Singleton.h

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#pragma once
//------------------------------------------------------------------------------
//------------------------------------------------------------------------------

// 3. Standard Library Headers

// 4. External Library Headers
#include <boost/shared_ptr.hpp>

// 5. Shared Headers

// 6. Non-shared Headers

//----- Namespace declaration --------------------------------------------------

namespace xms
{
template <typename T>
class Singleton
{
public:
  //----------------------------------------------------------------------------
  //----------------------------------------------------------------------------
  static T& Instance(bool a_delete = false, T* a_new = NULL)
  {
    static boost::shared_ptr<T> theSingleInstance;

    if (a_delete)
    {
      if (theSingleInstance)
      {
        theSingleInstance.reset();
      }
    }
    else
    {
      if (NULL == theSingleInstance)
      {
        theSingleInstance.reset(a_new ? a_new : new T);
      }
    }
    if (theSingleInstance)
    {
      return *theSingleInstance;
    }
    else
    {
      return *theSingleInstance.get();
    }
  } // Instance

  //----------------------------------------------------------------------------
  //----------------------------------------------------------------------------
  static void Delete() { Instance(true, boost::shared_ptr<T>()); }
}; // class Singleton
template <typename T>
class SharedSingleton
{
public:
  typedef boost::shared_ptr<T> Ptr; 

  //----------------------------------------------------------------------------
  static Ptr Instance(Ptr a_new = Ptr(), bool a_delete = false)
  {
    static boost::shared_ptr<T> theSingleInstance;

    if (a_delete)
    {
      if (theSingleInstance)
      {
        theSingleInstance.reset();
      }
    }
    else
    {
      // assert((NULL == theSingleInstance) ^ (NULL == a_new)); // init iff not
      // Above commented because need to return whether initialized or not
      if (NULL == theSingleInstance) // init because not
      {
        theSingleInstance = a_new;
      }
    }
    return theSingleInstance;
  } // Instance

  //----------------------------------------------------------------------------
  //----------------------------------------------------------------------------
  static void Delete() { Instance(Ptr(), true); }
}; // class SharedSingleton

} // namespace xms