how to return numpy.array from boost::python?

Question

I would like to return some data from c++ code as a numpy.array object. I had a look at boost::python::numeric, but its documentation is very terse

Answer 1

A solution that doesn't require you to download any special 3rd party C++ library (but you need numpy).

#include  // ensure you include this header

boost::python::object stdVecToNumpyArray( std::vector const& vec )
{
      npy_intp size = vec.size();

     /* const_cast is rather horrible but we need a writable pointer
        in C++11, vec.data() will do the trick
        but you will still need to const_cast
      */

      double * data = size ? const_cast(&vec[0]) 
        : static_cast(NULL); 

    // create a PyObject * from pointer and data 
      PyObject * pyObj = PyArray_SimpleNewFromData( 1, &size, NPY_DOUBLE, data );
      boost::python::handle<> handle( pyObj );
      boost::python::numeric::array arr( handle );

    /* The problem of returning arr is twofold: firstly the user can modify
      the data which will betray the const-correctness 
      Secondly the lifetime of the data is managed by the C++ API and not the 
      lifetime of the numpy array whatsoever. But we have a simple solution..
     */

       return arr.copy(); // copy the object. numpy owns the copy now.
  }

Of course you might write a function from double * and size, which is generic then invoke that from the vector by extracting this info. You could also write a template but you'd need some kind of mapping from data type to the NPY_TYPES enum.