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.

C++ Function Binding

Welcome to the first 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.

I would like to thank John Edwards for his contribution to my education in the areas of reflection and function binding. You can thank him too by checking out their games at thatgamecompany!

Function binding in C++ is the act of being able to trigger a function given some form of input. Usually this applies to C or C++ by means of calling any function in a generic way. This can be achieved easily during compile-time in C++ by using some templates along with decltype. This is useful for:

• Script binding
• Many others

The idea is to capture the pointer to a function (or method) and pass its type around in code as a template parameter. An instance of the template type is created as a template constant. This is possible with the usual compilers as function pointers are of integral type.

The rest of the work involves creating a nice wrapper to pack arguments together and get them to the template constant pointer in a generic fashion.

I’ve created some nice slides on the topic and some demo source code. There is one slide with a video that will not play from the pdf, I attached the video to the bottom of this post. Hope this helps someone out there.

Source code demo is here. If you wish to view a fully featured example, please see my project SEL.

Live Enumeration Editing in C++

Taron Millet, the programmer for Volgarr the Viking, created an interesting enumeration editor for their game editor used in the creation of Volgarr the Viking. This enumeration editor sparked my interest as somehow enumerations could contain within them an enumeration type. This forms a sort of tree hierarchy of enumerations! I actually emailed Taron about the editor, and he threw together a quick demo for me! If you’d like to see the demo just email me and I can send it to you.

Imagine you have an enumeration of types of items, things like breast plates, helmets, boots. Now imagine within each enumeration, lies another enumeration. You can enumerate types of helmets, types of boots and types of breast plates. Now imagine that this tree-like hierarchy is recursive with no depth boundary!

Not only was this enumeration tree really cool, but it also could be live-editted and commit back to C++ code. This is a very interesting idea and can be applied to custom editors for C++ game engines.

I’ve created my own terminal enumeration editor for a proof of concept. Here’s a video demo:

This sort of editor could be implemented in a fully featured editor, perhaps like the one Volgarr the Viking used! This is great for quick changes in gameplay and the like, and can greatly reduce the time required to setup type-safe enumerations. I myself use this editor to also reflect all constructed enumerations within a custom C++ introspection database. This allows all enumeration types to be passed to/from scripting languages, and serialized.

The implementation of such is actually super simple, and a proof of concept can be seen here: https://github.com/RandyGaul/Serialization_C. The idea is to use a single data file full of macro calls. This data file is then intentionally imported into multiple locations. Each time this import occurs different definitions of the macros are defined, thus interpreting the data in various ways upon each import. For more information about this see the link within this paragraph.

Powerful C++ Messaging

A prerequisite to this information is most of the previous C++ type introspection stuff I have been writing about for a while now. Assuming the previous information has been covered, lets move on:

There exists a design of messaging, specifically for C++, of which has minimal downsides and many positive advantages. Ideally messaging should not involve any polling or implicitly required searching (as in searching through game space to see who to message, which requires expensive collision queries). It should also have a very intuitive usage, and not be very complex to work with.

If such a messaging system can be achieved then inter-object communication can be setup, to create game logic, within a scripting language.

Here are some slides I wrote on this topic for my university, but are available for public viewing:

C++ Reflection Part 6: Lua Binding

Binding C/C++ functions to Lua is a tedious, error prone and time consuming task when done by hand. A custom C++ introspection system can aide in the automation of binding any callable C or C++ function or method a breeze. Once such a functor-like object exists the act of binding a function to Lua can look like this, as seen in a CPP file:

The advantage of such a scheme is that only a single CPP file would need to be modified in order to expose new functionality to Lua, allowing for efficient pipe-lining of development cycles.

Another advantage of this powerful functor is that communication and game logic can be quickly be created in a script, loaded from a text file, or even setup through a visual editor. Here is a quick example of what might be possible with a good scripting language:

In the above example a simple enemy is supposed to follow some target object. If the target is close enough then the enemy damages it. If the target dies, the enemy flashes a bright color and then acquires a new target.

The key here is the message subscription within the initialization routine. During run-time objects can subscribe to know about messages emitted by any other object!

So by now hopefully one would have seen enough explanation of function binding to understand how powerful it is. I’ve written some slides on the topic available in PDF format here (do note that these slides were originally made for a lecture at my university):

Templates and Metaprogramming

Recently I’ve begun to take over a club called Game Engine Architecture Club at DigiPen. The founder of the club graduated and asked me to try running it. I’ve found it be to a lot of fun! The first lecture I did was on template metaprogramming in C++ (TMP).

