exposing method with unsigned char & argument using Boost.Python - c++

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)