how to access a singleton data member from another class

singleton.cpp

#include <iostream>

class GlobalClass
{
    int m_value;
    static GlobalClass *s_instance;
    GlobalClass(int v = 0){
        std::cout << "global singleton being born at " << this << std::endl;
        m_value = v;
    };
    ~GlobalClass(){
        std::cout << "global singleton being killed at " << this << std::endl;
    };
public:
    int get_value()
    {
        return m_value;
    }
    void set_value(int v)
    {
        m_value = v;
    }
    void kill_me(){
        std::cout << "global singleton is dying!" << std::endl;
        GlobalClass::~GlobalClass();
    }
    static GlobalClass *instance()
    {
        if (!s_instance)
            s_instance = new GlobalClass;
        return s_instance;
    }
};

// Allocating and initializing GlobalClass's
// static data member.  The pointer is being
// allocated - not the object inself.
GlobalClass *GlobalClass::s_instance = nullptr;

class accessor {
public:
    accessor(int _id) : id(_id){std::cout << "birthing accessor: " << id << std::endl;};
    ~accessor(){std::cout << "killing accessor: " << id << std::endl;};
    
    void get_par(){
        std::cout << "im accessing the global thing " << GlobalClass::instance()->get_value() << std::endl;
    };
private:
    int id;
};

void foo(void)
{
    std::cout << "entering foo" << std::endl;
    std::cout << "foo: global_ptr is " << GlobalClass::instance()->get_value() << '\n';
    GlobalClass::instance()->set_value(1);
    std::cout << "foo: global_ptr is " << GlobalClass::instance()->get_value() << '\n';
    std::cout << "exiting foo" << std::endl;
}

void bar(void)
{
    std::cout << "entering bar" << std::endl;
    std::cout << "bar: global_ptr is " << GlobalClass::instance()->get_value() << '\n';
    GlobalClass::instance()->set_value(2);
    std::cout << "bar: global_ptr is " << GlobalClass::instance()->get_value() << '\n';
    std::cout << "exiting bar" << std::endl;
}

int main()
{
    std::cout << "main: global_ptr is " << GlobalClass::instance()->get_value() << std::endl;
    
    GlobalClass::instance()->set_value(5);
    
    accessor* accessor1 = new accessor(1);
    accessor1->get_par();
    
    foo();
    bar();
    delete accessor1;
    std::cout << "main: global_ptr is " << GlobalClass::instance()->get_value() << std::endl;
    GlobalClass::instance()->kill_me();
}