The lecture was recorded live and is currently up on DigiPen’s youtube channel for public viewing. Though the slides are visible during the video, I don’t think I can share the slides themselves publicly.

Hope you enjoy the video!

C++ Reflection: Type MetaData: Part 3 – Improvements

In our last article we learned how to store information about a classes’s members, however there are a couple key improvements that need to be brought to the MetaData system before moving on.

The first issue is with our RemQual struct. In the previous article we had support for stripping off qualifiers such as *, const or &. We even had support for stripping off an R-value reference. However, the RemQual struct had no support for a pointer to a pointer. It is weird that RemQual would behave differently than RemQual, and so on. To solve this issue we can cycle down, at compile time, the type through the RemQual struct recursively, until a type arrives at the base RemQual definition. Here’s an example:

As you can see, this differs a bit from our previous implementation. The way it works is by passing in a single type to the RemQual struct via typename T. Then, the templating matches the type provided with one of the overloads and feeds the type back into the RemQual struct with less qualifiers. This acts as some sort of compile-time “recursive” qualifier stripping mechanism; I’m afraid I don’t know what to properly call this technique. This is useful for finding out what the “base type” of any given type.

It should be noted that the example code above does not strip pointer qualifiers off of a type. This is to allow the MetaData system to properly provide MetaData instances of pointer types; which is necessary to reflect pointer meta.

It should be noted that in order to support pointer meta, the RemQual struct will need to be modified so it does not strip off the * qualifier. This actually applies to any qualifier you do not wish to have stripped.

There’s one last “improvement” one could make to the RemQual struct that I’m aware of. I don’t actually consider this an improvement, but more of a feature or decision. There comes a time when the user of a MetaData system may want to write a tidbit of code like the following:

Say the user wants to send a message object from one place to another. Imagine this message object can take three parameters of any type, and the reflection system can help the constructor of the message figure out the types of the data at run-time (how to actually implement features like this will be covered once Variants and RefVariants are introduced). This means that the message can take three parameters of any type and then take them as payload to deliver elsewhere.

However, there’s a subtle problem with the “Message ID” in particular. Param1 and Param2 are assumed to be POD types like float or int, however “Message ID” is a const char * string literal. My understanding of string literals in C++ is that they are of the type: const char[ x ], x being the number of characters in the literal. This poses a problem for our templated MetaCreator, in that every value of x will create a new MetaData instance, as the templating treats each individual value of x as an entire new type. Now how can RemQual handle this? It gets increasingly difficult to actually manage Variants and RefVariant constructors for string literals for reasons explained here, though this will be tackled in a later article.

There are two methods of handling string literals that I am aware of; the first is to make use of some form of a light-weight wrapper. A small wrapper object can contain a const char * data member, and perhaps an unsigned integer to represent the length, and any number of utility functions for common string operations (concat, copy, compare, etc). The use of such a wrapper would look like:

The S would be the class type of the wrapper itself, and the constructor would take a const char *. This would require every place in code that handles a string literal to make use of the S wrapper. This can be quite annoying, but has great performance benefits compared to std::string, especially when some reference counting is used to handle the heap allocated const char * data member holding the string data in order to avoid unnecessary copying. Here’s an example skeleton class for such an S wrapper:

As I mentioned before, I found this to be rather annoying; I want my dev team and myself to be able to freely pass along a string literal anywhere and have MetaData handle the type properly. In order to do this, a very ugly and crazy solution was devised. There’s a need to create a RemQual struct for every [ ] type for all values of x. This isn’t possible. However, it is possible to overload RemQual for a few values of x, at least enough to cover any realistic use of a string literal within C++ code. Observe:

The macro ARRAY_OVERLOAD creates a RemQual overload with a value of x. The __COUNTER__ macro (though not standard) increments by one each time the macro is used. This allows for copy/pasting of the ARRAY_OVERLOAD macro, which will generate a lot of RemQual overloads. I created a file with enough overloads to cover any realistically sized string literal. As an alternative to the __COUNTER__ macro, __LINE__ can be used instead, however I imagine it might be difficult to ensure you have one definition per line without any gaps. As far as I know, __COUNTER__ is supported on both GNU and MSVC++.

Not only will the ARRAY_OVERLOAD cover types of string literals, but it will also cover types with array brackets [ ] of any type passed to RemQual.

