Boost.Python: Unable to wrap C++ "operator<<" to expose print functionality in Python

Question

I am trying to wrap a C++ class that handles binary values using Boost.Python. For this class, the "<<" operator has been defined as

std::ostream &operator<<(std::ostream &output, const bin &inbin);

I tried wrapping it with

class_<bin>("bin", init<>())
    .def(str(self));

However, the compilation throws this error:

boost/python/def_visitor.hpp:31:9: error: no matching function for call to
‘boost::python::api::object::visit(boost::python::class_<itpp::bin>&) const’

I am not sure how to resolve this error, anyone have an idea?

Reference: http://www.boost.org/doc/libs/1_31_0/libs/python/doc/tutorial/doc/class_operators_special_functions.html

Answer 1

This somehow seems to be covered in the following SO posts:
what is wrong with c++ streams when using boost.python?
Build problems when adding `__str__` method to Boost Python C++ class

So, according to the previous posts, you should try to substitute

.def(str(self));  

with either

.def(self_ns::str(self)); 

or

.def(self_ns::str(self_ns::self))  

It seems to have resolved the same problem in those cases.

If the above doesn't work, try writing a custom wrapper. For example, you may define a print_wrap function as:

#include <sstream>
std::string print_wrap(const classname &c)
{
  std::ostringstream oss;
  oss << "HelloWorld " << c.var1 << " " << c.var2;
  return oss.str();
}

And then in the class_<classname>("py_classname") definition use,

.def("__str__", &print_wrap)

Then in Python you should be able to get

>>> obj = py_classname(1,2)  #var1=1, var2=2
>>> print obj
>>> HelloWorld 1 2