Need help getting started with Boost.Python

Question

I'm trying to build my first Boost.Python example.

#include <iostream>
#include <boost/python.hpp>

using namespace boost::python;


class Hello {

public:
    std::string greet() {
        std::cout << "Hello World" << std::endl;
    }
};


BOOST_PYTHON_MODULE(hello)
{
    class_<Hello>("Hello")
        .def("greet", &Hello::greet);
}

int main() {
    std::cout << "Boost.Python Test" << std::endl;
    Hello hello;
    hello.greet();
    return 0;
}

EDIT: Python development headers were missing, as @cdhowie has pointed out. I have found and included the required header files. Now the linker is complaining:

   10:43:58 **** Build of configuration BoostPythonTest-DPar for project BoostPythonTest 

****
make all 
Building file: ../src/BoostPythonTest.cpp
Invoking: GCC C++ Compiler
/usr/local/bin/g++-4.7 -I/usr/local/Cellar/python3/3.3.0/Frameworks/Python.framework/Versions/3.3/include/python3.3m -I/usr/include -I/usr/local/Cellar/gcc/4.7.2/gcc/include/c++/4.7.2 -O0 -g3 -p -pg -Wall -c -fmessage-length=0 -std=c++11 -MMD -MP -MF"src/BoostPythonTest.d" -MT"src/BoostPythonTest.d" -o "src/BoostPythonTest.o" "../src/BoostPythonTest.cpp"
Finished building: ../src/BoostPythonTest.cpp

Building target: libBoostPythonTest-DPar.dylib
Invoking: MacOS X C++ Linker
/usr/local/bin/g++-4.7 -L/usr/local/Cellar/python3/3.3.0/Frameworks/Python.framework/Versions/3.3/lib/python3.3/config-3.3m -L/usr/local/Cellar/python3/3.3.0/Frameworks/Python.framework/Versions/3.3/lib -L/usr/local/Cellar/boost/1.51.0/lib -std=c++11 -Xlinker -ldl -framework CoreFoundation -lpython3.3m -dynamiclib -o "libBoostPythonTest-DPar.dylib"  ./src/BoostPythonTest.o   -lpython3.3m -lboost_python-mt -lpython3.3
Undefined symbols for architecture x86_64:
  "boost::python::detail::init_module(PyModuleDef&, void (*)())", referenced from:
      _PyInit_hello in BoostPythonTest.o
ld: symbol(s) not found for architecture x86_64
collect2: error: ld returned 1 exit status
make: *** [libBoostPythonTest-DPar.dylib] Error 1

I've already linked to -lpython3.3m -lboost_python-mt -lpython3.3 - what else is missing?

EDIT: I think I've linked to everything which python3.3-config lists. Linking still does not work because of missing symbols.

Answer 1

When this particular linker error occurs, it is often the result of the application building against one version of Python, such as Python 3.x header files, while the boost_python library was built against a difference version, such as 2.x.

In boost/python/module_init.hpp, the init_module function has the following signature when building against Python 3.x:

PyObject* boost::python::detail::init_module(PyModuleDef&, void(*)());

and the following signature when building against Python 2.x:

PyObject* boost::python::detail::init_module(char const* name, void(*)());

As can be seen in the implementation , only one of the functions will be present in the Boost.Python library. Thus, given the Boost.Python library is being linked in, and the linker is only complaining about not being able to resolve the 3.x init_module function, then it is very likely that the Boost.Python library was built against a Python 2.x version, while the application code has been built against Python 3.x header files. You can verify this by dumping the Boost.Python library's symbols and check the init_module signature.

To resolve this, build the application with the same version of Python from which Boost.Python was built. In this case, either:

• Build the application with Python 2.x header files and link against Python 2.x libraries.
• Build Boost.Python against Python 3.x. This documentation describes the steps to build Boost, and this documentation goes into detail for Boost.Python. It may be necessary to explicitly provide the Python executable from which Boost.Python will build against during the bootstrap process by using the --with-python argument.