The second issue is the ability to properly reflect private data members. There are three solutions to reflecting private data that I am aware of. The first is to try to grant access to the MetaData system by specifying that the MetaCreator of the type in question is a friend class. I never really liked the idea of this solution and haven’t actually tried it for myself, and so I can’t really comment on the idea any further than this.

The next possible solution is to make use of properties. A property is a set of three things: a gettor; a settor; a member. The gettor and settor provide access to the private member stored within the class. The user can then specify gettors and/or settors from the ADD_MEMBER macro. I haven’t implemented this method myself, but would definitely like if I find the time to create such a system. This solution is by far the most elegant of the three choices that I’m presenting. Here’s a link to some information on creating some gettor and settor support for a MetaData system like the one in this article series. This can potentially allow a MetaData system to reflect class definitions that the user does not have source code access to, so long as the external class has gettor and settor definitions that are compatible with the property reflection.

The last solution is arguably more messy, but it’s easier to implement and works perfectly fine. I chose to implement this method in my own project because of how little time it took to set up a working system. Like I said earlier, if I have time I’d like to add property support, though right now I simply have more important things to finish.

The idea of the last solution is to paste a small macro inside of your class definitions. This small macro then pastes some code within the class itself, and this code grants access to any private data member by using the NullCast pointer trick. This means that in order to reflect private data, you must have source code access to the class in question in order to place your macro. Here’s what the new macros might look like, but be warned it gets pretty hectic:

The META_DATA macro is to be placed within a class, it places a couple declarations for NullCast, AddMember and RegisterMetaData. The DEFINE_META macro is modified to provide definitions for the method declarations created by the META_DATA macro. This allows the NullCast to retrieve the type to cast to from the DEFINE_META’s TYPE parameter. Since AddMember method is within the class itself, it can now have proper access to private data within the class. The AddMember definition within the class then forwards the information it gathers to the AddMember function within the MetaCreator.

In order for the DEFINE_META api to remain the same as before, the META_DATA macro creates a RegisterMetaData declaration within the class itself. This allows the ADD_MEMBER macro to not need to user to supply to type of class to operate upon. This might be a little confusing, but imagine trying to refactor the macros above. Is the RegisterMetaData macro even required to be placed into the class itself? Can’t the RegisterMetaData function within the MetaCreator call AddMember on the class type itself? The problem with this is that the ADD_MEMBER macro would require the user to supply the type to the macro like this:

This would be yet another thing the user of the MetaData system would be required to perform, thus cluttering the API. I find that by keeping the system as simple as possible is more beneficial than factoring out the definition of RegisterMetaData from the META_DATA macro.

Here’s an example usage of the new META_DATA and DEFINE_META macros:

The only additional step required here is for the user to remember to place the META_DATA macro within the class definition. The rest of the API remains as intuitive as before.

Here’s a link to a compileable (in VS2010) example showing everything I’ve talked about in the MetaData series thus far. The next article in this series will likely be in creating the Variant type for PODs.

C++ Reflection: Type MetaData: Part 2 – Type Reduction and Members

In the last post we learned the very basics of setting up a reflection system. The whole idea is that the user manually adds types into the system using a single simple macro placed within a cpp file, DEFINE_META.

In this article I’ll talk about type deduction and member reflection, both of which are critical building blocks for everything else.

First up is type deduction. When using the templated MetaCreator class:

Whenever you pass in a const, reference, or pointer qualifier an entire new templated MetaCreator will be constructed by the compiler. This just won’t do, as we don’t want the MetaData of a const int to be different at all from an int, or any other registered type. There’s a simple, yet very quirky, trick that can solve all of our problems. Take a look at this:

I’m actually not familiar with the exact terminology to describe what’s going on here, but I’ll try my best. There’s many template overloads of the first RemQual struct, which acts as the “standard”. The standard is just a single plain type T, without any qualifiers and without pointer or reference type. The rest of the templated overloaded version all contain a single typedef which lets the entire struct be used to reference a single un-qualified type by supplying any of the various overloaded types to the struct’s typename param.

Overloads for the R-value reference must be added as well in order to strip down to the bare type T.

Now that we have our RemQual (remove qualifiers) struct, we can use it within our META macros to refer to MetaData types. Take a look at some example re-writes of the three META macros:

The idea is you feed in the typedef’d type from RemQual into the MetaCreator typename param. This is an example of using macros well; there’s no way to screw up the usage of them, and they are still very clean and easy to debug as there isn’t really any abuse going on. Feel free to ignore specific META macros you wouldn’t actually use. I use all three META_TYPE, META and META_STR. It’s a matter of personal preference on what you actually implement in this respect. It will likely be pretty smart to place whatever API is created into a namespace of it’s own, however.

