Overview. Rationale Division of labour between script and C++ Choice of language(s) Interfacing to C++ Performance, memory

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Lauren Carpenter
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1 SCRIPTING

2 Overview Rationale Division of labour between script and C++ Choice of language(s) Interfacing to C++ Reflection Bindings Serialization Performance, memory

3 Rationale C++ isn't the best choice for all problems Complicated feature set, syntax Low-level, dangerous Lengthy shutdown/compile/link/rerun development process Many game projects use scripting to address some of these problems

4 Advantages of Scripting Expressiveness Expressiveness Scripting languages employ higher level constructs than C++ Reduced iteration time Interpreted, or byte-code compiled on-the-fly Useful features Automatic resource management (garbage collection) No pointers Safe data structures Dynamic typing Accessibility Accessibility Easier for casual programmers to grok than C++ Customizable Write your own language, or change an existing one to suit your needs

5 Problems Additional work to bind the chosen language to C++ These bindings are often non-trivial Standard C++ tools are not script aware Debugger, compiler, profiler Script is a black hole to C++ Multi-language debugging is harder Script code runs slower Harder to keep memory usage in check Having non-programmers write code can be a disaster

6 Division of Labour Script can can be employed with varying levels of commitment Loading only, scripts just wire up C++ at load time Script high level game flow, only one script running Event responders, game updates from C++, but scripts are called in response to exceptional events Entity behaviour scripted. Each entity is running a little script (usually a state machine of some sort). Write the whole game in the script language Not really scripting any more then, is it? Where a team stands on this depends on the type of game they are writing, the programming ability of the scripters, and how much performance they are willing to trade for the benefits of scripting

7 Concurrency It is often convenient to model an entities behaviour/brain/state machine as an independent thread of execution Scripts provide a convenient mechanism to support this Often built in language support (e.g. Lua co-routines, UnrealScript latent functions ) Script threads are co-operatively scheduled, so there are fewer issues compared to pre-emptive OS-level threads No locking primitives, race conditions, etc. Better performance Cheap context switch

8 Choice of Languages Lua Lua Widely used in the game industry Designed for embedding Byte-code interpreted Dynamically typed Very flexible tables (associative arrays) as the fundamental data structure Simple, clean syntax Garbage collection Not natively object-oriented, but can fake it effectively Good performance (for a scripting language)

9 Choice of Languages Java / C# Basically, C++ with pointers removed, garbage collection added Statically typed Very rich standard libraries Object oriented Object reflection, serialization Properties (C#) Good performance Some safety benefit over C++, but not much gain in expressiveness Generally these languages are too close to C++ to offer huge benefits as a scripting language A decent choice as an alternative to C++

10 Choice of Languages Perl / Python / Ruby All have been used in games at one time or another Very different syntax, but similar feature sets Rich standard libraries Dynamically typed Byte-code interpreted Object oriented to one degree or another Garbage collection Generally pretty slow Large memory footprint All have ambitions to be full-blown application languages Fairly hostile to embedding in C++ Optimised for text processing, which isn't a big concern for games

11 Choice of Languages Lisp, Scheme, etc. Very expressive syntax Not terribly readable, though Dynamically typed Compiled, or interpreted Easy to write an interpreter for Garbage collection Decent performance (if you have a good implementation) Doesn't play well with C++ Hard for people trained in imperative programming styles to learn Full-blown Lisp works better as an application language No support on consoles though Lots of implementations out there

12 Choice of Languages PHP, JavaScript, Small, Squirrel, Io, interpreted C (LCC), Tcl, etc. There are a lot of choices out there Roll your own? Not for this project, but... Get exactly the mix of features you want Seamless C++ bindings, state-machines, concurrency, event handling, resource management Write your own native-code compiler! UnrealScript: Java with game-oriented features GOAL: Great until your Lisp guy retires... Writing a language is not a task to take lightly Be careful, or you will end up with Perl, but not as useful

13 Binding to C++ Simple attribute binding C-function binding Object reflection

14 How to Bind By hand No Macros Macros Clunky, but proven Templates Templates boost::python Hard to write, convenient to use IDL Complex, but language independent IDL Code parsers SWIG Complex, but convenient Symbol file parsers

15 Summary It's worth using multi-language development to gain some productivity back that C++ takes away Scripting in a game can take on many forms and this influences language selection Lua seems to be the most popular choice currently Custom languages are still popular, however

Overview. Rationale Division of labour between script and C++ Choice of language(s) Interfacing to C++

Overview. Rationale Division of labour between script and C++ Choice of language(s) Interfacing to C++ Scripting 1 Overview Rationale Division of labour between script and C++ Choice of language(s) Interfacing to C++ Rationale C++ isn't the best choice for all problems Complicated feature set, syntax Low-level,