what is wrong with c++ streams when using boost.py

问题:

Update 2: I'm not sure why this is still being upvoted (March 2014). This appears to be fixed since I asked this question many years ago. Make sure you're using a recent version of boost.

UPDATE: Perhaps C++ streams need to be initialized in order to format numbers, and the initialization is not happening when the shared library is loaded in Python?

I am calling

cout << 1 << "!" << endl; 

in a method that is exported to a shared library via boost.python. It doesn't print anything, but if I do

cout << "%" << "!" << endl; 

it works.

This is important because I want to do this:

ostream& operator <<(ostream &os, const Bernoulli& b) {
    ostringstream oss;
    oss << b.p() * 100.0 << "%";
    return os << oss.str();
}

I exposed that by doing this:

BOOST_PYTHON_MODULE(libdistributions)
{
    class_<Bernoulli>("Bernoulli")
        .def(init<>())
        .def(init<double>())

        .def("p", &Bernoulli::p)
        .def("set_p", &Bernoulli::set_p)
        .def("not_p", &Bernoulli::not_p)

        .def("Entropy", &Bernoulli::Entropy)
        .def("KL", &Bernoulli::KL)
        .def(self_ns::str(self))
    ;
}

but when I call the str method in python on a Bernoulli object, I get nothing. I suspect the simpler cout problem is related.

回答1:

I also run into this problem a while ago, using self_ns as outlined in the answers here Build problems when adding `__str__` method to Boost Python C++ class

The reason for using self_ns is explained by Dave himself here http://mail.python.org/pipermail/cplusplus-sig/2004-February/006496.html

---

Just for the sake of debugging, please try

inline std::string toString(const Bernoulli& b) 
{
   std::ostringstream s;
   s << b;
   return s.str(); 
}

And replace .def(self_ns::str(self)) with

class_<Bernoulli>("Bernoulli")
[...]
.def("__str__", &toString)

回答2:

Have you tried using boost::format instead? Since you are already using boost, it shouldn't be too much of a hassle.

boost::format( "%d%%" ) % ( b.p() * 100.0 )

Another thing, try passing std::endl explicitly to the output os.

回答3:

have you tried flushing the stream prior to using the .str() method?

oss << b.p() * 100.0 << "%" << std::flush;