[Boost-users] Emulating Python class variables in C++

Subject: [Boost-users] Emulating Python class variables in C++
From: Will Hall (boost_at_[hidden])
Date: 2015-08-02 04:05:43

Hi there,

I am porting a Python library to C++ - and re-exposing back to Python
with boost (suffice to say the Python API is fixed).

I am having problems emulating class variables from boost as C++ static
data members using inheritance. Here's an example:

using namespace boost::python;
using namespace std;

struct A {
    static string value;
};

struct B : public A {
    static string value;
};

string A::value("A");
string B::value("B");

BOOST_PYTHON_MODULE(test)
{
    class_<A>("A")
        .def_readonly("value", &A::value);
    class_<B>("B")
        .def_readonly("value", &B::value);
}

If I don't specify inheritance using bases<>, then this works. But I
need inheritance, so I write:

BOOST_PYTHON_MODULE(test)
{
    class_<A>("A")
        .def_readonly("value", &A::value);
    class_<B, bases<A> >("B");
}

But now in Python, I only have access to A:

    >>> from test import A, B
    >>> A.value
    'A'
    >>> B.value
    'A'

If try and overload B::value, I get an error:

 BOOST_PYTHON_MODULE(test)
{
    class_<A>("A")
        .def_readonly("value", &A::value);
    class_<B, bases<A> >("B")
        .def_readonly("value", &B::value);
}

    >>> from test import A, B
    Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
    AttributeError: can't set attribute

I've also tried switching .def_readwrite, but now the error is:

    >>> from test import A, B
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    Boost.Python.ArgumentError: Python argument types in
        None.None(Boost.Python.StaticProperty)
    did not match C++ signature:
        None(std::string)

So, to summarise, any idea how to provide polymorphic Python class
variables in boost::python?

Thanks in advance for your help.