Re: [boost] Boost.Python, embedding and extending, exposing instances

From: David Abrahams (dave_at_[hidden])
Date: 2002-11-25 17:55:19

thor.arne.johansen_at_[hidden] writes:

> Hello,
>
> I'm trying to embedd and extend a C++ application.
>
> I was not able to find any good examples of how to do this. 

http://cvs.sourceforge.net/cgi-bin/viewcvs.cgi/*checkout*/boost/boost/libs/python/test/embedding.cpp

> From the small tidbits of information I found, plus browsing the
> Boost.Python sources, I tried the following approach (starting from
> the "getting_started2" example, with an added main() + other
> functions):
>
> use bjam -omylog.txt -n -sTOOLS=msvc
>
> To get a log of what exactly goes on when bjam builds the examples. Looking
> at the log it seems we have to make som modifications:
>
> Compile step:
>      remove    -DBOOST_DYNAMIC_LIB

Has no effect. 

> Link step:
>      remove         /DLL
>      change         /out: (file extension should be .exe instead of .pyd)
>      remove         /implib:* (no need for an import lib for an .exe)
>
> Using these modifications to the command lines we can compile and link an
> application.

You might try adding -DBOOST_PYTHON_STATIC_LIB if you insist on
linking Booost.Python directly into your application, but there's no
reason you can't just use the Boost.Python DLL for this job.

> It seems to work fine. I manually call the init function (whatever
> init##name()  expands to in the BOOST_PYTHON_MODULE macro) sometime after
> Py_Initialize() init##name(). I can then instatiate objects of classes
> defined in C++ in my embedded python script. (There must be a better way to
> initialize the module??). I also suspect the application is now exporting
> functions and interfaces, but I'm not too worried about that right now.
>
> I would also like to have access to _instances_ of C++ objects. Ie. the
> object has already been instantiated in C++, and contains state. I would
> like to use this object in an embedded script.
>
> Are there anyone with  ideas, pointers, or examples showing how this can be
> done?

If you know the lifetime of the C++ object will persist past its use
in Python, you can pass it to some Python callable object with ref():

   void pass_to_python(object func, SomeCppClass& x)
   {
        // Invoke func with a Python object built around x
        func(boost::ref(x));
   }

HTH,
-- 
                       David Abrahams
   dave_at_[hidden] * http://www.boost-consulting.com
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