Hi Stefan,
first of all thanks for your quick response.
You put me on the right track. Your explanation clarified some
C++/ Boost.Python issues I wasn't aware of before.
Your sketched solution is exactly what I need.
Below is the complete running example.
Thanks
Rainer
##############################################################################
#
# const_char_ptr.cpp
#
##############################################################################
#include <boost/python.hpp>
#include <iostream>
// Container Class
struct ConstCharPtr
{
char const *x;
std::string operator()() const
{
std::string s(x);
return s;
};
};
const char* x= "A.B";
// Class 'A'
class A
{
public:
A(): m_ccp(NULL) { };
int getName(const char *& ccp) {m_ccpr = ccp; ccp = x; return 1; }
std::string operator()() const
{
std::string s(m_ccp);
return s;
};
private:
const char* m_ccp;
};
// Container class factory
ConstCharPtr make_ccp() { return ConstCharPtr(); };
// the A::getName, adjusted to work with a ConstCharPtr argument
int get_name(A &a, ConstCharPtr &ccp) { return a.getName(ccp.x);}
namespace bp = boost::python;
// expose to Python
void export_ccp()
{
bp::class_<ConstCharPtr, boost::noncopyable> ccp("ConstCharPtr");
ccp.def("__init__", &make_ccp);
// Make ConstCharPtr instances Python callable
ccp.def("__call__", &ConstCharPtr::operator());
bp::class_<A>("A", "This is class A", bp::init< >())
.def("getName", &get_name)
;
};
##############################################################################
#
# Boost.Python wrapper: samplemodule.cpp
#
##############################################################################
#include <boost/python.hpp>
#include <iostream>
using namespace boost::python;
// export function
void export_ccp(); // const_char_ptr.cpp
BOOST_PYTHON_MODULE(sample)
{
export_ccp();
};
##############################################################################
#
# Python usage
#
##############################################################################
>>> from sample import *
>>> a=A()
>>> x=ConstCharPtr()
>>> n=a.getName(x)
>>> n
1
>>> x()
'A.B'
// Bingo!
Von: Stefan Seefeld <ste...@seefeld.name>
An: cplusplus-sig@python.org
Datum: 12.05.2011 15:39
Betreff: Re: [C++-sig] from_python conversion for a const char*&
argument
Gesendet von: cplusplus-sig-bounces+rainer.kluger=lbbw...@python.org
Hi Rainer,
On 2011-05-12 08:57, Rainer Kluger wrote:
>
> Hi
>
> I try to expose the following class to python (using gcc 4.5.1, boost
> 1_44_0, Python 2.7.1):
>
>
> // C++
> class A
> {
> public:
> int getName(const char*& name); // suppose the implementation
> assigns the const char* x = "foo"; variable to the ref-pointer
> }
>
> I'd like to expose this API as is and not alter the method interface so I
> think I'll have to do s.th. like this:
>
> ====================================
>
> # intended usage in Python:
>
>>>> import test
>>>> a = A()
>>>> x = const_char_ptr_ref()
>>>> y = a.getName(x)
>>>> x()
> 'foo'
>
> ====================================
>
>
> I followed the "recipes" of the python test example, realizing that
> - m1.cpp
> -extract.cpp
> and tried to get the things together, knowing that those example do not
> really match my use case.
>
> My understanding for what I need to do, are the following steps:
>
> (1) encapsulate the const char*& variable in a container class,
let's
> say ConstCharPtrRef""
> (2) expose the ConstCharPtrRef class to Python
> (3) provide a factory function for the class exposed in the step
(2)
> (4) expose the factory function from step (3)
Right.
> (5) provide an extractor function which extracts the "const
char*&"
> variable from within the given python argument
> (6) register a from_python converter for the given type and the
> provided extractor function
I'm not sure I understand the need for (5) and (6). If you have defined
a ConstCharPtrRef C++ type, and exported it to Python, you will already
have converters between the two, so from within C++ you then can simply
access the ConstCharPtrRef members by whatever API you define in C++.
There is no need for extra converters at that point.
I'm also not sure the reference needs to be captured at that level, as
that's more of a call policy (for the A::getName member function) than
an aspect of the type.
Here is how I would approach it (caution: fully untested code):
struct ConstCharPtr
{
char const *x;
};
// A factory...
ConstCharPtr make_ccp(...);
// the A::getName, adjusted to work with a ConstCharPtr argument
int get_name(A const &a, ConstCharPtr const &ccp) { return a.getName(ccp);}
BOOST_PYTHON_MODULE(test)
{
// Export the ConstCharPtr type...
class_<ConstCharPtr, boost::noncopyable> ccp("ConstCharPtr", no_init);
// ...and define how to construct it
ccp.def("__init__", make_cpp);
// Export the A type...
class_<A> a("A");
// and export the getName member function
a.def("getName", get_name);
//...
}
I believe this should actually do what you want.
Stefan
--
...ich hab' noch einen Koffer in Berlin...
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