C ++和Boost python简单函数

Question

I built this .so

#include <vector>

#include <boost/python.hpp>
#include <boost/python/suite/indexing/vector_indexing_suite.hpp>

extern "C"
{
    // A function adding two integers and returning the result
    int SampleAddInt(int i1, int i2)
    {
        return i1 + i2;
    }

    // A function doing nothing ;)
    void SampleFunction1()
    {
        // insert code here
    }

    // A function always returning zero
    int SampleFunction2()
    {
        // insert code here

        return 0;
    }

    char const* greet()
    {
        return "hello, world";
    }
}

#include <iostream>
#include <string>

class hello
{
public:
    hello(const std::string& country)
    {
        this->country = country;
    }
    std::string greet() const
    {
        return "Hello from " + country;
    }
private:
    std::string country;
};

// A function taking a hello object as an argument.
std::string invite(const hello& w)
{
    return w.greet() + "! Please come soon!";
}

boost::python::tuple HeadAndTail(boost::python::object sequence)
{
    return make_tuple(sequence[0], sequence[-1]);
}

namespace py = boost::python;

BOOST_PYTHON_MODULE(hello_ext)
{
    using namespace boost::python;

    def("greet", greet);
    def("SampleAddInt", SampleAddInt);
    def("HeadAndTail", HeadAndTail);

    // Create the Python type object for our extension class and define __init__ function.
    boost::python::class_<hello>("hello", init<std::string>())
    .def("greetclass", &hello::greet)  // Add a regular member function.
    .def("invite", invite)  // Add invite() as a regular function to the module.
    ;

    def("invite", invite); // Even better, invite() can also be made a member of module!!!
}

Linked it with boost_python.

In python, I then say: (with the correct path to the .so)

from ctypes import cdll
mydll = cdll.LoadLibrary('libHelloWorldPythonCpp.so')

#    import hello_ext

print mydll.greet()
vec = [-4, -2, 0, 2, 4]
print vec
print mydll.HeadAndTail(vec)

However I get a strange value when I call mydll.greet() of

-2015371328

The import hello_ext code that is commented out gives an error if I remove the comment, ImportError: No module named hello_ext .

However

print mydll.SampleAddInt(6, 3)

Works, but I can't access the rest of the code like invite HeadAndTail etc. For example

AttributeError: /home/idf/Documents/BOOST_Python/HelloWorldPythonCpp/bin/Release/libHelloWorldPythonCpp.so: undefined symbol: HeadAndTail

If I move

boost::python::tuple HeadAndTail(boost::python::object sequence)
{
    return make_tuple(sequence[0], sequence[-1]);
}

inside the extern "C" then it seems to work. However then I get an error when I pass it vec:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python2.7/dist-packages/spyderlib/widgets/externalshell/sitecustomize.py", line 540, in runfile
    execfile(filename, namespace)
  File "/home/idf/.spyder2/.temp.py", line 17, in <module>
    print mydll.HeadAndTail(vec)
ctypes.ArgumentError: argument 1: <type 'exceptions.TypeError'>: Don't know how to convert parameter 1
>>> 

I did a

idf@C55t-A:~/Documents/BOOST_Python/HelloWorldPythonCpp/bin/Release$ readelf -Ws libHelloWorldPythonCpp.so | grep -i sample
   118: 0000000000008430     2 FUNC    GLOBAL DEFAULT   11 SampleFunction1
   120: 0000000000008440     3 FUNC    GLOBAL DEFAULT   11 SampleFunction2
   234: 0000000000008410     4 FUNC    GLOBAL DEFAULT   11 SampleAddInt
idf@C55t-A:~/Documents/BOOST_Python/HelloWorldPythonCpp/bin/Release$ 

And even

idf@C55t-A:~/Documents/BOOST_Python/HelloWorldPythonCpp/bin/Release$ readelf -Ws libHelloWorldPythonCpp.so | grep -i head  
   230: 0000000000008560   483 FUNC    GLOBAL DEFAULT   11 _Z11HeadAndTailN5boost6python3api6objectE
idf@C55t-A:~/Documents/BOOST_Python/HelloWorldPythonCpp/bin/Release$ 

So HeadAndTail seems to be mangled.

1. What am I missing in the greet etc case?
2. Why does import hello_ext give an error?
3. How do I call HeadAndTail which appears to be c++
4. What type is boost::python::object sequence in Python?
5. How do I call the function "greetclass" inside the class?

EDIT:

I just downloaded this

https://github.com/TNG/boost-python-examples

and everything compiles and seems to run fine. I don't understand what I am doing wrong, but when I find out I will post an answer.

Answer 1

This is pretty dumb on my part.

Ok, here are the problems

1. I am using spyder IDE without setting the working directory to the path of the .so. If run from the command line, there are probably complicated PYTHONPATH type of things that could work, but having .so in the same directory from where your run the .py file works.
2. You don't need any of this from ctypes import cdll mydll = cdll.LoadLibrary crap. Probably name your .so the same name you give in BOOST_PYTHON_MODULE(same_name_as_lib_name). So if your lib is named hello.so, put hello inside of BOOST_PYTHON_MODULE(). Then jus say import hello and then call the functions like print hello.greet()
3. I didn't need to say extern "C" at all.