Re: [Boost-users] When using singleton, I get "undefined symbol: _ZN9Singleton8just_oneE" error message

6 Feb 2007


      You must initilize the static variable:

#include 
#include 
#include 
#include 
#include 
#include <utility>

// classes to wrap
class Singleton
{
public:
    static Singleton* get_it() {return &just_one;}

private:
    Singleton() : x(0) {}
public:
    int exchange(int n)  // set x and return the old value
    {
        std::swap(n, x);
        return n;
    }

private:
    int x;
    static Singleton just_one;
};

//initialization
Singleton Singleton::just_one;


On Tue, February 6, 2007 17:01, Jin Sun wrote:
...
I am learning boost python. I have the following code:
//test.cpp------------------------------------------------------------------
-----------------------------
#include 
#include 
#include 
#include 
#include 
#include <utility>
// classes to wrap
class Singleton
{
public:
    static Singleton* get_it() {return &just_one;}
private:
    Singleton() : x(0) {}
public:
    int exchange(int n)  // set x and return the old value
    {
        std::swap(n, x);
        return n;
    }
private:
    int x;
    static Singleton just_one;
};
// Wrapper code
using namespace boost::python;
BOOST_PYTHON_MODULE(singleton)
{
    class_("Singleton", no_init)
        .def("get_it", &Singleton::get_it,
return_value_policy()).staticmethod("get_it")
        .def("exchange", &Singleton::exchange)
        ;
}
//--------------------------------------------------------------------------
------------------------------------------------
My Jamfile is as following:
//--------------------------------------------------------------------------
------------------------------------------------
# This is the top of our own project tree
project-root ;
#   Include definitions needed for Python modules
import python ;
extension singleton                     # Declare a Python extension called
hello
:   hello.cpp                       # source
    # requirements and dependencies for Boost.Python extensions
    <template>@boost/libs/python/build/extension
    ;
//--------------------------------------------------------------------------
----------------------------------------------
When I run it, I get the following error message:
Python 2.5 (r25:51908, Jan 30 2007, 15:03:58)
[GCC 4.0.2 20051125 (Red Hat 4.0.2-8)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
...
...
...
import singleton
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ImportError: ./singleton.so: undefined symbol: _ZN9Singleton8just_oneE
Can anyone help me?
Thanks!!!
Jin Sun
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With Kind Regards,

Ovanes Markarian