Automated Lua Binding

Welcome to the fifth post in a series of blog posts about how to implement a custom game engine in C++. As reference I’ll be using my own open source game engine SEL. Please refer to its source code for implementation details not covered in this article. The folder of interest would be LuaInterface.


Introduction

Binding things to Lua is twofold: objects and functions must be able to be sent to and retrieved from Lua. Functions can be either static C or struct/class methods. Objects can be sent “by value” or “by reference”. As you can imagine it is important to be able to unify and simplify the binding process as much as possible to reduce all manual dev-work and upkeep.

Generic C++ Functor

As with many things in a modern C++ game engine it is critical to have a generic C++ functor. Ideally this functor can wrap around class/struct methods (not only static functions). It is also possible have this functor able to refer to a function within Lua as well.

Please see my article and slides on C++ Function Binding for implementation details not covered here.

Prerequisites

This article is on the topic of automatic Lua binding; if you’re unfamiliar with how to bind simple C functions to Lua please do a little research and come back later. The deep end of the pool is actually pretty deep!

I also suggest a working knowledge of C++ templates before trying to implement these sort of features. A working knowledge of Lua is also essential.

Setting the Boundaries

With a scripting language it’s important to clearly define what you want to expose to script. Is the entire game in Lua? Are only specific parts accessible? What are the boundaries. It’s all too easy to get very caught up in what to send, what to implement, what not to do. Having clear boundaries of exactly what you want to do is the best way to start coding.

Passing Objects to Lua

Objects can be passed to Lua by reference and value. A reference would consist of 4 bytes of memory to contain a pointer to some C++ memory. This allows Lua to store a “reference” to an object in C++. Most of the work involved in this type of object binding is in allow Lua to call C++ methods or functions on the pointer its storing.

The benefits of this approach are such that: calling class methods is pretty fast and shouldn’t be a worry; fairly simple to implement as most of the work is finished by creating a generic functor in C++; no hassle or upkeep when wanting to send new types of objects to Lua -each object is just a 4 byte pointer.

Passing by Reference with lightuserdata

There are two ways I’d recommend to pass an object to Lua with: userdata and lightuserdata. A lightuserdata represents a void * in Lua and can hold a reference to an object in C++.

Here’s how one might send and retrieve lightuserdata from Lua:

This method is very fast, simple to implement and has very minimal memory overhead. Additionally lightuserdata can be compared to one another, and are equal if the underlying address is equivalent. However, one cannot attach metatables to lightuserdata and there is no sense of type safety what so ever. A lack of type safety means that if someone passes a lightuserdata into an incorrect C function the host program will likely crash.

With lightuserdata the following code is possible:

This solution will work for one, maybe two people working on a smaller project or minimal amount of code. I can imagine that the lack of type safety will be the biggest issue as time goes on.

Reflection for Type Safety

It is possible to implement type-safety in Lua. However this requires Lua code to be maintaining type information. Lua is a scripting language meaning it ought best be used to script things. Something so integral and common as type-safety might better be implemented in lower-level C++ code.

Implementing type safety on the C++ side has two benefits: efficiency of implementation; type-safety can optionally be compiled away in release mode.

I highly recommend building yourself a simple, custom introspection library in C++. All that is really needed to start is the ability to query a type’s size and name efficiently. Please see my older article on custom Introspection or the game engine SEL for examples on how to implement such a system.

With a simple macro-based registration system one can register and lookup type information via introspection like so:

After this is complete and working (if you don’t have an implementation of introspection yet this is fine, just think of it as a black box) a small generic Variable object ought to be created. Sample code of a functional Variable object is in this post.

A Variable can be used like so:

It is important to note that the Variable itself is not a templated type!

When passing an object to Lua we can send a pointer to a Variable. As long as the Variable exist in memory in C++ the lightuserdata within Lua will point to a valid Variable. Upon retrieval of the Lua object back to C++ a type assertion can be run:

Generic Static Function Binding

Bind C-style static functions in a generic way makes heavy use of custom introspection. The way I was originally taught was to just throw the entire binding function (in C++) at you all at once and let you suffer. Prepare to suffer as I did!

This function isn’t doing the bind, it’s what is bound. Every time a function in C++ is called from Lua, this function is called first.

An upvalue in Lua is akin to static variables in C. Using this we can attach a pointer to a generic functor to a bound C function within Lua. As Lua calls a C function this upvalue is retrieved and eventually used to actually call the C function.

The rest is just a matter of handling variables to/from Lua. In the above example the Variable object contains some helper functions call ToLua and FromLua. The nice thing about my implementation of this within SEL is that no heap memory is used during this entire process! All this code boils down to a very efficient method of generically calling C functions.

I will leave binding C++ methods as an exercise for the reader. By now you ought to have an idea of where to look for example implementation! The idea is to handle type information for the “this pointer” of the method, and pass around an actual “this pointer” to call the method.

