Cython: overloaded constructor initialization using raw pointer

Question

I'm trying to wrap two C++ classes: Cluster and ClusterTree. ClusterTree has a method get_current_cluster() that instantiates a Cluster object, and returns a reference to it. ClusterTree owns the Cluster object, and manages its creation and deletion in C++.

I've wrapped Cluster with cython, resulting in PyCluster.

PyCluster should have two ways of creation:

1) By passing in two arrays, which then implies that Python should then automatically handle deletion (via __dealloc__)
2) By directly passing in a raw C++ pointer (created by ClusterTree's get_current_cluster()). In this case, ClusterTree then assumes responsibility of deleting the underlying pointer.

from libcpp cimport bool
from libcpp.vector cimport vector

cdef extern from "../include/Cluster.h" namespace "Terran":
    cdef cppclass Cluster:
        Cluster(vector[vector[double]],vector[int]) except +

cdef class PyCluster:
    cdef Cluster* __thisptr
    __autoDelete = True

    def __cinit__(self, vector[vector[double]] data, vector[int] period):
        self.__thisptr = new Cluster(data, period)  

    @classmethod
    def __constructFromRawPointer(self, raw_ptr):
        self.__thisptr = raw_ptr
        self.__autoDelete = False

    def __dealloc__(self):
        if self.__autoDelete:
            del self.__thisptr

cdef extern from "../include/ClusterTree.h" namespace "Terran":
    cdef cppclass ClusterTree:
        ClusterTree(vector[vector[double]],vector[int]) except +
        Cluster& getCurrentCluster()

cdef class PyClusterTree:

    cdef ClusterTree *__thisptr

    def __cinit__(self, vector[vector[double]] data, vector[int] period):
        self.__thisptr = new ClusterTree(data,period)

    def __dealloc__(self):
        del self.__thisptr

    def get_current_cluster(self):

        cdef Cluster* ptr = &(self.__thisptr.getCurrentCluster())
        return PyCluster.__constructFromRawPointer(ptr)

This results in:

Error compiling Cython file:
------------------------------------------------------------
...
    def get_current_cluster(self):
        cdef Cluster* ptr = &(self.__thisptr.getCurrentCluster())
        return PyCluster.__constructFromRawPointer(ptr)
                                                     ^
------------------------------------------------------------

terran.pyx:111:54: Cannot convert 'Cluster *' to Python object

Note I cannot cdef __init__ or @classmethods.

The function you are calling needs to be cdef. If Cython does not support a cdef classmethod, then you will need to use something other than a classmethod. — Jim Garrison
Also, your classmethod attempts to modify __thisptr, which is an instance variable, not a class variable. For this reason typically the first argument of a classmethod is cls, not self. — Jim Garrison
aren't cdef'd variables always instance variables? I thought cdef Cluster* __thisptr; declares __thisptr as an instance variable. Are @classmethods not allowed to modify instance variables? — proteneer
classmethods cannot modify instance variables; they don't even have access to an instance (only the class). — Jim Garrison

Managarm Managarm · Accepted Answer · 2018-05-08T08:47:10

Pointers can only be passed to cdef'd functions as arguments, and cinit has to be def'd. But providing a classmethod is almost the way to go!

cdef Cluster* __thisptr
cdef bool __wrapped  ## defaults to False

@staticmethod
cdef PyCluster wrap(Cluster* ptr):
    cdef PyCluster pc = PyCluster([], [])  ## Initialize as cheaply as possible
    del pc.__thisptr  ## delete the old pointer to avoid memory leaks!
    pc.__thisptr = ptr
    pc.__wrapped = True
    return pc

Cython: overloaded constructor initialization using raw pointer

2 Answers