Lua Syntax In C++

A Lua identifier is a name used to identify a variable, function, or any other user-defined item. An identifier starts with a letter ‘A to Z’ or ‘a to z’ or an underscore ‘’ followed by zero or more letters, underscores, and digits (0 to 9). Function object is a Lua object, which can represent either a Lua function or a C function. We can verify that the interesting entry is a function by checking that L-baseci5.func-tt equals 6 (LUATFUNCTION); after that we’ll check the type of function with L-baseci5.func-value.gc-cl.c.isC.

C++ With Lua
Lua Syntax In C++ 10
Lua Syntax Checker
Lua Syntax In C++ 2
Lua Syntax Check

wiki

Lua can be embedded and extended[1] with code or applications written in other languages. Code and values in another language can be exposed to Lua and vice-versa. The following lists low and high level solutions for binding between Lua and other languages.

Lua C API

The most direct method to bind to Lua is with the Lua C API. The C API consists of two parts: the basic API (lua.h) provides the primitive functions for all interactions between C and Lua, while the auxiliary library (lauxlib.h) provides higher-level functions for some common tasks.[2]

[The Lua 5.1 Reference Manual: The Application Program Interface] (5.1) describes Lua's native C API for calling Lua from C and vice-versa.

See also Lua book [PIL2]. Or read about the C API online in the 1st edition of PIL: [PIL1 - The C API]

An introductory video on embedding Lua in C++: [Embedding Lua in C++ #1]

A tutorial for embedding Lua in C: [Embedding Lua in C]

Enabling API checking

By default, the Lua C API does almost no sanity checking of arguments passed to it. Passing incorrect stack indexes, for example, can result in segfaults or random data corruption. You should always enable API checking in any debug build. You can do this by compiling with the option -DLUA_USE_APICHECK. luaconf.h uses this C macro to define luai_apicheck to call assert() in various places (you can also edit this definition to do something possibly more useful).

Example

Some examples of using the C API can be found by examining the source code of Lua's own standard libraries (src/*lib.c}. For example, the math library (math.*) is implemented in the file src/lmathlib.c. The basic form of this file is outlined below. First we import various headers, including the C API (lua.h) and the auxiliary library (lauxlib.h):

Then various Lua functions implemented in C are defined. These all have the same signature, and arguments are passed via Lua's own stack. For example, the sin function is defined as follows. luaL_check_number() is used to check the correct type of the sin function argument. lua_pushnumber() is used to return the sine calculated. Note, the return value of the math_sin() function is the number of values returned (lua functions can return many values).

These functions are registered into Lua by building a table of function pointers and names and then calling luaL_register(). Constants pi and huge are set separately. This registration code is placed in a function named luaopen_math(), which can be called statically (e.g. from linit.c) or dynamically (via Lua's shared library loading mechanism via require).

Binding C/C++ with Lua

C

[LuaAutoC] (5.2) - Automatically wrap C functions and structs at runtime.
[LuaNativeObjects] (5.1) - A C bindings generator written in Lua. It turns C structs into objects.
[luapi] (5.0) - a C API over the official Lua API.
[CaLua] (5.0) - A way to bind C functions and structures to Lua, and work with C pointers, arrays and functions in Lua. (uses x86 assembly)

C Foreign Function Interfaces (FFI)

[Alien] (5.1) - a foreign function interface (FFI) for Lua. An [FFI] lets Lua code call C functions directly without having to write C 'glue', so you can use Alien to write C extensions purely in Lua. (Wraps libffi)
[C/Invoke for Lua] (5.1) - Use [C/Invoke] from Lua to call C libraries (DLLs, .so files) directly, like Microsoft's P/Invoke and Python's ctypes. (Similar to Alien)
[The LuaJIT FFI] allows calling external C functions and using C data structures from Lua code. It parses plain C declarations and supports C99 plus some GCC/MSVC/C++ extensions.
[luaffi] (5.1) - an implementation of LuaJIT FFI for Lua.
[cffi-lua] (5.1, LuaJIT, 5.2, 5.3, 5.4) - Mostly compatible with the LuaJIT FFI (but integrates with new Lua versions), but portable (libffi based) and implemented from scratch.

