The Dead Reference Problem

• Circular Singleton Interactions: KDL
1. Keyboard and Display singletons created
2. Log records errors, including errors in constructing/destroying K and D
3. First error instantiates L, registers it for destruction ...
• Illustrates that when multiple singletons interact, we cannot ensure correct order of destruction
• At least: detect the dead reference

// Singleton.h
class Singleton
{
public:
  static Singleton& Instance()
  {
    if (!pInstance)
    {
      if (destroyed)  // check for dead reference
        OnDeadReference();
      else            // create on first call
        Create();
    }
    return *pInstance_;
  }
  virtual ~Singleton()
  {
    pInstancce_ = 0;
    destroyed_ = true;
  }

private:
  static void Create()
  {
    static Singleton theInstance;
    pInstance_ = &theInstance;
  }
  static void OnDeadReference()
  {
    throw std::runtime_error("Dead Reference Detected");
  }
  // variables
  static Singleton* pInstance_;
  static bool       destroyed_;
  // disabled constructors, assignment
  ...
};

// Singleton.cpp
Singleton* Singleton::pInstance_ = 0;
bool       Singleton::destroyed_ = false;