Question

I am sorry if this question has been asked before (I tried to find it but in vain).
I was going through Singleton design pattern at http://sourcemaking.com/design_patterns/singleton/cpp/1 (code is copied from there)

class GlobalClass
{
    int m_value;
    static GlobalClass *s_instance;
    GlobalClass(int v = 0)
    {
        m_value = v;
    }
  public:
    int get_value()
    {
        return m_value;
    }
    void set_value(int v)
    {
        m_value = v;
    }
    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 = 0;

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

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

int main()
{
  cout << "main: global_ptr is " << GlobalClass::instance()->get_value() << '\n';
  foo();
  bar();
}

My question is in the lines
if (!s_instance)
s_instance = new GlobalClass;

s_instance = new GlobalClass will call the constructor but constructor is non-static and we are calling it from a static function. How does that work? Is Constructor “special” in this regard?

Thanks!

Answer 1

You can call a constructor from anywhere. If you couldn’t call a constructor without an instance, how would you get any instances in the first place?

You’ve probably called them from plain functions countless times. Static functions are no different.