C Inline

[lua-tcc] (5.1) - Simple interface to TCC, a fast runtime C compiler by Fabrice Bellard, it allows a Lua script to compile C code and register it at runtime as Lua-callable C funcions. Intentionally limited to avoid TCC's bug with multi-environments, that would be too tempting in Lua.
[Luatcc] (5.1) - another, more complete, Lua binding for libtcc, which is the core library of the [Tiny C Compiler]. It allows compiling and loading C code directly from Lua, includes the ability to load C modules directly from C source.
InlineCee provides a similar approach, invoking other compilers out-of-process.

C++

Various C++ or C++ template bindings have been developed to simplify the process in C++:

[MiniLua (5.3, 5.4)] - Minimal Lua interface for loading a lua-file and getting values into your C++-application.
[Lua-Adapter] (5.3, 5.4) - Use this lightweight wrapper-/adapter-class as a simple 'interface' between Lua and C++.
[CppLua] (5.0 & 5.1) - a C++ wrapper of the Lua API; handles class member functions.
[LuaCppInterface] (5.2) - a C++ wrapper of the Lua API. Uses TMP to make calling/passing functions, handling coroutines and filling tables easy and type-safe.
[sol2] (5.1, LuaJIT, 5.2 & 5.3) - a fast, powerful and easy C++14 wrapper for the API including support for table indexing, user-defined types, metamethods, coroutines and more.
[sol] (5.2) - a C++11 easy to use and type safe wrapper of the Lua API.
[Diluculum] (5.1) - A library that intends to make the coexistence of C++ and Lua more harmonious.
[Tomaka17's lua wrapper] (5.2) - Simple bindings for Lua using C++11 supporting class and function registration, custom member functions, multiple return values, tables and more.
[Luabind] (5.1 & [5.2]) - a template-based binding of C++ classes and functions which uses the Boost library. The library seems to be abandoned by the [original authors], but some more or less actively maintained [forks] exist, e.g. [3] or [4].
[LuaBridge] (5.1 & 5.2) - lightweight, dependency-free, template-based library for exporting C++ classes/functions to Lua environments.
[LuaBridge3] (5.1 & 5.2 & 5.3 & 5.4) - lightweight, dependency-free, template-based library for exporting C++ classes/functions to Lua environments, next generation with more feature and fixes.
[SLB] (5.2) - Simple Lua Binder, a cross platform, small, easy to integrate, template-based library. Very similar to Luabind or boost-python but SLB doesn't require boost or any other dependency.
[Luna] (4.0), LunaWrapper (5.1) and LunaFive (5.2) - clean, template-based method of binding a C++ class to Lua. See also [LTN5] and SimplerCppBinding.
LunaWrap (5.2 & 5.3) - clean, template based way of pushing arbitrary types to and getting them from Lua, as well as a COMPILE-TIME 'proxy'-function ('int foo(lua_State*)') generator for C++ member functions!
[MLuaBind] (5.1) - a template-based binding of C++ classes and functions which uses the Loki library.
[MultiScript] (5.1) - a simple library that demonstrates Lua independent C++ interface to bind C++ classes and functions.
[OOLua] (5.1, 5.2 & 5.3) - Cross platform template generator binding which has no dependencies.
[Selene] (5.1, 5.2, 5.3) - Dead simple and intuitive C++11 bindings supporting class registration and functions.
[Sweet Lua] (5.1) - a template-based binding of C++ classes and functions. (MSVC)
[lux] - emits functions at compile time via C++ templates and argument overloading
[nlua] - 'namespace lua' binding of C/C++ functions, table-like usage, templated based (no boost)
[LuaWrapper] - A single header library that provides type-safe and intuitive functions such as luaW_to<T> and luaW_push<T> for arbitrary types, to make managing C++ classes with Lua simple.
[Lutok] - Lightweight C++ API for Lua. Lutok provides provides thin C++ wrappers around the Lua C API to ease the interaction between C++ and Lua. These wrappers make intensive use of RAII to prevent resource leakage, expose C++-friendly data types, report errors by means of exceptions and ensure that the Lua stack is always left untouched in the face of errors. The library also provides a small subset of miscellaneous utility functions built on top of the wrappers.
[integral] (5.1, LuaJIT, 5.2, 5.3, 5.4) - C++ library with no dependencies for creating Lua bindings.
[lua-intf] (5.1, 5.2 & 5.3) - Pure C++11 API and binding for Lua (headers only), supports lua stack API, a higher level API that can refer lua object just like a C++ class object, and can also export C++ class and functions for lua script. The export binding supports custom C++ type, multiple return values, C++ shared pointer, in/out arguments, and can also make argument optional or with default value. The new version also comes with an optional Qt data type mapping (QString, QVariant, QByteArray, etc...).
[LuaState] (5.1 & 5.2) - C++11 template library (clang, gcc and VS 12) no boost. Made to be really easy to use and as fast as possible. Lambda based bindings from Lua to C++.
[Kaguya] (5.1 , 5.2 , 5.3) - C++03 with boost or C++11 template library.
[Lua-Primer] (5.2, 5.3, Eris) - C++11 template library. Not only creates bindings to lua, but does it in a way that supports easy, seamless serialization using Lua Eris. See also [github] descriptions of eris.
[Ponder] (5.3) Ponder is a C++ reflection API that can expose the API declared to Lua. Easy to use.
[Automagical Bindings] Using C++11 templates to deduce C function arguments to automagically create wrappers to expose native C functions to lua code.

