未定义的引用错误消息-C++[重复]

可能重复:
Undefined reference - C++ linker error

现在,我在以下语句上分别收到“未定义的引用错误消息,指向'GlobalClass::s_instance'和'GlobalClass::instance()':

GlobalClass *GlobalClass::s_instance = 0;

GlobalClass::instance()->set_value(myAddress);  \\ <== undefined reference    
std::cout << "====>address is is " << GlobalClass::instance()->get_value() 

================================================

#ifndef GLOBALCLASS_H_
#define GLOBALCLASS_H_

#include <string>
class GlobalClass {

public:

    std::string get_value();

    void set_value(std::string);

    static GlobalClass *instance();

    static GlobalClass *s_instance;

private:

    std::string m_value;
};

#endif /* GLOBALCLASS_H_ */

===============================================

#include <string>
#include "GlobalClass.h"



/*
 GlobalClass(int v = 0)
 {
 m_value = v;
 }
 */

    static GlobalClass *s_instance;

    std::string GlobalClass::get_value()
    {
        return m_value;
    }

    void GlobalClass::set_value(std::string v)
    {
        m_value = v;
    }

    static GlobalClass *instance() {
        if (!s_instance)
                s_instance = new GlobalClass;
        return s_instance;
    }

===========================================================

#include <iostream>
#include "GlobalClass.h"

using namespace std;

int main() {

    GlobalClass::s_instance = 0; \\ <== undefined reference 


    std::string myAddress = "abc";
    GlobalClass::instance()->set_value(myAddress);  \\ <== undefined reference

    std::cout << "====>address is is " << GlobalClass::instance()->get_value() 
    \\ <== undefined reference

                 << std::endl;
    return 0;

}