CODEWITHSUNDEEP
cython
DikobrAz
DikobrAz

Reputation: 3707

convert python object to cython pointer

I'm trying to expose c++ classes to python using cython. I wrote their definitions in *.pxd file and implemented a wrappers in *.pyx files. But I got stuck trying to pass to a function pointer to the extension type. Here is simplified example.

foo.pyx

from c_foo cimport cFoo
cdef class Foo:
    cdef cFoo* _impl

c_foo_holder.pxd

cdef extern from "FooHolder.h":
    cdef cppclass cFooHolder:
        cFooHolder(cFoo* foo)

foo_holder.pyx

from c_foo_holder cimport cFooHolder
from c_foo cimport cFoo

cdef class FooHolder:
    cdef cFooHolder* _impl
    def __init__(self, foo):
        self._impl = new cFooHolder(<cFoo*>(foo._impl)) # error here

But on the last line I get error "Python objects cannot be cast to pointers of primitive types". I also tried several other approaches, but nothing worked:

# error: 'Foo' is not a type identifier 
from foo import Foo
def __init__(self, Foo foo):
    self._impl = new cFooHolder(foo._impl)

# error: 'Foo' is not a type identifier 
def __init__(self, foo):
   self._impl = new cFooHolder(<Foo>(foo)._impl)

Upvotes: 8

Views: 8441

Answers (2)

DikobrAz
DikobrAz

Reputation: 3707

I found the solution. You have to tell cython that foo._impl is really cFoo* instance. That can be achieved by providing Foo definition (e.g. in foo.pxd). Afterwards you are able to cast python object to Foo and cython will know that its _impl field has type cFoo*.

foo.pxd

from c_foo cimport cFoo
cdef class Foo:
    cdef cFoo* _impl

foo.pyx

from c_foo cimport cFoo
cdef class Foo:
    # methods implementation

c_foo_holder.pxd

cdef extern from "FooHolder.h":
    cdef cppclass cFooHolder:
        cFooHolder(cFoo* foo)

foo_holder.pyx

from c_foo_holder cimport cFooHolder
from c_foo cimport cFoo
from foo cimport Foo

cdef class FooHolder:
    cdef cFooHolder* _impl
    def __init__(self, foo):
        self._impl = new cFooHolder((<Foo?>foo)._impl)

Upvotes: 3

Czarek Tomczak
Czarek Tomczak

Reputation: 20675

__init__ constructor can accept only pure python objects, there is also __cinit__ but it can accept only C primitive types.

When you pass Foo to __init__ it cannot see the _impl member, only members of python types can be seen.

The solution would be to pass cFoo* explicitly to an Init() method which emulates a constructor:

cdef class FooHolder:
    cdef cFooHolder* _impl
    cdef void Init(self, cFoo* foo_impl):
        self._impl = new cFooHolder(foo_impl)

Call it this way:

fooHolder = new FooHolder()
fooHolder.Init(foo._impl)

Upvotes: 0

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