And that covers type deduction. There are some other ways of achieving the same effect, like partial template specialization as covered here, though I find this much simpler.

Next up is register members of structures or classes with the MetaData system. Before anything continues, lets take a look at an example Member struct. A Member struct is a container of the various bits of information we’d like to store about any member:

This member above is actually almost exactly what implementation I have in my own reflection as it stands while I write this; there’s not a lot needed. You will want a MetaData instance to describe the type of data contained, a name identifier, and an unsigned offset representing the member’s location within the containing object. The offset is exceptionally important for automated serialization, which I’ll likely be covering after this article.

The idea is that a MetaData instance can contain various member objects. These member objects are contained within some sort of container (perhaps std::vector).

In order to add a member we’ll want another another very simple macro. There are two big reasons a macro is efficient in this situation: we can use stringize; there’s absolutely no way for the user to screw it up.

Before showing the macro I’d like to talk about how to retrieve the offset. It’s very simple. Take the number zero, and turn this into a pointer to a type of object (class or struct). After the typecasting, use the -> operator to access one of the members. Lastly, use the & operator to retrieve the address of the member’s location (which will be offset from zero by the -> operator) and typecast this to an unsigned integer. Here’s what this looks like:

This is quite the obtrusive line of code we have here! This is also a good example of a macro used well; it takes a single parameter and applies it to multiple locations. There’s hardly any way for the user of this macro to screw up.

NullCast is a function I’ll show just after this paragraph. All it does is return a pointer to NULL (memory address zero) of some type. Having this type pointer to address zero, we then use the ADD_MEMBER macro to provide the name of a member to access. The member is then accessed, and the & operator provides an address to this member with an offset from zero. This value is then typecasted to an unsigned integer and and passed along to the AddMember function within the macro. The stringize operator is also used to pass a string representation of the member to the AddMember function, as well as a MetaData instance of whatever the type of data the member is.

Now where does this AddMember function actually go? Where is it from? It’s actually placed into a function definition. The function AddMember itself resides within the MetaCreator. This allows the MetaCreator to call the AddMember function of the MetaData instance it holds, which then adds the Member object into the container of Members within the MetaData instance.

Now, the only place that this AddMember function can be called from, building from the previous article, is within the MetaCreator’s constructor. The idea is to use the DEFINE_META macro to also create a definition of either the MetaCreator’s constructor, or a MetaCreator method that is called from the MetaCreator’s constructor. Here’s an example:

As you can see this formation is actually very intuitive; it has C++-like syntax, and it’s very clear what is going on here. A GameObject is being registered in the Meta system, and it has members of ID, active, and components being added to the Meta system. For clarity,  here’s what the GameObject’s actual class definition might look like (assuming component based architecture):

// This boolean should always be true when the object is alive. If this is
// set to false, then the ObjectFactory will clean it up and delete this object
// during its inactive sweep when the ObjectFactory’s update is called.
bool active;
std::vector components;

Now lets check out what the new DEFINE_META macro could look like:

The RegisterMetaData declaration is quite peculiar, as the macro just ends there. What this is doing is setting up the definition of the RegisterMetaData function, so that the ADD_MEMBER macro calls are actually lines of code placed within the definition. The RegisterMetaData function should be called from the MetaCreator's constructor. This allows the user to specify what members to reflect within a MetaData instance of a particular type in a very simple and intuitive way.

Last but not least, lets talk about the NullCast function real quick. It resides within the MetaCreator, as NullCast requires the template's typename MetaType in order to return a pointer to a specific type of data.

And that's that! We can now store information about the members of a class and deduce types from objects in an easily customizeable way.

Here's a link to a demonstration program, compileable in Visual Studio 2010. I'm sure this could compile in GCC with a little bit of house-keeping as well, but I don't really feel like doing this as I need to get to bed! Here's the output of the program. The format is . For the object, members and their offsets are printed:

Now you might notice at some point in time, you cannot reflect private data members! This detail will be covered in a later article. The idea behind it is that you require source code access to the type you want to reflect, and place a small tiny bit of code inside to gain private data access. Either that or make the MetaCreator a friend class (which sounds like a messy solution to me).

And here we have all the basics necessary for automated serialization! We can reflect the names of members, their types, and offsets within an object. This lets the reflection register any type of C++ data within itself.