See also:

DoItYourselfCppBinding - provides a few simple code fragments for helping Lua and C++ work together
LuaGenPlusPlus - luagen++, a C++ header file to generate efficient sequences of Lua C API calls at compile-time via template metaprogramming techniques
CppStreamChunkReader - lua_load chunk reader for any C++ std::istream (function)
[A comparison of LuaBind, SWIG, OOLua, and toLua++] - One developer's attempt to find a good binding framework for use in a memory-constrained environment.
[C++ Binding Benchmarks] - performance benchmarks for various C++:Lua bindings. [Github benchmark project here]

Calling Lua from C/C++

These frameworks and articles are one-way bindings: to call Lua functions from C/C++ passing arguments and getting return values.

[A Generic Call Function] (5.0) - from PiL uses a format string to call a global function with a variable number of arguments.
[LuaGenericCall] (5.1) - uses code snippets and printf/scanf like format strings to support many types and features.
[LuaClassBasedCall] (5.1) - uses code snippets along with C++ constructor overloads and templates, supports nearly all C++ types.
[lua_icxx] (5.1) - embeds a Lua interpreter in a C++ application, using an OO API. See also SimpleLuaIcxxExample.
SimpleLuaApiExample - example using the C API: the C program loads/runs a Lua script file, sets Lua variables and retrieves return value.
CppLuaDataPassing - another more advanced example using regular C API to run a Lua script file with arguments.
GettingValuesFromLua - explanation of how to get return values from lua_dostring.
CallingLuaFromCpp - example to use the Luna binding (see above) to call Lua from C++.

Embedding Lua in C++

C++ With Lua

These frameworks are a little broader than the previous ones: they allow your C++ program full interaction with a Lua interpreter, but leave the task of extending Lua to other binding systems (SWIG, tolua++, and others).

[lua_icxx]: (pronounced 'lua-ix') Lua interpreter for C++; call Lua functions and User Data (class) methods, use tables, create function sandboxes, use multiple return values, evaluate expressions and scripts, and more. The missing link for use of bindings like SWIG and tolua++.

Automatic binding generators

[toLua] (5.2) - If there is a lot of information to bind then automating the process using a binding generator can help. toLua is one such tool. A package file is prepared, which is a cleaned up version of the C/C++ interface. Another advantage of this technique is that less work may be required between Lua versions if the Lua C API changes significantly.
[tolua++] (5.1) - an extended version of tolua, with some extra features oriented to c++. See also CompilingToluappWithoutScons (compiling tolua++ without SCons).

