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Unity scripting language api - exposed class naming

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I'm having a bit of difficulty deciding exactly how the C++ classes will be exposed in my scripting language. I'm currently designing my scripting api, so this thread is very interesting. I would quite enjoy hearing the actual details of why the current api's aren't so hot. Also if anyone has any ideas for how to nicely bind C++ and a scripting language it would be much appreciated. My current plan is to have an xml file that is hand edited to explain what you want to expose and how. This file is then read by a command line program that generates a cpp file with all the class function wrapper methods and a function with the same name as the file that, when run dynamically adds these definitions to the virtual machine runtime. I think this method isn't too bad because the xml file really contains exactly the information that you need. Comments and suggestions welcome. If everyone thinks that this way of doing things is okay, then on to my problem at the moment. Do you want to expose a C++ class into the scripting language with a different name? And if so, do you want to do the same with it's methods? Is this important functionality that I should include in my api?

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Might be okay, if you can get this command line utility integrated in your build process it probably wouldn't be too hard to keep the scripting language bindings up to date at all times.

However, you would still be required to update two places each time you change or add a function that is to be exposed to the scripting language. That's why I like the approaches of LuaBind and Boost.Python where the scripting language bindings are generated by template magic solely in C++ without a seperate command line utility.


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Yes that was the idea, a make file will be able to generate them easily.

mmm, template magic. Sounds interesting, but I distrust templates. I will have a look into them though.

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As a result of that thread, I'm playing with something new to see how it works out. I'm basically playing around with meta-classes. You declare a class definition in your application, attaching methods and properties to it. What you do then is ask the environment to make you an instance of that class; this is then usable in both the scripting environment and your native application. What's more, your 'native' classes can also be extended via the scripting langauge to add new methods, overrides or properties. The main drawback of this is that although you can access the meta-class instance in C++, it'd obviously be slower than using native code - the benefits are that there's not really a 'script' API any longer as the meta-class system and the scripting system share the same interface.

Again, not sure how this will pan out though - but it keeps me occupied at least.

Look at my journal for snippets...

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Original post by umbrae
mmm, template magic. Sounds interesting, but I distrust templates. I will have a look into them though.

Oh, don't. Templates are wonderful things =)

I used a heavy template approach when writing gmBind, a C++ binding library for GameMonkey Script.

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Original post by umbrae
I'm currently designing my scripting api, so this thread is very interesting. I would quite enjoy hearing the actual details of why the current api's aren't so hot.

1. Verbosity - if I have to type a lot to expose a function, it gets boring quickly. Similarly, if I have to check the number of parameters to each function, then the type of each of them, and write a 2-clause if statement for each, I'm gonna get bored and make mistakes.

2. Error-proneness - if the API is verbose or offers many different functions, it takes longer to look up what is needed for the binding and subsequently debug it.

3. Error-reporting facilities - generally you want the API to be able to detect when you misuse it (eg. you write a binding for a function that takes a double but you bind it as if it takes an integer), so ideally you want checked conversion operations and a well-defined place to report any errors.

4. Documentation quality - for example, the Lua documentation is quite comprehensive but poorly organised. A lot of the best binding functions are hidden in the extra libraries and not fully explained. Make sure your binding helper functions are clearly documented with examples.

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I think for the moment that I will stay with this xml method and see how far it goes. It might be an interesting idea if I also generate the class's .h file from the xml source as well. That way there would only be one place to make any changes.

I think it is a bit of a toss up between being able to expose already written C++ code (api's etc) or writing C++ for the scripting language directly. If you are trying to expose already written C++ code you don't want to make many changes, but C++ code for the language can be written it in a way that makes it easy to expose.

Since I'm starting with this language the latter is fine for me, but I also want other people to use it which means that it should be easy to bind. But if I want to expose any type of C++ class I need to support all the weird methods and heirarchy features of C++. My language doesn't have multiple inheritance is it okay to leave out support for this and other C++ language features? My plan was to just be able to expose one class, not a heirarchy. You will be able to setup a heirarchy in the scripting language, but it will behave like an object in my language, not a C++ object. ie every method is virtual etc.

Anyway, my original question is whether you want to expose C++ classes into the scripting language as a different name, and the same with methods. ie C++ class called "Scene" is actually exposed as "Scene3D" in the language. I think this would be good for libraries and cleaning up naming conventions.

[Edited by - umbrae on March 16, 2005 9:15:17 PM]

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Kylotan: Thanks heaps for your comments.

This is currently how I am envisioning binding classes to my language. This xml file is parsed and a .h file is generated. A C method with the same name as the file is also made and when called it registers the wrapper functions to the virtual machine.

<?xml version="1.0" standalone="no" ?>
<class name="bob" alias="bob_tyre">
<method name="print_a" type="void" />
<method name="number_id" type="int" />
<method name="distance" type="float">
<parameter name="x" type="int" />
<parameter name="y" type="int" />

<class name="dave">
<method name="print_a" type="void" />

So now in the scripting environment you can reference a class called "bob_tyre" and call methods from it. I still have to add a few more options to the xml file (to allow for a few of the cool features of my language) but it won't change much.

My code isn't commented, and it is all in the main function. But it does generate stuff.

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CPB is now working with C++, it should do what you need. Expect to see a new version that includes the new features in a few days.


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Toes: I'm guessing that the technique that you use in CPB only works with x86? For me at least this isn't an option, my binding library needs to work on more platforms than just windows. Especially because I don't currently have a windows machine.

Interesting technique, but too low level for me.

edit: ooh, and I am not using lua, this is for my own scripting language.

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