passing structure from c++ to python

Question

C++ code

typedef struct Box { public: int length; // Length of a box int breadth; // Breadth of a box int height; // Height of a box }; extern "C" { //struct Box __declspec(dllexport) GetAllInfo(); TESTAPI struct Box * GetAllInfo(); } extern "C" { TESTAPI struct Box * GetAllInfo() { Box *Box1 = new Box; Box1->height = 5; Box1->length = 6; Box1->breadth = 7; cout << "Info is : " << Box1->height<<Box1->length<<Box1->breadth <<endl; return Box1; } }

Python code

import ctypes from ctypes import Structure, c_int, c_double, windll from ctypes import * from ctypes import wintypes from collections import namedtuple #astdll = windll.CDll class Box(object): def __init__(self,height,length,breadth): self.height = height self.length = length self.breadth = breadth #class Box(Structure): # _fields_ = [ # ("height", ctypes.c_int), # ("length", ctypes.c_int), # ("breadth", ctypes.c_int) # ] Box = namedtuple("Box", "height length breadth") lib = cdll.LoadLibrary("F:\\QT SCADA\\forPythonDLL\\Neha_Test\\Debug\\Neha_Test.dll") #lib.GetAllInfo.restype = POINTER(Box) s = Box global result result = lib.GetAllInfo() #s = result s.height = 20 print (result.height) print (s.height)

This is the error:

Running script: "C:\Users\Administrator\Desktop\test.py"

Info is : 567

Traceback (most recent call last):

File "C:\Users\Administrator\Desktop\test.py", line 41, in

print (result.height)

AttributeError: 'int' object has no attribute 'height'

this is pretty illegible. please see this help which explains how to format your code — scytale
– scytale, Commented Aug 20, 2015 at 16:59

kurtzmarc · Accepted Answer · 2019-07-31 15:10:26Z

Thanks fr all the help....:)

I got this code

its working fine

import ctypes from ctypes import Structure, c_int, c_double, windll from ctypes import * from ctypes import wintypes from collections import namedtuple lib = cdll.LoadLibrary(DLL_PATH) class Box(Structure): _fields_ = [ ("length", c_int), ("breadth", c_int), ("height", c_int)] lib.GetAllInfo.restype = POINTER(Box) result = lib.GetAllInfo() print ("result type: ", type(result)) print ("-"*30) print ("result: ",result) print ("-"*30) print (result.contents) Box1=result[0] print(Box1.height)

bawjensen · Accepted Answer · 2015-08-20 17:27:37Z

Not 100% sure on this, but at a guess I'd say your issues lies in the fact that the C++ code for GetAllInfo returns a Box pointer (type Box*), which is effectively just an integer referring to a location in memory. You're then trying to get the height attribute of this integer pointer, which is resulting in the error AttributeError: 'int' object has no attribute 'height'.

Try returning a Box object (as below), not a Box pointer, and see if that fixes it.

TESTAPI struct Box * GetAllInfo() { Box *Box1 = new Box; Box1->height = 5; Box1->length = 6; Box1->breadth = 7; cout << "Info is : " << Box1->height<<Box1->length<<Box1->breadth <<endl; return Box1; }

becomes

TESTAPI struct Box GetAllInfo() { Box *Box1 = new Box; Box1->height = 5; Box1->length = 6; Box1->breadth = 7; cout << "Info is : " << Box1->height<<Box1->length<<Box1->breadth <<endl; return (*Box1); }

That is not a good idea since it creates a memory leak, Box1 is not deallocated since there is no corresponding delete.

Tim · Accepted Answer · 2016-07-24 12:09:49Z

You are mixing C and C++. Your updated code works because ctypes.Structure fixes this issue for you. You need to also release the memory you allocate. How you do this depends on what your program needs to do with a Box. One possibility is to use an in/out parameter in GetAllInfo (implementation left as an exercise for the reader).

struct Box { int length; // Length of a box int breadth; // Breadth of a box int height; // Height of a box }; extern "C" TESTAPI Box* GetAllInfo() { Box *Box1 = new Box; Box1->height = 5; Box1->length = 6; Box1->breadth = 7; cout << "Info is : " << Box1->height << Box1->length << Box1->breadth << endl; return Box1; }

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