[CPB] (5.0) - A simple, fast, and powerful binding library for the Lua scripting language. It takes the information from the compilers symbols and generates binding code directly from it. (MSVC)

[SWIG] (5.0/5.1) - Simplified Wrapper and Interface Generator, it can generate bindings between your C/C++ code and a variety of scripting languages (Python, Perl, Tcl/Tk, Ruby, ...), including Lua. A big advantage might be that *one* interface file, similar to that used in tolua/tolua++, is used for bindings to all languages supported by SWIG. See also the SWIG docs pertaining to Lua [5].

[luna-gen] - lua binding-code generator that supports features such as properties, inheritance (both from lua and c++), namespace, enum, type-checking, c++ exception handling, adopt-policy, g++ and MSVS, operator overloading, user-configurable custum string and number/enum types and so on. Fast compilation and execution speed (much faster than luabind and SWIG), human-readable output codes, no boost dependency.

[dub] - Uses Doxygen to parse C++ headers. Supports features such as properties, inheritance, type casting, Lua wrapping, enum, exception handling, operators, custom type binding, C++ template resolution, C++ callbacks with error handling, overloaded methods with runtime type resolution, etc. The generated code is optimized to be as fast as possible (return value optimization, inline objects, etc).

[Ponder] - After declaring an API using Ponder it can be automatically exposed to Lua. Ponder is a C++ reflection system that also supports Lua binding, and XML/JSON serialisation.

Proxy wrapper for DLL

Function information can be exported from dynamically linked libraries. [FuBi] describes a method to call these functions from script (or RPC). This functionality can be used from Lua [6].

Miscellaneous

[wxScript] (5.0) - A set of abstract classes to add script-interpreter support to your wxWidgets applications/libraries.

Other languages

Ada

[Lua-Ada] (5.0) - Lua binding to Ada 95. (link broken)
[lua-ada] (5.1) - Ada bindings to the Lua language (public domain) (link broken)
[AdaCore ada-lua] (5.2) - Ada bindings to the Lua Language with generic packages to ease binding of Ada types to Lua

Bash Shell

The luabash tool is a dynamically loadable module for the bash shell that provides a bridge between the bash shell and the lua scripting language. This enables complex embedded code segments to be replaced with efficient lua code and the speed of shell scripts to be increased without the drastic need to rewrite existing code.

Basic

[FreeBASIC] - A open-source BASIC compiler that comes with the Lua headers and libraries.
[PBLua] (5.0.2) - a Purebasic library wrapping Lua 5.0.2. (deprecated)
[PowerBLua] (5.0) - PowerBASIC include & source for wrapping Lua (work in progress).
[BlitzMax] (5.1.2) - a BlitzMax module wrapping Lua 5.1.2 (note: the link currently points into this wiki (which is usually a bad idea in this list) but this will change as soon as the package has its own web site)

COBOL

[OpenCOBOL] (5.1) - Using Lua with OpenCOBOL 1.1
[OCLua] - Generated documentation from the OpenCOBOL interface source code

D

[LuaD] (5.1) - Lua bindings and high-level interfaces for the D programming language.
[DLua] (5.1) - Lua bindings for the D programming language.

Erlang

[erlua] (5.1) - aims to enable seamless interoperability between Erlang and Lua.
[erl-lua] (5.1) - an Erlang linked-in driver that allows embedding Lua into the Erlang VM.
[erluna] (5.1) - Lua bindings for Erlang.

Fortran

[Aotus] (5.3) - aims to enable seamless usage of Lua scripts for configuration of Fortran applications, uses the ISO-C-Binding.
[f2k3-lua] - Interaction with Lua via the Fortran 2003 ISO-C-Binding.

Go

[golua] (5.1) - Lua bindings for Go.
[luar] (5.1) - a reflection layer on top of golua API providing a simplified way to publish go functions to a Lua VM..
[go-lua-test](5.1) - Examples using various Lua bindings for Go

Haskell

[HsLua] (5.1) - Scripting.Lua binding for Haskell.

