返回C ++ double到Python？ [英] Return C++ double to Python?

问题描述

所以我使用python在共享的C ++库中调用方法。我有一个问题从C ++返回一个双重的python。我有一个玩具示例展示了这个问题。

这里是python代码（soexample.py）：

 ＃Python从ctypes导入
 import CDLL 
 import numpy as np 
 
＃打开共享CPP库：
 cpplib = CDLL './libsoexample.so'）
 cppobj = cpplib.CPPClass_py（）
 
＃转换为短**？ 
x = cpplib.func_py（cppobj）
 print'x ='，x 
  

这里是C ++（soexample.cpp）：

  #include< iostream> 
 
 using namespace std; 
 
 class CPPClass 
 {
 public：
 CPPClass（）{} 
 
 void func（double& x）
 { 
x = 1.0; 
} 
}; 
 
 //用于python：
 externC{
 CPPClass * CPPClass_py（）{return new CPPClass（）; } 
 double func_py（CPPClass * myClass）
 {
 double x; 
 myClass-> func（x）; 
 return x; 
} 
} 
  

编译：

  g ++ -fPIC -Wall -Wextra -shared -o libsoexample.so soexample.cpp 
  

当我运行时我得到：

  $ python soexample.py 
x = 0 
  

因此结果是类型为整数，值为0.发生了什么？ / p>

我也很好奇通过引用填充数组。

解决方案

从 ctypes文档：

默认情况下，函数返回C int 类型。其他
返回类型可以通过设置
函数对象的 restype 属性来指定。

如果您将 func_py 的使用更改为以下内容，则此方法有效：

  import ctypes 
 
 func_py = cpplib.func_py 
 func_py.restype = ctypes.c_double 
x = func_py（cppobj）
 print 'x ='，x 
  

虽然它可能适用于这种简单的情况， code> CPPClass_py.restype 。

So I am using python to call methods in a shared C++ library. I am having an issue returning a double from the C++ to the python. I have a created a toy example that exhibits the problem. Feel free to compile and try it out.

Here is the python code (soexample.py):

# Python imports
from ctypes import CDLL
import numpy as np

# Open shared CPP library:
cpplib=CDLL('./libsoexample.so')
cppobj = cpplib.CPPClass_py()

# Stuck on converting to short**?
x = cpplib.func_py(cppobj)
print 'x =', x

Here is the C++ (soexample.cpp):

#include <iostream>

using namespace std;

class CPPClass
{
  public:
  CPPClass(){}

  void func(double& x)
  {
    x = 1.0;
  }
};

// For use with python:
extern "C" {
    CPPClass* CPPClass_py(){ return new CPPClass(); }
    double func_py(CPPClass* myClass)
    {      
      double x;  
      myClass->func(x);
      return x;    
    }
}

Compile with:

g++ -fPIC -Wall -Wextra -shared -o libsoexample.so soexample.cpp

When I run I get:

$ python soexample.py
x = 0

So the result is an integer in type and of value 0. What's going on?

I'm also curious about filling arrays by reference.

解决方案

From ctypes documentation:

By default functions are assumed to return the C int type. Other return types can be specified by setting the restype attribute of the function object.

It works if you change your use of func_py to the following:

import ctypes

func_py = cpplib.func_py
func_py.restype = ctypes.c_double
x = func_py(cppobj)
print 'x =', x

While it will probably work for this simple case, you should also specify CPPClass_py.restype as well.

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