What is the error in this boost.python program?

Question

#include <iostream>
using namespace std; 
char const* greet()
{
   return "hello, world";
}

#include <boost/python.hpp>

BOOST_PYTHON_MODULE(hello_ext)
{
    using namespace boost::python;
    def("greet", greet);
}

I got the following error:

/usr/lib/gcc/x86_64-linux-gnu/5/../../../x86_64-linux-gnu/crt1.o: In function _start':
(.text+0x20): undefined reference to `main'
/tmp/ccg2v42f.o: In function `inithello_ext':
hello_world.cpp:(.text+0x1a): undefined reference to `boost::python::detail::init_module(char const*, void (*)())'
/tmp/ccg2v42f.o: In function `boost::python::type_info::name() const':
hello_world.cpp:(.text._ZNK5boost6python9type_info4nameEv[_ZNK5boost6python9type_info4nameEv]+0x1f): undefined reference to `boost::python::detail::gcc_demangle(char const*)'
/tmp/ccg2v42f.o: In function `boost::python::to_python_value<char const* const&>::operator()(char const* const&) const':
hello_world.cpp:(.text._ZNK5boost6python15to_python_valueIRKPKcEclES5_[_ZNK5boost6python15to_python_valueIRKPKcEclES5_]+0x1b): undefined reference to `boost::python::converter::do_return_to_python(char const*)'
/tmp/ccg2v42f.o: In function `boost::python::to_python_value<char const* const&>::get_pytype() const':
hello_world.cpp:(.text._ZNK5boost6python15to_python_valueIRKPKcE10get_pytypeEv[_ZNK5boost6python15to_python_valueIRKPKcE10get_pytypeEv]+0x9): undefined reference to `PyString_Type'
/tmp/ccg2v42f.o: In function `boost::python::api::object::object()':
hello_world.cpp:(.text._ZN5boost6python3api6objectC2Ev[_ZN5boost6python3api6objectC5Ev]+0xd): undefined reference to `_Py_NoneStruct'
/tmp/ccg2v42f.o: In function `void boost::python::def<char const* (*)()>(char const*, char const* (*)())':
hello_world.cpp:(.text._ZN5boost6python3defIPFPKcvEEEvS3_T_[_ZN5boost6python3defIPFPKcvEEEvS3_T_]+0x50): undefined reference to `boost::python::detail::scope_setattr_doc(char const*, boost::python::api::object const&, char const*)'
/tmp/ccg2v42f.o: In function `boost::python::api::object boost::python::detail::make_function_aux<char const* (*)(), boost::python::default_call_policies, boost::mpl::vector1<char const*> >(char const* (*)(), boost::python::default_call_policies const&, boost::mpl::vector1<char const*> const&)':
hello_world.cpp:(.text._ZN5boost6python6detail17make_function_auxIPFPKcvENS0_21default_call_policiesENS_3mpl7vector1IS4_EEEENS0_3api6objectET_RKT0_RKT1_[_ZN5boost6python6detail17make_function_auxIPFPKcvENS0_21default_call_policiesENS_3mpl7vector1IS4_EEEENS0_3api6objectET_RKT0_RKT1_]+0x67): undefined reference to `boost::python::objects::function_object(boost::python::objects::py_function const&)'
/tmp/ccg2v42f.o: In function `boost::python::objects::py_function_impl_base::py_function_impl_base()':
hello_world.cpp:(.text._ZN5boost6python7objects21py_function_impl_baseC2Ev[_ZN5boost6python7objects21py_function_impl_baseC5Ev]+0x9): undefined reference to `vtable for boost::python::objects::py_function_impl_base'
/tmp/ccg2v42f.o:(.rodata._ZTVN5boost6python7objects23caller_py_function_implINS0_6detail6callerIPFPKcvENS0_21default_call_policiesENS_3mpl7vector1IS6_EEEEEE[_ZTVN5boost6python7objects23caller_py_function_implINS0_6detail6callerIPFPKcvENS0_21default_call_policiesENS_3mpl7vector1IS6_EEEEEE]+0x30): undefined reference to `boost::python::objects::py_function_impl_base::max_arity() const'
/tmp/ccg2v42f.o: In function `boost::python::objects::caller_py_function_impl<boost::python::detail::caller<char const* (*)(), boost::python::default_call_policies, boost::mpl::vector1<char const*> > >::~caller_py_function_impl()':
hello_world.cpp:(.text._ZN5boost6python7objects23caller_py_function_implINS0_6detail6callerIPFPKcvENS0_21default_call_policiesENS_3mpl7vector1IS6_EEEEED2Ev[_ZN5boost6python7objects23caller_py_function_implINS0_6detail6callerIPFPKcvENS0_21default_call_policiesENS_3mpl7vector1IS6_EEEEED5Ev]+0x20): undefined reference to `boost::python::objects::py_function_impl_base::~py_function_impl_base()'
/tmp/ccg2v42f.o:(.rodata._ZTIN5boost6python7objects23caller_py_function_implINS0_6detail6callerIPFPKcvENS0_21default_call_policiesENS_3mpl7vector1IS6_EEEEEE[_ZTIN5boost6python7objects23caller_py_function_implINS0_6detail6callerIPFPKcvENS0_21default_call_policiesENS_3mpl7vector1IS6_EEEEEE]+0x10): undefined reference to `typeinfo for boost::python::objects::py_function_impl_base'
/tmp/ccg2v42f.o: In function `boost::python::converter::expected_pytype_for_arg<char const*>::get_pytype()':
hello_world.cpp:(.text._ZN5boost6python9converter23expected_pytype_for_argIPKcE10get_pytypeEv[_ZN5boost6python9converter23expected_pytype_for_argIPKcE10get_pytypeEv]+0x1b): undefined reference to `boost::python::converter::registry::query(boost::python::type_info)'
hello_world.cpp:(.text._ZN5boost6python9converter23expected_pytype_for_argIPKcE10get_pytypeEv[_ZN5boost6python9converter23expected_pytype_for_argIPKcE10get_pytypeEv]+0x32): undefined reference to `boost::python::converter::registration::expected_from_python_type() const'
collect2: error: ld returned 1 exit status

Answer 1

using namespace boost::python;
// Change to
using namespace; 
boost::python;

You've used the namespace std; function twice. You should only add using namespace std; at the beginning of the program or wherever you are putting it, although you should only use it once.

The problem here seems that you have used that function twice. The program seems to be clashing those two functions.

You are also receving two of these errors where there is an undefined reference.

(.text+0x20): undefined reference to `main'

and

hello_world.cpp:(.text+0x1a): undefined reference to `boost::python::detail::init_module(char const*, void (*)())'

Either you are meant to have the int main() function within the program as this allows you to declare a set of functions, or then there is another issue.

Please give this a try and let me know how it goes.