Java

[LuaJava] (5.1) - allows scripts written in Lua to manipulate components developed in Java (JNI to native Lua) and vice-versa.
[JNLua] (5.2, 5.1) - another Java JNI <-> Lua bridge. Includes JSR 223 (Scripting for the Java Platform) provider.
[jna-lua] - Java JNA <-> native Lua bridge.
[java-lua] - Java JNI Lua bridge. Calls Lua from Java as well as Java callbacks from Lua

LabVIEW

[Lua for LabVIEW] (5.0, 5.1) - embedding Lua into National Instruments LabVIEW software.

Objective-C

[LuaObjCBridge] (5.1) - calling Lua from Objective-C and vice-versa (MacOS X Cocoa).
[LuaCore] (5.1) - a LuaObjCBridge-based framework, makes it easy to setup and run Lua scripts.
[LuaCocoa] (5.1) - spiritual successor to LuaObjCBridge/LuaCore?, bridge between Lua & Obj-C, and uses Apple's BridgeSupport? to fill in remaining non-ObjC areas (e.g. structs, C functions, inline functions) to access all APIs on the platform. Also includes a command line tool suitable for writing AppleScript? in Lua via ScriptingBridge?. (Mac OS X).
[iPhone Wax] (5.1) - a bridge between Lua and all the Objective-C/CocoaTouch classes.
[Objective Lua] (5.1) - brings Objective-C-like syntax to Lua.
[TLC] (LuaJIT 2) - A lightweight (Under 400 lines, pure Lua) bridge that exposes Objective-C objects&methods to Lua.

OCaml

[ocaml-lua] (5.1.x) - calling Lua from OCaml and vice-versa (tested only in Linux, should work with OS X).

Pascal

Lua Syntax In C++ 10

[VerySimple.Lua] (5.3) - Delphi XE5-XE7 Lua interface with OOP callback functions for Win32, Win64, Mac OS X, iOS and Android.
[Freepascal bindings] (5.1 to 5.3.4) - Free Pascal bindings for the Lua library.
[Pascal bindings] (5.1 to 5.3work1) - Pascal bindings for the Lua library, both for Delphi and Freepascal.
[LuaPascal] (5.1) - Integrates Lua and Object Pascal languages. Allows Lua to manipulate components developed in Pascal/Delphi.
[Lua4Delphi] (5.1) - Delphi and VCL Lua interface and Util functions.
LuaDelphi (5.1) - calling Lua from Delphi/Pascal.
[Lua4Delphi] (5.1) - for Delphi 2010 and XE.
[THlua] (5.0) - Lua distribution for Delphi/Kylix/FreePascal. Includes header files, samples, some utils. For Windows, there are also LuaBinaries and project files for Visual Studio 6. Last updated in 2003.
[Delphi-Lua] (5.0) - calling Lua from Delphi/Pascal.
[DLua] (5.0) - calling Lua from Delphi/Pascal.
[Free Pascal] Pascal units lua, lualib, lauxlib corresponding closely to the usual C API and axiliary library. See LuaInFreePascal.

Perl

[LuaPerl] (5.1) - calling Perl (5.8 and 5.10 streams) from Lua.
[Inline::Lua] (5.1) - calling Lua from Perl (inline).
[Data::Lua] (5.1) - parse variables out of Lua code. Relies on Inline::Lua.
[re::engine::Lua] (5.1) - supplies the Lua regular expression dialect for Perl.

PHP

[LuaPhpExtension] (5.1) - PHP extension which allows embedding the Lua interpreter in a PHP application.
[Nutria] (5.1) - PHP Standard Library written in Lua.

Python

LunaticPython[7] (5.1) - Lua/Python two-way bridge. Allows embedding of Lua interpreter in Python programs, and embedding of Python interpreter in Lua programs.
[Lupa] (LuaJIT2)(5.1, 5.2, ?5.3) - Embedded Lua in Python. Allows two-way communication between Python and Lua code and tries to adhere to common behaviour on both sides.
ffilupa [8] (5.2, 5.3) - A modern two-way bridge between Python and Lua. Integrate Lua into Python using CFFI as backend, which runs fast on both CPython and PyPy?. Inspired by Lupa.

