Re: static pointer is always null used in the plugin

From:

Saeed Amrollahi <amrollahi.saeed@gmail.com>

Newsgroups:

comp.lang.c++

Date:

Sun, 23 May 2010 23:15:09 -0700 (PDT)

Message-ID:

<4d1e1c28-757a-4813-af59-66c84fd39c31@k31g2000vbu.googlegroups.com>

On May 24, 8:50 am, ken <rencanji...@gmail.com> wrote:

hello,
class App {

};

class Scene {
static Scene *s_pScene;
static Scene * instance() {
  if (!s_pScene)
    s_pScene = new Scene;
  return s_pScene;};

Scene * Scene::s_pScene = 0;

//---
the App *app is passed to a plugin loaded by LoadLibrary...
but the Scene::instace() return null always, why?

Hi

I think, because you didn't call the Instance member function in app.
Here is a typical use of Singleton pattern and calling Instance
inside App ctor:

#include <iostream>
struct Singleton {
  static Singleton* pSingleton;
  static Singleton* Instance()
  {
    if (!pSingleton)
      pSingleton = new Singleton();
    return pSingleton;
  }
};

struct App {
  App() { Singleton::Instance(); }
};

Singleton* Singleton::pSingleton = 0;
int main()
{
  using namespace std;
  App app;
  if (!Singleton::pSingleton)
    cout << "Imposibble ..." << '\n';
  return 0;
}

Regards,
  -- Saeed Amrollahi