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How to Expose C++ Classes to Lua in Ultra Engine


Josh

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I'm using the excellent sol2 library to interface C++ and Lua in the upcoming Turbo Game Engine. I've decided not to create an automatic header parser like I did for tolua++ in Leadwerks 4, for the following reasons:

  • There are a lot of different options and special cases that would probably make a header parser a very involved task with me continually discovering new cases I have to account for.
  • sol2 is really easy to use.

Each class I want available to Lua will have a static function the Lua virtual machine code can call when a new Lua state is created. The new_usertype method is able to expose a C++ class to Lua in a single command. At a minimum, the name of the class and the base class should be defined. This method can accept a lot of arguments, so I am going to break it up over several lines.

void Vec3::InitializeClass(sol::state* luastate)
{
	//Class
	luastate->new_usertype<Vec3>
	(			
		//Name
		"Vec3",

		//Hierarchy
		sol::base_classes, sol::bases<Object>()
	);
}

We can export members to Lua very easily just by adding more arguments in the call to new_usertype:

//Members
"x", &Vec3::x,
"y", &Vec3::y,
"z", &Vec3::z,

Metamethods are special operations like math operands. For example, adding these arguments into the method call will set all the metamethods we want to use.

//Metamethods
sol::meta_function::to_string, &Vec3::ToString,
sol::meta_function::index, [](Vec3& v, const int index) { if (index < 0 or index > 2) return 0.0f; return v[index]; },
sol::meta_function::new_index, [](Vec3& v, const int index, double x) { if (index < 0 or index > 2) return; v[index] = x; },
sol::meta_function::equal_to, &Vec3::operator==,
sol::meta_function::less_than, &Vec3::operator<,
sol::meta_function::subtraction, sol::resolve<Vec3(const Vec3&)>(&Vec3::operator-),
sol::meta_function::addition, sol::resolve<Vec3(const Vec3&)>(&Vec3::operator+),
sol::meta_function::division, sol::resolve<Vec3(const Vec3&)>(&Vec3::operator/),
sol::meta_function::multiplication, sol::resolve<Vec3(const Vec3&)>(&Vec3::operator*),
sol::meta_function::unary_minus, sol::resolve<Vec3()>(&Vec3::operator-),
sol::meta_function::modulus, &Vec3::operator%,	

And then finally the class methods we want to use can be exposed as follows:

//Methods
"Length", &Vec3::Length,
"Cross", &Vec3::Cross,
"Normalize", &Vec3::Normalize,
"Inverse", &Vec3::Inverse,
"Distance", &Vec3::DistanceToPoint

In C++ you can not retrieve a pointer to a function, so we are going to create a quick Lambda expression and expose it as follows:

//Constructor
luastate->set_function("Vec3", [](float x, float y, float z) {return Vec3(x, y, z); } );

This allows us to create a Vec3 object in Lua the same way we would with a constructor in C++.

Here is the complete Vec3 class initialization code, which makes Lua recognize the class, exposes the members, adds math operations, and exposes class methods:

void Vec3::InitializeClass(sol::state* luastate)
{
	//Class
	luastate->new_usertype<Vec3>
	(			
		//Name
		"Vec3",

		//Hierarchy
		sol::base_classes, sol::bases<Object>(),

		//Members
		"x", &Vec3::x,
		"y", &Vec3::y,
		"z", &Vec3::z,
		"r", &Vec3::x,
		"g", &Vec3::y,
		"b", &Vec3::z,

		//Metamethods
		sol::meta_function::to_string, &Vec3::ToString,
		sol::meta_function::index, [](Vec3& v, const int index) { if (index < 0 or index > 2) return 0.0f; return v[index]; },
		sol::meta_function::new_index, [](Vec3& v, const int index, double x) { if (index < 0 or index > 2) return; v[index] = x; },
		sol::meta_function::equal_to, &Vec3::operator==,
		sol::meta_function::less_than, &Vec3::operator<,
		sol::meta_function::subtraction, sol::resolve<Vec3(const Vec3&)>(&Vec3::operator-),
		sol::meta_function::addition, sol::resolve<Vec3(const Vec3&)>(&Vec3::operator+),
		sol::meta_function::division, sol::resolve<Vec3(const Vec3&)>(&Vec3::operator/),
		sol::meta_function::multiplication, sol::resolve<Vec3(const Vec3&)>(&Vec3::operator*),
		sol::meta_function::unary_minus, sol::resolve<Vec3()>(&Vec3::operator-),
		sol::meta_function::modulus, &Vec3::operator%,			
			
		//Methods
		"Length", &Vec3::Length,
		"Cross", &Vec3::Cross,
		"Normalize", &Vec3::Normalize,
		"Inverse", &Vec3::Inverse,
		"Distance", &Vec3::DistanceToPoint
	);

	//Constructor
	luastate->set_function("Vec3", [](float x, float y, float z) {return Vec3(x, y, z); } );
}

To add your own C++ classes to Lua in Turbo, you will create a similar function as above and call it at the start of the program.

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If you want to expose multiple versions of the same function you can do it like this:

sol::meta_function::subtraction, sol::overload( sol::resolve<Vec3(const Vec3&)>(&Vec3::operator-), sol::resolve<Vec3(const float)>(&Vec3::operator-) ),

 

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sol doesn't do default function arguments, so you have to create an overload with a Lambda expression. The World::Update function has the following syntax:

void World::Update(const int ups = 30)

This exposes the function but requires a single argument be supplied:

L->new_usertype<World>
(
	"World",
	sol::base_classes, sol::bases<SharedObject, Object>(),
	"Update", &World::Update
);

We will use sol::overload and a Lambda expression to add a version of the function that does not require any arguments.

L->new_usertype<World>
(
	"World",
	sol::base_classes, sol::bases<SharedObject, Object>(),
	"Update", sol::overload( &World::Update, [](shared_ptr<World> world) { world->Update(); } )
);

It seems kind of complicated, but that is literally ALL you have to do to add new classes. There are no funny headers or other files to deal with. That's it.

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This script code now works:

--Create a window
local window = CreateWindow("Game",0,0,1024,768,WINDOW_TITLEBAR)

--Create a rendering context
local context = CreateContext(window)

--Create the world
local world = CreateWorld()

--Create a camera fpr rendering
local camera = CreateCamera(world)
camera:SetPosition(0,0,-3)

--Create a box
local box = CreateBox(world)

for n=1,#box.lods do
    local lod = box.lods[n]
    for k=1,#lod.surfaces do
        local material = lod.surfaces[k].material
        if material ~= nil then
            local texture = material.textures[1]
        end
    end
end

while window:KeyHit(KEY_ESCAPE)==false and window:Closed()==false do

    --Make the box spin
    box:Turn(0,1,0)    

    --Update physics and other stuff
    world:Update()

    --Send a frame to the rendering thread
    world:Render(context)

end

 

  • Upvote 1
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As much as I can.

We also have getters/setters:

L->new_usertype<Entity>
(
	"Entity",
	"position", sol::property(&Entity::GetPosition, &Entity::SetPosition)
);

This will allow Lua code like this:

local box = CreateBox(world)
box.position = Vec3(10,0,0)

And it will actually call the SetPosition() functions, which updates the entity matrix, triggers changes in surrounding shadows, etc., instead of just simply setting a variable, which would not work.

https://sol2.readthedocs.io/en/latest/api/property.html

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Yes. They have to be because they have additional parameters for global / local and you need that option available.

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I just realized that with this system, any object could have a script attached to it now and it would work just fine.

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