Embedding Lua in the Source Engine
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Note:External files listed here are now gone, Consider viewing Adding Lua instead.What is Lua?
Lua is a powerful, fast, lightweight, embeddable scripting language.
Lua combines simple procedural syntax with powerful data description constructs based on associative arrays and extensible semantics. Lua is dynamically typed, runs by interpreting bytecode for a register-based virtual machine, and has automatic memory management with incremental garbage collection, making it ideal for configuration, scripting, and rapid prototyping.
Find out more about Lua on here.
Overview
Lua is currently the leading scripting language in games, and for Steam users, is synonymous with Garry's Mod. Unfortunately, Garry's Mod isn't open source. However, 
Half-Life 2: Sandbox is currently the leading open source project dedicated to bringing the developer community a method for embedding Lua into their mod nearly seamlessly, and this article will explain how.
Half-Life 2: Sandbox uses a pure variant of Lua, different than Facepunch's implementation of the language, which has restrictions on many libraries, and modified syntax for C++ styled comments. Half-Life 2: Sandbox goal is to provide a neutral, clean, standardized and well documented code base for interfacing with Lua, unlike articles seen within the Valve Developer Community such as the LuaBind oriented Lua tutorial or Adding Lua article.
Implementation
Files
Todo: Is it necessary to link the lua library file? Or can't people just build it?
Installation
The Lua manager scripts found in the Half-Life 2: Sandbox source code were written generically so they could be added to any mod.
- While working with a mod's source code, rather than a plugin, the files above should be placed within src/game/shared
- In the project files, place both the luamanager.cpp and luamanager.h in the Source Files filter
- Compile the binaries
- Simply add the precompiled lua51.dll to the mod's bin folder!
Note:Half-Life 2: Sandbox has went through several revisions extending its Lua API greatly including many classes and libraries for real-world usage with Source. As a result of these expansions, to be able to use these classes, mod creator should need to add the individual class .cpp and .h files, which are all listed in luamanager.cpp by headers that correspond to .cpp files.