I think @FlorianWeimer’s answer provides a decent solution (allocate a vector
and pass that into your C++ function) but it should be possible to return a vector from doit
and avoid copies by using the move constructor.
from libcpp.vector cimport vector
cdef extern from "<utility>" namespace "std" nogil:
T move[T](T) # don't worry that this doesn't quite match the c++ signature
cdef extern from "fast.h":
vector[int] doit(int length)
# define ArrayWrapper as holding in a vector
cdef class ArrayWrapper:
cdef vector[int] vec
cdef Py_ssize_t shape[1]
cdef Py_ssize_t strides[1]
# constructor and destructor are fairly unimportant now since
# vec will be destroyed automatically.
cdef set_data(self, vector[int]& data):
self.vec = move(data)
# @ead suggests `self.vec.swap(data)` instead
# to avoid having to wrap move
# now implement the buffer protocol for the class
# which makes it generally useful to anything that expects an array
def __getbuffer__(self, Py_buffer *buffer, int flags):
# relevant documentation http://cython.readthedocs.io/en/latest/src/userguide/buffer.html#a-matrix-class
cdef Py_ssize_t itemsize = sizeof(self.vec[0])
self.shape[0] = self.vec.size()
self.strides[0] = sizeof(int)
buffer.buf = <char *>&(self.vec[0])
buffer.format="i"
buffer.internal = NULL
buffer.itemsize = itemsize
buffer.len = self.v.size() * itemsize # product(shape) * itemsize
buffer.ndim = 1
buffer.obj = self
buffer.readonly = 0
buffer.shape = self.shape
buffer.strides = self.strides
buffer.suboffsets = NULL
You should then be able to use it as:
cdef vector[int] array = doit(length)
cdef ArrayWrapper w
w.set_data(array) # "array" itself is invalid from here on
numpy_array = np.asarray(w)
Edit: Cython isn’t hugely good with C++ templates – it insists on writing std::move<vector<int>>(...)
rather than std::move(...)
then letting C++ deduce the types. This sometimes causes problems with std::move
. If you’re having issues with it then the best solution is usually to tell Cython about only the overloads you want:
cdef extern from "<utility>" namespace "std" nogil:
vector[int] move(vector[int])