R

RClient [9] (LuaJIT) - LuaJIT client for Rserve which is used to host a (local or remote) R session toward which a connection is established. A library that allows to execute arbitrary R programs within LuaJIT.
Lupa (python packaged mentioned above) + reticulate (package for and installed from CRAN within R: https://rstudio.github.io/reticulate/) - workaround that allows one to access Lua language and objects while in R session by passing through python session as an intermediary set up by the Reticulate. Best to review examples in both LunaticPython and Lupa to understand how to set up and use this workaround.

Ruby

[RubyLuaBridge] (5.1) - calling Lua from Ruby (calling Ruby from Lua is planned)
[rub-lua5] (5.0) - calling Lua from Ruby.

Tcl

[ltcltk] (5.1) - a binding of Tcl and Tk for Lua.
[LuaTclBridge] (5.0) - a Lua<->Tcl bridge.
[TcLux] (4.0) - Tcl extension which embeds Lua into Tcl.
See also [Lua] on Tcl.tk wiki.

Terra

[Terra] is a new low-level system programming language that is designed to interoperate seamlessly with the Lua programming language.

Like C, Terra is a simple, statically-typed, compiled language with manual memory management. But unlike C, it is designed from the beginning to interoperate with Lua. Terra functions are first-class Lua values created using the terra keyword. When needed they are JIT-compiled to machine code.

You can use Terra and Lua as:

A scripting-language with high-performance extensions. While the performance of Lua and other dynamic languages is always getting better, a low-level of abstraction gives you predictable control of performance when you need it.

A JIT compiler for your software's DSLs. Meta-program Terra using Lua. allows you to compile domain-specific languages (DSLs) written in Lua into high-performance Terra code. Embedding Terra-Lua programs in other software as a library allows you to add a JIT-compiler into your existing software.

A stand-alone low-level language. Terra was designed so that it can run independently from Lua. In fact, if your final program doesn’t need Lua, you can save Terra code into a .o file or executable. In this use-case, Lua serves as a powerful meta-programming language. You can think of it as a replacement for C++ template metaprogramming with better syntax and nicer properties.

Frameworks

CORBA

[OiL] (5.1) - ORB in Lua ([CORBA])

Windows COM

LuaCom - (5.1/5.0) Lua interface to Microsoft's Component Object Model (COM). Calling COM objects from Lua and implementing COM objects in Lua.

Windows .NET

[dotLua] (5.0) - A .NET/CLI library, wrapping the Lua API.
LuaInterface[10] (5.1) - a library for integration between the Lua language and Microsoft .NET platform's Common Language Runtime (CLR). Lua scripts can use it to instantiate CLR objects, access properties, call methods, and even handle events with Lua functions.
[LuaNETInterface][11] (5.1) - a modification of LuaInterface to provide definition of Lua functions using attributes and loaded into the Lua engine using reflection.
[NLua] (5.2) - a fork of LuaInterface which is active as of 2014
[Eluant] (5.1) - a library with a similar purpose to LuaInterface/NLua, but fixing the memory management problems with LuaInterface/NLua. Active as of 2015.
[LunaRoad] - Lua C API (v5.1, 5.2, 5.3, ...) for .NET / represents a flexible platform to work with Lua. Provides Lua interfaces, Lambda expressions, Dynamic features that can be used to generate new API-Layer automatically at runtime, etc. Active as of 2016.

Firefox

[Moonshine] - embedded Lua environment for the Firefox web browser. Allows Firefox extensions to be developed in Lua rather than JavaScript. Implemented as an extension (XPI), written in XUL, JavaScript, and C++.

XML-RPC / SOAP

See LuaXml.

