[C++-sig] Expose a C++ function with an argument of template vector to Python: Boost.Python.ArgumentError

[Liam Herron]

What happens if you do:

print canupo.read_msc_header(mscfile, scales, float(ptnparams))

?

From: Cplusplus-sig [mailto:cplusplus-sig-bounces+herron=ellington.com at python.org] On Behalf Of Zhan Li
Sent: Tuesday, November 03, 2015 5:13 PM
To: cplusplus-sig at python.org
Subject: [C++-sig] Expose a C++ function with an argument of template vector to Python: Boost.Python.ArgumentError

Greetings,

I'm new to Boost.Python. I have a C++ library (written and shipped by another party) that I want to wrap into Python modules so that I can use several functions in Python. One function takes an argument that is a C++ template vector. How can I export this function to a python function?

In a nutshell, here is what I'm doing now but without luck. I wrap the template vector into a Python class using Boost.Python's vector_indexing_suite. Then in the Python script, I first declare this Python class and then pass it to the function. But I got no luck and the following error message,

"
Boost.Python.ArgumentError: Python argument types in
    canupo.read_msc_header(MSCFile, FloatVec, int)
did not match C++ signature:
    read_msc_header(MSCFile {lvalue}, std::vector<float, std::allocator<float> > {lvalue}, int {lvalue})
"

More specifically, here are the C++ code and Python script I have.

In the C++ library code file "helper.hpp", it has a struct MSCFile and a function read_msc_header.

struct MSCFile
{
  // definition of MSCFile
};

int read_msc_header(MSCFile& mscfile, std::vector<FloatType>& scales, int& ptn)
{
  // implementation of read_msc_header
}

The "FloatType" is just a alias for float.

In the C++ wrapper code file "wrapper.cpp"
#include "helpers.hpp"
#include <boost/python.hpp>
#include <boost/python/module.hpp>
#include <boost/python/def.hpp>
#include <boost/python/implicit.hpp>
#include <boost/python/suite/indexing/vector_indexing_suite.hpp>

BOOST_PYTHON_MODULE(canupo)
{
  using namespace boost::python;

  class_<std::vector<FloatType> >("FloatVec")
    .def(vector_indexing_suite<std::vector<FloatType> >())
    ;

  class_<MSCFile>("MSCFile", init<const char*>())
    ;

  def("read_msc_header", read_msc_header);
}

In the python script, I tried to use the function as following but failed.

import canupo
mscfile = canupo.MSCFile("msc_file_name")
scales = canupo.FloatVec()
ptnparams = 0
print canupo.read_msc_header(mscfile, scales, ptnparams)

And the error goes like this:

Boost.Python.ArgumentError: Python argument types in
    canupo.read_msc_header(MSCFile, FloatVec, int)
did not match C++ signature:
    read_msc_header(MSCFile {lvalue}, std::vector<float, std::allocator<float> > {lvalue}, int {lvalue})

I googled around but didn't find a good solution or a solution that I can understand well. Most requires rewrite the function with Boost.Python. But I don't want to change the C++ library code itself. I think there may be a good way to expose such a function to Python. Thanks a lot!

Zhan.

--
Zhan Li, 李展;
Earth & Environment, Boston University, USA;

2008 - 2011, in Chinese Academy of Sciences, China;
2004 - 2008, in Nanjing University, China;

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