在 C++ 中创建对象并将指针传递给 python

Posted 2023-02-23

【中文标题】在 C++ 中创建对象并将指针传递给 python

【英文标题】：Create object in C++ and pass pointer out to python

【发布时间】：2019-02-08 14:06:28

【问题描述】：

我正在尝试使用 ctypes 与 Python 共享 C++ 对象，方法是在 C++ 中创建对象，并通过 C 包装器将指针传递给 Python。我希望稍后能够使用 Python 类中的其他函数对这个对象进行操作，do_something 在下面的代码中。

我尝试了以下代码，但出现了段错误。我是 C 与 C++ 和 C 与 Python 接口的新手，所以我不确定在传递指针时我是否在做一些根本性错误，或者内存是否被 Python 垃圾收集清除/移动，之后我创建对象？

This question 讨论了类似的 boost 问题，但答案对 ctypes 不是很有用。

object.h

class object   
public:

  // constructor 
  object()  
    pointer = nullptr;
  

  // destructor
  virtual ~object() 
    delete pointer;
    pointer = nullptr;
  

  // get member functions of object_pointer
  // from C++
  double do_something();

protected:

  // pointer to the object
  object_pointer *pointer;

;

extern "C" 
  object* object_new();
  void object_delete(object *Ob);
  double object_do_something(object *Ob);

object.cpp

#include "object.h"
double object::do_something()  return pointer->do_something(); 

extern "C" 
    object *object_new()  return new object(); 
    void object_delete(object *Ob)  delete Ob; 
    double object_do_something(object *Ob)  return Ob->do_something(); 

object.py

​​>

from ctypes import *

lib = cdll.LoadLibrary('./lib_object.so')
lib.object_new.argtypes = ()
lib.object_new.restype = c_void_p
lib.special_delete.argtypes = c_void_p,
lib.special_delete.restype = None
lib.object_pointer.argtypes = c_void_p
lib.object_pointer.restype = c_void_p

class Object:
    def __init__(self):
        self.obj = lib.object_new()
        print self.obj
    def __del__(self):
        lib.object_delete(self.obj)
    def do_something(self):
        lib.object_do_something(self.obj)

s = Object()
>> 94549743086144
s.do_something()
>> Segfault

任何帮助将不胜感激！

【问题讨论】：

你用什么命令将object.cpp编译成lib_object.so？

你有没有换过pointer？否则 pointer->do_something() 取消引用空指针。另外，为什么是S->do_something() 而不是Ob->do_something？

抱歉S->do_something() 打错了，我已经修正了

指针确实发生了变化：一个对象是从一个字符缓冲区重建的，然后pointer指向该对象

argtypes 应该是一个列表（或 tuple）：lib.object_pointer.argtypes = [c_void_p].

【参考方案1】：

注意事项：

没有编译问题中的文件（.cpp、.py）。它们包含语法错误和语义错误 我不知道 pointer 角色的用途是什么（它会产生 syntax 错误）。我只能假设尝试了 singleton 实现 考虑到上述情况，我没有在现有文件中指出错误（其中有很多错误），而是创建了一个全新的基本示例 虽然 ctypes 不是唯一需要改进的地方，但我也指出：[Python 2]: ctypes - A foreign function library for Python

object.h:

class Object 
    public:

    Object() 
        m_double = 2.718282;
    

    virtual ~Object() 

    double do_something();

    private:

    double m_double;
;


extern "C" 
    Object* object_new();
    void object_delete(Object *ob);
    double object_do_something(Object *ob);

object.cpp:

#include <iostream>
#include "object.h"


using std::cout;

double Object::do_something() 
    std::cout << "Doing something in C++\n";
    return m_double;



extern "C" 
    Object *object_new()  return new Object(); 
    void object_delete(Object *pObj)  delete pObj; 
    double object_do_something(Object *pObj)  return pObj->do_something(); 

object.py:

#!/usr/bin/env python2

import sys
import ctypes


lib = ctypes.cdll.LoadLibrary('./lib_object.so')

lib.object_new.argtypes = []
lib.object_new.restype = ctypes.c_void_p
lib.object_delete.argtypes = [ctypes.c_void_p]
lib.object_do_something.argtypes = [ctypes.c_void_p]
lib.object_do_something.restype = ctypes.c_double


class Object:
    def __init__(self):
        self.obj = lib.object_new()
        print("`Object` instance (as a `void *`): 0x:016X".format(self.obj))

    def __del__(self):
        lib.object_delete(self.obj)

    def do_something(self):
        return lib.object_do_something(self.obj)


def main():
    obj = Object()
    ret = obj.do_something()
    print(ret)


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

输出：

[cfati@cfati-5510-0:/cygdrive/e/Work/Dev/***/q054594122]> ls
object.cpp  object.h  object.py
[cfati@cfati-5510-0:/cygdrive/e/Work/Dev/***/q054594122]> gcc -shared -fPIC -o lib_object.so object.cpp -lstdc++
[cfati@cfati-5510-0:/cygdrive/e/Work/Dev/***/q054594122]> python2 object.py
Python 2.7.14 (default, Oct 31 2017, 21:12:13)
[GCC 6.4.0] on cygwin

`Object` instance (as a `void *`): 0x0000000600078870
Doing something in C++
2.718282

【讨论】：

谢谢，这很有帮助。将来我会尝试提供一个合适的独立示例，而不是从我的代码中提取随机位...

不客气！ [SO]: How to create a Minimal, Complete, and Verifiable example (mcve).