Once you have the latest lua source, it's time to build it. (Yes, you could just download header files and the .dll, but what's the fun in that? When available, it's always better to have the source, that way you can see how things are really working -- and you can debug more easily). Using visual studio:

Open up Visual Studio, and create a new project

Create a win 32 console application
In the Project Creation wizard, check the 'static library' radio button to build as a static library.

Add the lua source to the project. Just dragging the files directly from their folders into the project works well for this.
There are two source files that you don't need: lua.c and luac.c. These files contain the main program for the command line interpreter and the bytecode compiler, respectively. You can remove this files from the project if you wish. If you are building as a static library, having more than one main won't be a problem. However, if you are building as an executable, then you will need to exclude one of these files from the project. Right-click on them in the project explorer and select 'Exclude from Project'

You now have a project that you can add to any solution that needs lua. Once you have added the lua project to your game, you need to do a few more steps:

In the properties for the project that is using lua, be sure that the directory that contains the lua .h files is included in the set of include paths. If at all possible, make these paths relative, and not absolute
Your project will need to get at the .lib files for the lua. There are a few ways to do this:
- Add a reference from your project to the lua project. Right-click on your project in the project explorer, and select References:
  Add a new reference:
  Select the lua project:
- Or, add Lua.lib to 'Additional Dependencies' (in the Librarian/Genrerl tab of the Project Property Pages) and add the directory where Lua.lib lives to 'Additional Library Directories' (also in the Librarian/Genrerl tab of the Project Property Pages).

Once we have build Lua, and set up our project dependencies correctly, we are ready to use in on our application,

First off, we need to include the proper header files. Since Lua is ANSI C, if we are coding in C++, we will need to enclose the #includes in extern 'C':

Everything that we need to mantain the current state of the interprer (all global tables, etc) is stored in a variable of type lua_State. The first thing we need to do is create an initial state (essentially, an instantiation of the lua interpreter), which is done by the call:

Lua Syntax Checker

Every time we want to access the interpreter, we need to pass in this state variable. We can actually have as many instances of Lua state variables as we like (for instance, if we wanted to run separate instances of the interpreter on separate processors, or we wanted different scripts to have different global namespaces), though typically people just use one instance of the interpreter that all scripts use. The global tables in this state variable contain all 'core' functions, but none of the libraries. The libraries contain a bunch of important functions -- including everything in math, and i/o functions like print -- so we'd like to load them. We can load libraries with the call:

So, now we have the skeleton of a main function:

We are ready to start using lua!

We will start with the simplest way to use lua -- have the interpreter execute a file. This is functionally equivalent to the dofile command from within lua (and unsurprisingly, has the same name!). To execute the file 'test.lua', we can use the call:

Note that if we are using relative paths other than absolute paths under windows, then the system will look in the current working directory -- which is the directory that the executable file is in (if you double-click on the executable), or the directory that the project is in (if you run from within Visual Studio) You can, of course, change the working directory that the debugger uses under the project settings in Visual Studio.

So, if we use the following myFile.lua:

when the command luaL_dofile(L, 'myFile.lua') is executed, the following will be printed out

Note that the dofile command not only computes these values and prints them out, it also adds the fib function to the global namespace of our lua enviornment (stored in the C variable L).

We can also call lua functions directly from C/C++, and get back the return values to use in our C/C++ code. To call a lua function we:

Push the function on the top of the lua stack, using a call to

lua_getGlobal(lua_state *L, char *globalName)

Push the arguments to the function on the lua stack, using the functions:

lua_pushnumber(lua_state *L, float number)
lua_pushstring(lua_state *L, char *str)
lua_pushboolean(lua_state *L, bool)
lua_pushuserdata(lua_state *L, void *userdata)
lua_pushnil(lua_state *L)

Call the function, using the function:

lua_call(lua_state *L, int numArguments, int numReturnValues)
We need to pass in both the number of arguments that we are passing in, and the number of arguments that we expect in return. Lua is very loose about the number of arguments and return value, but C/C++ is less so -- hence we need to be explicit when calling lua from C/C++.

