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python - 如何使用 ctypes 将类对象从 c++ (DllExport) 返回到 python，反之亦然？

问题描述

我在 c++ 中有一个函数，它将类对象返回给 python，我在下面的代码片段中有一个 python(ctypes import) 的包装器，当我从 python 调用函数(Get_Student) 时，学校有在 c++ 中创建的新实例，我无法检索学生班级的数据，

//c++代码是：

 #include"stdafx.h"
 #include<string>
 using namespace std;

class Student
{
    int ID;
public:
  void SetId(int id)
    {
      ID = id;
    }
  int GetId()
    {
     return ID;
    }
};
class School{
    Student* stud;
    public:

    void* CreateStudent()
    {
      stud = new Student();
      return stud;
    }

    Student* GetStudent()
    {
      return stud;
    }
};

extern "C"
{
    __declspec(dllexport) void Student_SetId(Student* stud ,int id)
   {
     stud->SetId(id);
   }
    __declspec(dllexport) int Student_GetId(Student* stud)
    {
       return (stud->GetId());
    }
    __declspec(dllexport) School* School_new(){
    return new School();
    }

    __declspec(dllexport) void* School_CreateStudent(School* school){
    return (school->CreateStudent());
    }

    __declspec(dllexport)Student* School_GetStudent(School* school){ 
    return (school->GetStudent());
    }
}


#The Python code is:
from ctypes import*
mydll=cdll.LoadLibrary("D:\\shameel\\Cpp\\Sample_class\\Debug\\Sample_class.dll")

class Student():
    def __init__(self):
        self.nativeStudent = mydll.School_CreateStudent()
    def Set_Id(self,arg):
        mydll.Student_SetId(self.nativeStudent,arg)

    def Get_Id(self):
        return (mydll.Student_GetId(self.nativeStudent))

class School():
    def __init__(self):
        self.nativeSchool = mydll.School_new()

    def CreateStudent(self):
        return Student()
    def Get_Student(self):
        mydll.School_GetStudent.restype =  c_void_p
        self.nativestd = mydll.School_GetStudent(self.nativeSchool)
        return (self.nativestd)

#The Python call is as follows:

fobj = School()
std_new = fobj.CreateStudent()
std_new.Set_Id(3)
Id = std_new.Get_Id()
getobj =fobj.Get_Student()
print(getobj)

当我打印 getobj 时，它的输出是“无”。

我如何从 CPP 获得相同的对象？

标签： pythonc++interopctypesdllexport

解决方案

ctypesctypes.c_int除非另有说明，否则假定参数和返回值是 32 位整数 ( )。特别是如果您使用 64 位 Python，指针将被截断为 32 位。首先要做的是正确声明函数参数和返回值：

mydll.Student_SetId.argtypes = c_void_p,c_int
mydll.Student_SetId.restype = None
mydll.Student_GetId.argtypes = c_void_p,
mydll.Student_GetId.restype = c_int
mydll.School_new.argtypes = ()
mydll.School_new.restype = c_void_p
mydll.School_CreateStudent.argtypes = c_void_p,
mydll.School_CreateStudent.restype = c_void_p
mydll.School_GetStudent.argtypes = c_void_p,
mydll.School_GetStudent.restype = c_void_p

这也将帮助您捕获错误，例如School_CreateStudent()需要传递 aSchool*才能正常工作。

要捕获更多错误，请声明特定的指针类型。这是一个显示其类型和示例的小类：

class Ptr(c_void_p):
    def __repr__(self):
        cls_name = self.__class__.__name__
        if self.value is not None:
            return f'{cls_name}({self.value:#x})'
        return f'{cls_name}(None)'

class pStudent(Ptr): pass
class pSchool(Ptr): pass

演示：

>>> p = pSchool()
>>> p
pSchool(None)

然后像这样声明函数参数和结果：

mydll.Student_SetId.argtypes = pStudent,c_int
mydll.Student_SetId.restype = None
mydll.Student_GetId.argtypes = pStudent,
mydll.Student_GetId.restype = c_int
mydll.School_new.argtypes = ()
mydll.School_new.restype = pSchool
mydll.School_CreateStudent.argtypes = pSchool,
mydll.School_CreateStudent.restype = pStudent
mydll.School_GetStudent.argtypes = pSchool,
mydll.School_GetStudent.restype = pStudent

现在，在 Python 中错误使用代码的任何地方都应该给你一个很好的错误消息。

修复 C++ 和 Python 代码后，您的输出print(getobj)将类似于：

pStudent(0x1b54b89b350)

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