Calling Methods from Lua

Lets say you have an implementation that allows Lua code like the following:

A few things need to happen here. The first is that the object in question should only call methods that are actually methods of that specific type of class; one cannot simply bind all C++ methods and place functions in Lua within the global scope. Any object type could call any method type making for a lack of type-safety and dangerous code.

At this point the lightuserdata will have to be upgraded to a full userdata. Full userdata in Lua enjoy benefits such as the ability to set and modify metatables. If you’re not familiar with Lua metatables please do a little research on the topic and come back later.

A full userdata allows us to place a copy of a Variable within Lua memory, instead of just a void *. This means a temporary Variable can be used to call ToLua, instead of requiring that the Variable sent stays valid in C++ for the duration of usage within Lua.

Currently a way to create metatables for all of our C++ types is required. Assuming a linked list of all TypeInfo objects from the introspection system is available:

This loop is just creating metatables given the string names of what each metatable should be called.

After the tables are created the actual C++ methods and functions should be bound. This turns out to be really simple! It is assumed that each function and method registered within the introspection system can be passed to the function at some point (perhaps during registration of the type information):

And that’s really all there is to it! The idea here is to make sure that a type with methods sent to Lua has its userdata fixed with a metatable containing the available methods to call. When the __index metamethod is called it will search within the metatable itself for an appropriate member. Members of the metatable are the functions we bound to Lua. After they are fetched they can be called. This is what happens behind the scenes when we do:

Passing Object by Value

Passing objects by value is actually much more difficult. The idea is to utilize tables to to store representations of the members associated with a class or struct. A table can be used to represent state of an object.

The __index and __newindex metamethods of a userdata should be set to look into the state table first. This lets users assign new values, and lets your ToLua and FromLua functions copy members from C++ to/from this Lua state table.

If a member is not found in the state table the metatable can then be searched by setting the __index metamethod of the state table to refer to the proper metatable.

All of this table indirection does incur significant overhead, however it allows objects in Lua to be used like so:

I myself have not implemented this type of Lua binding, though it is entirely possible and can be quite nice to work with. I reiterate that adding this many tables incurs both memory and performance overhead not seen with the other styles. This seems to be the only drawback.

Conclusion

Well this post turned out longer than I expected -over 2k words! Hopefully the information was clear. It’s really nice being able to refer people to a complete and working example such as the SEL engine; it makes writing articles much easier and simpler.

Hopefully this can help someone out there! As always feel free to ask questions or provide comments right here on this page.

Please see Game Programming Gems 6 ch. 4.2 for more information about binding C++ objects to Lua,

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4 thoughts on “Automated Lua Binding

  1. AnonymousAndy

    This is really good stuff. Personally I’d like to get some scripting for my own project but all that’s out there for lua is some kind of bloated boost monstrosity. I’m looking at AngelScript right now which provides similar macros to what you have done I guess. Though it doesn’t have all the lua fancy stuff like dynamic types and multiple return values and all the other nice things that come with it. Have you considered sharing your work in some kind of library? Maybe since there’s a lot of template code you could make it header only which would be amazing.

    Also with vs2013 its it not possible to use variadic templates to generate a wrapper to anything? I’m no expert on variadic functions though ^^

    cheers

    Reply
  2. Tobias Kammerer

    Hey Randy,

    i am really impressed by your Introspection and LUA System. One question still bugs me and i was unable to locate the missing piece in our source code: How do you manage hot-(re)loading of your LUA scripts?
    I thought about something, but i am not sure if this will work (beware: I am a LUA Noob):
    Create a warpper for the luaState and load all LUA scripts in the beginning (with lual_loadFile). Then i’ll take it from the stack (as a function) and save a reference to it in the LUA registry (with lual_ref). Now i save all those references together with their filenames in a list of structs and monitor changes made to all files in a specific directory (with a OS specific function for example). If a change happens to a file i remove the reference from the registry and execute the whole process from above again, while updating the affected struct with the new reference value.
    Now in each game-update (or how often i like to run the lua scripts) i iterate over this list of structs, get the referenced function (with lua_rawgeti) from the registry, and execute it with lua_pcall.

    With this method i would get as little as possible i/o overhead while still being able to hot-(re)load .lua files.

    Is this feasiable, or even possible? You also only use ONE luaState for all Scripts, aren’t you?

    Thank you for all your effort you put into your blogs and tutorials!

    Reply
    1. Randy Gaul Post author

      Well when you do hotloading this is a dev tool, so worrying about file i/o efficiency is a moot point, since it doesn’t matter as long as the hot-load is quick enough to be useful. Since Lua files are themselves functions, and functions are first-class types in Lua, loading and running a file will re-assign all functions in that file to their names. Does this make sense?

      Reply

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