new extension generator for C++

[Samuel Williams]

Dear Rouslan,

It looks interesting. I say go for it. You will learn something and might make some improvements on existing ideas.

I recommend putting the code on www.github.com

Kind regards,
Samuel

On 4/05/2010, at 8:44 AM, Rouslan Korneychuk wrote:

> Hi, I'm new here. I'm working on a program that exposes C++ declarations to Python and I was wondering if there is any interest in it.
> 
> It's a hobby project. I was originally using Boost.Python on another project but found a couple of things I didn't like. Eg: you can't really have private constructors. If you want a Python object created, you either have to expose the constructor in Python, or create an instance of the object somewhere else in memory, and have it either copied to the PyObject or have the PyObject hold a reference to it. I was working on a rudimentary 3D game engine that was a Python extension. It was important to avoid copying potentially huge amounts of data, but to have almost every PyObject just be a pointer to another object (and the object already is just a few or even one pointer) seemed like an pointless compromise, considering that I was writing code that was specifically designed to be a Python extension (There was also another issue I came across but don't remember now).
> 
> So I looked for other solutions and noticed that Py++ (which simply generates Boost.Python code for you) was based on a seperate program called GCCXML. I figured I could use GCCXML and generate code however I wanted. So I did.
> 
> My program generates human-readable code (it even uses proper indentation). The program still has a lot of work to be done on it, but it can already generate working Python extensions.
> 
> It takes an input file like this:
> <?xml version="1.0"?>
> <module name="testmodule" include="main.h">
>    <doc>module doc string</doc>
> 
>    <class name="AClass" type="MyClass">
>        <doc>class doc string</doc>
>        <init/>
>        <member cmember="value"/>
>        <def func="greet"/>
>        <propery get="getX" set="setX"/>
>    </class>
> </module>
> 
> You can probably figure out what the resulting code does. The goal is to generate code that is as close to hand-written code as possible. It even forgoes using PyArg_ParseTupleAndKeywords and has the argument checks hard-wired. It also generates a header file that contains a PyObject implementation of MyClass called obj_AClass, with every constructor fowarded and with the new and delete operators overloaded to play nice with Python's memory handler:
> ...
> extern PyTypeObject obj_AClassType;
> 
> struct obj_AClass {
>    PyObject_HEAD
>    MyClass base;
>    bool initialized;
> 
>    void *operator new(size_t s) {
>        void *ptr = PyMem_Malloc(s);
>        if(!ptr) throw std::bad_alloc();
>        return ptr;
>    }
> 
>    void operator delete(void *ptr) {
>        PyMem_Free(ptr);
>    }
> 
>    obj_AClass(MyClass const & _0) : base(_0) {
>        PyObject_Init(reinterpret_cast<PyObject*>(this),&obj_AClassType);
>        initialized = true;
>    }
> 
>    obj_AClass(unsigned int _0) : base(_0) {
>        PyObject_Init(reinterpret_cast<PyObject*>(this),&obj_AClassType);
>        initialized = true;
>    }
> 
> };
> ...
> 
> If you want to expose one overload of a constructor or function but not the others, you can add overload="list,of,args" to <init/> or <def/>. Default values for arguments are handled automatically. When handling overloaded functions, it checks the types of the arguments to pick the best overload, taking into consideration polymorphism and the fact that basic types like ints can be converted from one-another. It will automatically detect if two overloads match to the same set of Python arguments (eg: void func(float) vs void func(double)). The only issue is it will not use keyword arguments for overloaded functions (I don't know if that can even be done reliably *and* efficiently. I would need to give it more thought).
> 
> If there is enough interest I can put this project up on SourceForge or Google Code.
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