Extract the return values, using the functions:

int lua_tointeger(lua_state *L, int stackLocation)
Grab an integer value off the lua stack. The first return value is at index -1, the second return value is at index -2, and so on.
double lua_tonumber(lua_state *L, int stackLocation)
Grab a double value off the lua stack. The first return value is at index -1, the second return value is at index -2, and so on.
const char *lua_tolstring(lua_state *L, int stackLocation)
Grab a string value of the lua stack
int lua_toboolean(lua_state *L, int stackLocation)
Grab a boolean value off the lua stack
Note that these functions 'peek' at the lua stack, they don't actually remove any values from the stack. (That is done by the next function)

Pop the return value(s) off the top of the sack (this is just to clean up) with a call to

lua_pop(lua_state *L, int numReturnValsToPop);

Let's take a look at how we might call the fib function defined in the previous section to find the 13th Fibonacci number:

Let's look at a second example. Assume that we had defined the following lua function add (that we could define by calling lua_dofile):

We could call this function to add from C/C++ with the code:

So now you could write scripts that you can call from your game engine. However, these scripts won't be very useful if they can't interact with your game objects -- you need a way for your scripts to affect the game world. To do that, you need to be able to call C/C++ functions from lua. Since Lua and C++ have very different conventions for how functions are called, there is a little bit of wrapper work that needs to be done. First, write a C function that takes as input a Lua state variable (the parameters for the function will need to be extracted by hand from the lua stack -- this is slightly tedious, but not too difficult.) Then, the function will need to be registered with lua.

Step 1: Writing a C function that Lua can call

C/C++ functions that are called from lua need to take as an input parameter the lua state. The parameters can be examined on the call stack using the functions lua_tointeger, lua_tonumber, etc (described above). The first parameter is at index 1, the second parameter is at index 2, and so on. Once we have extracted the parameters, we do our calculation, and then push the result on the top of the stack.

Let's look at a slightly more complicated C function. We can write C functions that takes a variable number of parameters, and returns more than one return value. While the previous function assumed that we were passed in two parameters, we can instead query the lua state to see how many parameters were actually passed into the function. The number of parameters is stored on the top of the stack, which can be accessed by a call to lua_gettop(lua_state *L). Let's look at a function that takes in multiple parameters, and calculates the sum and average of all parameters that were passed in:

Step 2: Registering the C function for lua

Once we have written the function, we just need to register it with lua. That is, we need to add the name of the function to the global lua namespace, and provide a pointer to the function so that lua cal access it. There is a helpful macro for this: lua_register(lua_state *L, const char *name, functionPointer fn). So, to register the above two functions:

Step 3: Calling the C funcion from lua

This part is easy -- once the function is registered, lua can call it like any other function.

So, the complete round trip is:

Start the lua interpreter
Register the C functions you want lua to use
Call a lua function from within C, either by a call to luaL_dofile, or by calling a lua function directly
The lua function that you called from C can access the C function

We can also send a string straight to the lua interpreter, and it will be executed just as if that string was in a file that was executed with a dofile. So, we could do something like:

and we would get the output:

Lua Syntax In C++ 2

We could thus create a braindead interpreter as follows:

Note that this would not work at all in a game environment! We will look at how to embed a command-line lua interpreter within a game next time. For now, this is enough for us to play around with a bit.

Now we are ready to get our fingers dirty!

Lua Syntax Check

Download the following project, which contains a basic skeleton lua framework
Write a function in C (exposed to Lua, much like average and cAdd) that takes as input an integer n, and returns all prime numbers less than or equal to n.

Test your primes function in the interpreter with 'print(primes(100))'

Write a function in lua nthPrime, that takes as input a number n, and returns the nth prime number. So, nthPrime(6) should return 13. We'll define nthPrime(1) to be 2, and nthPrime(0) will be undefined. Your lua function should call the c primes function
- Small change in how lua does variable numbers of arguments: From the notes
  Alas, this does not work. But we can fix it with a simple change:
Write C code that computes the 100th prime number by calling the lua nthPrime function, and prints the value out using printf. So, C calling Lua that calls C.