G4ThreadLocalSingleton.hh

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// $Id$
//
// ---------------------------------------------------------------
// GEANT 4 class header file
//
// Class Description:
//   This class implements a thread-private "singleton". Being thread
//   private the singleton is not a singleton in the term, but a different
//   instance existis for each thread.
//   This class is a wrapper around the real object that we need to
//   make singleton.
//
// Limitation:
//   The object that is made thread-private singleton, should not
//   contain any G4ThreadLocal data member. Note that in general,
//   if object is to be thread-private it is unnecessary to mark
//   any data-member as G4ThreadLocal.
//   
// Performance issues:
//   This class uses locks and mutexes.
//
// Example:
//   This is the singleton patter often found in G4 (sequential):
//   class G4Class {
//      private:
//         static G4Class* instance;
//         G4Class() { ... }
//      public:
//         static G4Class* GetInstance() {
//              static G4Class theInstance;
//              if ( instance == 0 ) instance = &theInstance;
//              return instance;
//         }
//  };
//  This is transformed to the following to implement a thread-local
//  singleton:
//   class G4Class {
//      private:
//         static G4ThreadLocal G4Class* instance;
//         G4Class() { ... }
//      public:
//         static G4Class* GetInstance() {
//              if ( instance == 0 ) instance = new G4Class;
//              return instance;
//         }
//  };
//  Note that this class also has a memory leak.
//
//  This class can be used as follows:
//   class G4Class {
//      friend class G4ThreadLocalSingleton<G4Class>;
//      private:
//         G4Class() { ... }
//      public:
//         static G4Class* GetInstance() {
//              static G4ThreadLocalSingleton<G4Class> instance;
//              return instance.Instance();
//         }
//  };
//  Each thread has its own instance of G4Class.
//  Deletion of G4Class instances is done at end of program.
//  Note the "friend" statement.
//
// History:
//  28 October 2013: A. Dotti - First implementation

#ifndef G4TLSSINGLETON_HH
#define G4TLSSINGLETON_HH

//Debug this code
//#define g4tlssdebug 1

#include "G4Cache.hh"
#include <list>

//Forward declaration. See G4AutoDelete.hh
namespace G4AutoDelete {
  template<class T>
  void Register(T*);
}

template<class T>
class G4ThreadLocalSingleton : private G4Cache<T*> {
  friend void G4AutoDelete::Register<T>(T*);
public:
  G4ThreadLocalSingleton();
  //Creates thread-local singleton manager

  ~G4ThreadLocalSingleton(); 

  T* Instance() const; 
  //Returns a pointer to a thread-private instance of T

private:
  G4ThreadLocalSingleton( G4ThreadLocalSingleton& rhs);// {}
  void Register(T* i) const; 

  void Clear();

  mutable std::list<T*> instances;
  mutable G4Mutex listm;
};


//=============================================================
// Implementation details follow
//=============================================================
#include "G4AutoLock.hh"
template<class T>
G4ThreadLocalSingleton<T>::G4ThreadLocalSingleton() : G4Cache<T*>() {
  G4MUTEXINIT(listm);
  G4Cache<T*>::Put(static_cast<T*>(0)); 
}

template<class T>
G4ThreadLocalSingleton<T>::~G4ThreadLocalSingleton() {
  Clear();
}

template<class T>
T* G4ThreadLocalSingleton<T>::Instance() const {
    T* instance = G4Cache<T*>::Get();
    if ( instance == static_cast<T*>(0) ) {
      instance = new T;
      G4Cache<T*>::Put( instance );
      Register(instance);
    }
    return instance;
  }

template<class T>
G4ThreadLocalSingleton<T>::G4ThreadLocalSingleton( G4ThreadLocalSingleton&) {}
template<class T>
void G4ThreadLocalSingleton<T>::Register(T* i) const {
  G4AutoLock l(&listm);
  instances.push_back(i);
}


template<class T>
void G4ThreadLocalSingleton<T>::Clear() {
    G4AutoLock l(&listm);
    while ( ! instances.empty() ) 
    {
      T* thisinst = instances.front();
      instances.pop_front();
      if ( thisinst != 0 ) delete thisinst;
    }
  }

#endif //G4TLSSINGLETON_HH