetStyle, &Widget::SetStyle> },

    { NULL, NULL }
};

int luaopen_Widget(lua_State* L)
{
    luaW_register<Widget>(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);
}

Answer 2

Personally, I am a fan of AngelScript.

It does exactly the things you want, it has a close to C++ syntax.

You can easily create different scripting contexts, and for some types of scripts, have functionality x, for others, y. For example if you want to have a game loop in a script, and AI specific functions in other, while hiding the other one's API.

You can call functions by their name, you can export global functions and class members easily. You can define global properties as well.

So you could basically configure your script to know a Game object, which is your actual Game instance. Then call functions exported on it.

Its really straightforward, but if you want I can add little snippets of how it could be done. Though, we re talking about 1-line setups !