Return C++ double to Python?

Question

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.

Answer 1

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.