exposing method with unsigned char & argument using Boost.Python

Question

I've closed-source C++ library, which provides header files with code equivalent to:

class CSomething
{
  public:
      void getParams( unsigned char & u8OutParamOne, 
                      unsigned char & u8OutParamTwo ) const;
  private:
      unsigned char u8OutParamOne_,
      unsigned char u8OutParamTwo_,
};

I'm trying to expose that to Python, my wrapper code is something like this:

BOOST_PYTHON_MODULE(MySomething)
{
    class_<CSomething>("CSomething", init<>())
        .def("getParams", &CSomething::getParams,(args("one", "two")))

}

Now I'm trying to use that in Python, which fails horribly:

one, two = 0, 0
CSomething.getParams(one, two)

Which results in:

ArgumentError: Python argument types in
    CSomething.getParams(CSomething, int, int)
did not match C++ signature:
    getParams(CSomething {lvalue}, unsigned char {lvalue} one, unsigned char {lvalue} two)

What do I need to change either in the Boost.Python wrapper code or Python code to make it work? How do I add some Boost.Python magic to automatically cast PyInt to unsigned char and vice-versa?

Answer 1

Boost.Python is complaining about a missing lvalue parameter, a concept which does not exist in Python:

def f(x):
  x = 1

y = 2
f(y)
print(y) # Prints 2

The x paramter of the f function is not a C++-like reference. In C++ the output is different:

void f(int &x) {
  x = 1;
}

void main() {
  int y = 2;
  f(y);
  cout << y << endl; // Prints 1.
}

You have a few choices here:

a) Wrap the CSomething.getParams function to return a tuple of the new parameters values:

one, two = 0, 0
one, two = CSomething.getParams(one, two)
print(one, two)

b) Wrap the CSomething.getParams function to accept a class instance as parameter:

class GPParameter:
  def __init__(self, one, two):
    self.one = one
    self.two = two

p = GPParameter(0, 0)
CSomething.getParams(p)
print(p.one, p.two)