[C++-sig] [boost.python] char arrays in structs

[Ralf W. Grosse-Kunstleve]

--- Markus Schöpflin <markus.schoepflin at comsoft.de> wrote:

> Using gcc-3.4.3 and boost.python 1.32.0. Full example:
> 
> ---%<---
> #include <boost/python/module.hpp>
> #include <boost/python/class.hpp>
> 
> typedef char bar_t[4];

Could you make it a boost::array<char, 4>?

> struct foo
> {
>    bar_t bar;
>    foo() {
>      bar[0] = 'b'; bar[1] = 'a';
>      bar[2] = 'r'; bar[3] = 0;
>    }
> };
> 
> BOOST_PYTHON_MODULE(test)
> {
>    using namespace boost::python;
> 
>    class_<foo>("foo")
>      .def_readonly("bar", &foo::bar)
>      ;
> }
> --->%---
> 
> The compiles and links ok, but when using it from python I get the 
> following error:
> 
> Python 2.3 (#1, Jul 30 2003, 14:14:00)
> [GCC 3.3 20030226 (prerelease) (SuSE Linux)] on linux2
> Type "help", "copyright", "credits" or "license" for more information.
>  >>> from test import *
>  >>> print foo().bar
> Traceback (most recent call last):
>    File "<stdin>", line 1, in ?
> TypeError: No to_python (by-value) converter found for C++ type: char [4]
> 
> So I went ahead and added a custom converter for bar_t like so:
> 
> struct bar_t_to_python_str
> {
>    static PyObject* convert(bar_t const &s)

Here bar_t is equivalent to a plain char*. The size information is lost. This
due to the C heritage.

>    {
>      return boost::python::incref(boost::python::object(s).ptr());
>    }
> };
> 
> and in BOOST_PYTHON_MODULE(test):
> 
>    to_python_converter<bar_t, bar_t_to_python_str>();
> 
> Is it to be expected that I do need a custom type converter for this? Or do 
> I miss something?

You'd need a custom converter for boost::array<char, 4> if that is an option.
The converter would also do the from_python conversion.

If the C++ interface is fixed, you'd have to work with thin wrappers. As long
as you stay exclusively within the framework of C++, as Boost.Python does, this
is the only option.

Cheers,
        Ralf


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