Example on how two expose special functions to Python does not compile

[Volker-Weissmann]

The documentation contains many examples on how to use boost for python-c++ interoperation, but one of them does not compile:

#include <boost/python.hpp>
#include <ostream>
using namespace boost::python;
using namespace std;

class Rational {
public:
  operator double() const;
};

Rational pow(Rational, Rational);
Rational abs(Rational);
ostream &operator<<(ostream &, Rational);

BOOST_PYTHON_MODULE(hello) {

  class_<Rational>("Rational")
      .def(float_(self))               // __float__
      .def(pow(self, other<Rational>)) // __pow__
      .def(abs(self))                  // __abs__
      .def(str(self))                  // __str__
      ;
}

$ g++ -I/usr/include/python3.12 -fpermissive main.cpp -c
main.cpp: In function 'void init_module_hello()':
main.cpp:19:37: error: expected primary-expression before ')' token
   19 |       .def(pow(self, other<Rational>)) // __pow__
      |                                     ^

Removing the .def(pow(self, other<Rational>)) // __pow__ changes the error:

In file included from /usr/include/boost/python/object_core.hpp:20,
                 from /usr/include/boost/python/args.hpp:22,
                 from /usr/include/boost/python.hpp:11,
                 from main.cpp:1:
/usr/include/boost/python/def_visitor.hpp: In instantiation of 'static void boost::python::def_visitor_access::visit(const V&, classT&) [with V = boost::python::def_visitor<boost::python::api::object>; classT = boost::python::class_<Rational>]':
/usr/include/boost/python/def_visitor.hpp:67:34:   required from 'void boost::python::def_visitor<DerivedVisitor>::visit(classT&) const [with classT = boost::python::class_<Rational>; DerivedVisitor = boost::python::api::object]'
   67 |         def_visitor_access::visit(*this, c);
      |         ~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~
/usr/include/boost/python/class.hpp:221:22:   required from 'boost::python::class_<T, X1, X2, X3>::self& boost::python::class_<T, X1, X2, X3>::def(const boost::python::def_visitor<Derived>&) [with Derived = boost::python::api::object; W = Rational; X1 = boost::python::detail::not_specified; X2 = boost::python::detail::not_specified; X3 = boost::python::detail::not_specified; self = boost::python::class_<Rational>]'
  221 |         visitor.visit(*this);
      |         ~~~~~~~~~~~~~^~~~~~~
main.cpp:20:11:   required from here
   17 |   class_<Rational>("Rational")
      |   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   18 |       .def(float_(self)) // __float__
      |       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   19 |       .def(abs(self))    // __abs__
      |       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   20 |       .def(str(self))    // __str__
      |       ~~~~^~~~~~~~~~~
/usr/include/boost/python/def_visitor.hpp:31:34: error: no matching function for call to 'boost::python::api::object::visit(boost::python::class_<Rational>&) const'
   31 |         v.derived_visitor().visit(c);
      |         ~~~~~~~~~~~~~~~~~~~~~~~~~^~~
/usr/include/boost/python/object_core.hpp:160:12: note: candidate: 'template<class ClassT, class DocStringT> void boost::python::api::object_operators<U>::visit(ClassT&, const char*, const boost::python::detail::def_helper<DocStringT>&) const [with DocStringT = ClassT; U = boost::python::api::object]'
  160 |       void visit(ClassT& cl, char const* name, python::detail::def_helper<DocStringT> const& helper) const
      |            ^~~~~
/usr/include/boost/python/object_core.hpp:160:12: note:   candidate expects 3 arguments, 1 provided

I use boost 1.86.0 and gcc 14.2.1, both installed using the Arch Linux package manager. Adding --std=c++11, --std=c++14, --std=c++17 or --std=c++20 does not solve the issue.