SWIG Python C++ 结构作为输入/输出参数

Posted 2023-02-16

【中文标题】SWIG Python C++ 结构作为输入/输出参数

【英文标题】：SWIG Python C++ struct as in/out parameter

【发布时间】：2021-12-30 16:10:14

【问题描述】：

老实说，我在这个网站上阅读了很多关于 struct 主题的帖子。但我需要你的帮助。

我有 C 风格的结构

   struct Time
   
      uint16_t  year; //    year with four digits like 2016
      uint8_t   month; // 1 .. 12
      uint8_t   day; // 1 .. 31
      uint8_t   hour; // 0 .. 23, 24 hour representation
      uint8_t   minute; // 0 .. 59
      uint8_t   second; // 0 .. 59
   ;

以及具有成员函数的类，其实现在 DLL 中。

class DeviceInterface 
virtual uint32_t getTime(Time&  time) = 0;
;

其中 uint32_t 值是状态码。

这里是自动生成的 SWIG C++ 代码：

SWIGINTERN PyObject *_wrap_getTime(PyObject *SWIGUNUSEDPARM(self), PyObject *args) 
  PyObject *resultobj = 0;
  DeviceInterface *arg1 = (DeviceInterface *) 0 ;
  Time *arg2 = 0 ;
  void *argp1 = 0 ;
  int res1 = 0 ;
  void *argp2 = 0 ;
  int res2 = 0 ;
  PyObject *swig_obj[2] ;
  uint32_t result;
  
  if (!SWIG_Python_UnpackTuple(args, "getTime", 2, 2, swig_obj)) SWIG_fail;
  res1 = SWIG_ConvertPtr(swig_obj[0], &argp1,SWIGTYPE_p_DeviceInterface, 0 |  0 );
  if (!SWIG_IsOK(res1)) 
    SWIG_exception_fail(SWIG_ArgError(res1), "in method '" "getTime" "', argument " "1"" of type '" "DeviceInterface *""'"); 
  
  arg1 = reinterpret_cast< DeviceInterface * >(argp1);
  res2 = SWIG_ConvertPtr(swig_obj[1], &argp2, SWIGTYPE_p_Time,  0 );
  if (!SWIG_IsOK(res2)) 
    SWIG_exception_fail(SWIG_ArgError(res2), "in method '" "getTime" "', argument " "2"" of type '" "Time &""'"); 
  
  if (!argp2) 
    SWIG_exception_fail(SWIG_ValueError, "invalid null reference " "in method '" "getTime" "', argument " "2"" of type '" "Time &""'"); 
  
  arg2 = reinterpret_cast< Time * >(argp2);
  result = (uint32_t)(arg1)->getTime(*arg2);
  resultobj = SWIG_From_unsigned_SS_int(static_cast< unsigned int >(result));
  return resultobj;
fail:
  return NULL;

从上面的代码中，我看不到返回了什么时间值，即 arg2 变量。那么我需要在 SWIG 接口文件中编写什么来获取状态码和时间？

【参考方案1】：

您可以编写一个类型映射来附加Time 输出参数。 SWIG 为可以根据需要通过SWIG_NewPointerObj() 生成的结构生成代理。完整示例如下：

DeviceInterface.h（最小实现）

#include <stdint.h>

struct Time

    uint16_t year;   // year with four digits like 2016
    uint8_t  month;  // 1 .. 12
    uint8_t  day;    // 1 .. 31
    uint8_t  hour;   // 0 .. 23, 24 hour representation
    uint8_t  minute; // 0 .. 59
    uint8_t  second; // 0 .. 59
;

class DeviceInterface 
public:
    virtual uint32_t getTime(Time& time) 
        time.year = 2021;
        time.month = 12;
        time.day = 30;
        time.hour = 9;
        time.minute = 47;
        time.second = 30;
        return 0;
    
;

test.i

%module test

%
#include "DeviceInterface.h"
%

%include <stdint.i>

// Do not require an input parameter for Time.
// Instead, SWIG will allocate one.
%typemap(in,numinputs=0) Time& %
    $1 = new Time;
%

// After calling the function, process Time as an output.
// Convert the allocated pointer to a SWIG Python wrapper
// and pass ownership to Python.  Python will free the object
// when it goes out of scope.
%typemap(argout) Time& (PyObject* tmp) %
    // Convert C pointer to SWIG Python wrapper
    tmp = SWIG_NewPointerObj($1, $1_descriptor, SWIG_POINTER_OWN);
    // Append to existing uint32_t return value
    $result = SWIG_Python_AppendOutput($result, tmp);
%

%include "DeviceInterface.h"

演示：

>>> import test
>>> d=test.DeviceInterface()
>>> r=d.getTime()
>>> r
[0, <test.Time; proxy of <Swig Object of type 'Time *' at 0x000001ED4F866090> >]
>>> r[1].year
2021
>>> r[1].month
12

如果您想自定义 SWIG 包装器的显示以使 Time 更易于阅读，您可以使用以下方法扩展 Time 对象：

%module test

%
// Added to support the __repr__ implementation
#include <string>
#include <sstream>

#include "DeviceInterface.h"
%

%include <stdint.i>
%include <std_string.i> // SWIG support for std::string.

%typemap(in,numinputs=0) Time& %
    $1 = new Time;
%

%typemap(argout) Time& (PyObject* tmp) %
    tmp = SWIG_NewPointerObj($1, $1_descriptor, SWIG_POINTER_OWN);
    $result = SWIG_Python_AppendOutput($result, tmp);
%

// Extend time to suport Python's __repr__.
// It must return a string representing how to display the object in Python.
%extend Time 
    std::string __repr__()
    
        std::ostringstream ss;
        ss << "Time(year=" << $self->year << ", month=" << (unsigned)$self->month
           << ", day=" << (unsigned)$self->day << ", hour=" << (unsigned)$self->hour
           << ", minute=" << (unsigned)$self->minute << ", second=" << (unsigned)$self->second << ")";
        return ss.str();
    


%include "DeviceInterface.h"

演示：

>>> import test
>>> d=test.DeviceInterface()
>>> r=d.getTime()
>>> r
[0, Time(year=2021, month=12, day=30, hour=9, minute=47, second=30)]

【讨论】：

+100500 太棒了，谢谢。如果我返回由Py_BuildValue 打包的元组而不是由SWIG_Python_AppendOutput 创建的列表，可以吗？

@Den SWIG_Python_AppendOutput 很聪明。如果正确处理 void 返回值和类型图的多个实例，例如您之前的问题，它有两个输出参数。如果您正在考虑构建时间元组而不是代理 Time 对象，请注意 Time 代理可以传递给采用 Time 对象的其他函数，而元组必须再次转换回 Time 对象。

我的意思不是返回状态码和时间对象的列表，而是返回一个元组，以便获得以下status_code, time = d.getTime()

@Den 你也可以用一个列表来做到这一点。它不需要是一个元组