Python ctypes cdll.LoadLibrary, instantiate an object, execute its method, private variable address truncated

Always (CORRECTLY) specify argtypes and restype for functions defined in C, otherwise (C89 style) they will default to int (generally 32bit), generating !!! Undefined Behavior !!!. On 64bit, addresses (larger than 2 GiB) will be truncated (which is exactly what you're experiencing). Check [SO]: C function called from Python via ctypes returns incorrect value (@CristiFati's answer) for more details.

Also, when running into issues, don't forget about [Python 3.Docs]: ctypes - A foreign function library for Python.

Below it's an adapted version of your code.

detector.cpp:

#include <stdio.h>
#include <memory.h>
#include <fstream>

#define C_TAG "From C"
#define PRINT_MSG_2SP(ARG0, ARG1) printf("%s - [%s] (%d) - [%s]:  %s: 0x%0p\n", C_TAG, __FILE__, __LINE__, __FUNCTION__, ARG0, ARG1)


using std::endl;

std::ofstream outFile;


class Detector {
    public:
        Detector();
        void process(int *pIn, int *pOut, int n);

    private:
        int m_var;
};


Detector::Detector() 
: m_var(0) {
    outFile.open("addr_debug.txt");
    outFile << "m_var init address: " << &m_var << endl;
    PRINT_MSG_2SP("&m_var", &m_var);
}

void Detector::process(int *pIn, int *pOut, int n) {
    outFile << "m_var process address: " << &m_var << endl;
    outFile.close();
    PRINT_MSG_2SP("&m_var", &m_var);
}


#define SIM_EXPORT __declspec(dllexport)

#if defined(__cplusplus)
extern "C" {
#endif

    SIM_EXPORT Detector *DetectorNew() { return new Detector(); }
    SIM_EXPORT void DetectorProcess(Detector *pDet, int *pIn, int *pOut, int n) {
        pDet->process(pIn, pOut, n);
    }
    SIM_EXPORT void DetectorDelete(Detector *pDet) { delete pDet; }

#if defined(__cplusplus)
}
#endif

code.py:

import sys
from ctypes import CDLL, POINTER, \
    c_int, c_void_p
import numpy as np


sim_dll = CDLL("./sim.dll")

detector_new_func = sim_dll.DetectorNew
detector_new_func.restype = c_void_p

detector_process_func = sim_dll.DetectorProcess
detector_process_func.argtypes = [c_void_p, POINTER(c_int), POINTER(c_int), c_int]

detector_delete_func = sim_dll.DetectorDelete
detector_delete_func.argtypes = [c_void_p]


class Detector():
    def __init__(self):
        self.obj = detector_new_func()

    def process(self, pin, pout, n):
        detector_process_func(self.obj, pin, pout, n)

    def __del__(self):
        detector_delete_func(self.obj)


def main():
    detector = Detector()

    n = 1024
    a = np.arange(n, dtype=np.uint32)
    b = np.zeros(n, dtype=np.int32)

    aptr = a.ctypes.data_as(POINTER(c_int))
    bptr = b.ctypes.data_as(POINTER(c_int))

    detector.process(aptr, bptr, n)


if __name__ == "__main__":
    print("Python {:s} on {:s}\n".format(sys.version, sys.platform))
    main()

Notes:

• As I stated at the beginning, the problem was argtypes and restype not being specified (e.g. for DetectorNew: comment detector_new_func.restype = c_void_p, and you'll run into the problem again)
• Code in the question is missing parts (#includes, imports, ...), also there are some syntax errors, so it doesn't compile, and therefore doesn't follow [SO]: How to create a Minimal, Complete, and Verifiable example (mcve) guidelines. Please when make sure to have mcve when asking
• The object that you allocate (new Detector()), must also be deallocated (otherwise, it will generate a memory leak), so I added a function (DetectorDelete - to do that), which is called from (Python) Detector's destructor
• Other (non critical) changes (identifiers renaming, a bit of refactoring, printing to stdout, ...)

Output:

(py35x64_tes1) e:\Work\Dev\StackOverflow\q052268294>"c:\Install\x86\Microsoft\Visual Studio Community\2015\vc\vcvarsall.bat" x64

(py35x64_test) e:\Work\Dev\StackOverflow\q052268294>dir /b
code.py
detector.cpp

(py35x64_test) e:\Work\Dev\StackOverflow\q052268294>cl /nologo /DDLL /EHsc detector.cpp  /link /DLL /OUT:sim.dll
detector.cpp
   Creating library sim.lib and object sim.exp

(py35x64_test) e:\Work\Dev\StackOverflow\q052268294>dir /b
code.py
detector.cpp
detector.obj
sim.dll
sim.exp
sim.lib

(py35x64_test) e:\Work\Dev\StackOverflow\q052268294>"e:\Work\Dev\VEnvs\py35x64_test\Scripts\python.exe" ./code.py
Python 3.5.4 (v3.5.4:3f56838, Aug  8 2017, 02:17:05) [MSC v.1900 64 bit (AMD64)] on win32

From C - [detector.cpp] (28) - [Detector::Detector]:  &m_var: 0x0000020CE366E270
From C - [detector.cpp] (34) - [Detector::process]:  &m_var: 0x0000020CE366E270