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I have a 2D basic editor written in Qt, and I'm in the process of adding entities. I want the editor to be able to receive RTTI information from entities to change properties, create some logic being able to link published events to published actions (for example, A level activate event throws a door open action), etc... Because of this I guess my entity system should be written in a scripting language, in my case Lua.

On the other hand I want to use a component based design for my entities, and here starts my questions:

1) Should I define my componentes in C++? If I do this in C++ won't I lose all the RTTI information I want for my editor? On the other hand, I use box2d for physics, if I define all my components in script won't it be a lot of work to expose third party libs to lua?

2) Where should I place the messaging system for my game engine? Lua? C++?

I'm tempted to just have C++ object to behave as servers, offering services to lua business logic. Things like physics system, rendering system, input system, World class, etc... And for all the other things, lua. Creation/Composition of entities based on components, game logic, etc...

Could anyone give any insight on how to accomplish this? And which approach is better?

Sorry for the late response James, I did go to sleep. I have prefered to create a new response becaue I'm going to write a somehow long text.

I think you were not wrong and you did get the idea exactly as I wanted. Let me explain my ideas right now and key points after some sleep to clarify everything:

1) Components: At first I wanted to write them in lua. Now I have clear I'm going to write them in C++ and expose everything needed. Ussually only interfaces are exposed to lua, so I have come across that to export properties to be published in the editor we could get all "getXXXXX" methods (that have been exported to lua) and show them in the editor as XXXXX. Other things like data type, etc... should be thought.

2) Entities: They will be defined in lua through composition of C++ components (entity templates). They could be hardcoded too in the C++ side and have a factory for components and another for entities and let the lua side instantiate them.

All this, is what I can think of now. I know my editor can still be using game objects even if it is external, but the game am I into is for iphone using cocos2d and the editor is writtien in QT, so, I can't have the game running in a qt canvas and make qt<->game talk.

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1) Yes. While you want to expose the functionality via Lua for scripting you will in general want all the objects defined in C++. This will likely require either using the built in (and usually crappy) RTTI or writing your own system (If you look into it, for the basics, its really not that hard to do so dont let this deter you).

By keeping all of the objects within C++ then your game knows how to deal with them, and you can use the scripting to define them or give them small little action sets via the script and the like. And as you said, this keeps them available for other third party libs if need be.

2) C++ again. This will just be another thing that you want to expose through Lua so that other things can trigger or script logic to react to the messages and the like. This will also let the objects defined in C++ use the system as well with out having to go through Lua to do it.

In general I would say Always design your engine in the language its going to be running in. Hardware these days is very powerful compared to even a few years ago, but that doesnt mean you will not reach the ends of that power with your projects. A Scripting language will never run as fast as a compiled language.

Secondly this allows you to develop the entities/objects as completely as you want with out having to expose everything to the scripting interface. This is potentially another handy layer of abstraction that you can utilize to rework and upgrade or the like, parts of the system as you work on it with out necessarily having to redo every part at the same time.

Plus, Kudos on the Component system... Most people go straight tree hierarchy from the get go and then get overwhelmed with its upkeep and lack of extensibility.

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  • \$\begingroup\$ I have heard of enignes like cryengine, ut3, etc... (I know this are top notch game engiens) that write all the entities in script. So, they offer a real good information for the game editor, this is why I wanted to do something alike, but much simpler. And this is why I asked for define my entitiens un lua side. Could you suggest any doc or something about the fake RTTI in C++? \$\endgroup\$
    – Notbad
    Jan 6, 2011 at 21:15
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    \$\begingroup\$ Having worked with one of those and other engines, I can say that they do fully expose their system and you are able to make objects within their scripting interfaces as well as large amounts of game logic.. However.. The base core objects, the components if you will, are almost entirely C++ objects that are then exposed via RTTI and the scripting interface for people to muck with. On the RTTI side of things, give me a day or two and Ill see if I can not find something, maybe even a bit of a code example from one of my non-professional projects so I can give away the code with out worry :) \$\endgroup\$
    – James
    Jan 6, 2011 at 21:35
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    \$\begingroup\$ A second note, cause I ran out of space on that one.. Is that you do not strictly need a complicated RTTI just to expose things through a scripting language.. Having a generic way to define and wrap properties and methods may make exposing them through a scripting language easier, but especially with lua (Ive embedded it into 3 engines so far, but the last one was about 3 years ago so this may have changed) you can just go through and manually expose the parts that you want, no RTTI needed in the strictest sense of the word since you are defining the types from C++ to Lua yourself. \$\endgroup\$
    – James
    Jan 6, 2011 at 21:37
  • \$\begingroup\$ Hi James, and thanks a lot for your comments they are really good (sorry I can't give you credits, I'm new user :S). If I understand correctly, you mean that as I expose my components to lua, then from my editor, there's no need to support RTTI to get this information, I can directly get it from lua, am I right?. The problem, my editor is now an external editor, so I don't have this information exported from my game C++ part to lua.If you don't mind could we talk through IM or something to have a more fluently conversation?Don't wanna rush you with questions,just it is a pain to speak here :). \$\endgroup\$
    – Notbad
    Jan 6, 2011 at 22:05
  • \$\begingroup\$ I would have to say for exposing to the editor that some sort of a generic RTTI system for being able to look up properties and methods would be the way to go, this same system could also be used to expose the objects through Lua as well as I would assume your Editor is going to be using Lua when dropping in the entities and the like. \$\endgroup\$
    – James
    Jan 6, 2011 at 23:20

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