/* ScummVM - Graphic Adventure Engine
 *
 * ScummVM is the legal property of its developers, whose names
 * are too numerous to list here. Please refer to the COPYRIGHT
 * file distributed with this source distribution.
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License
 * as published by the Free Software Foundation; either version 2
 * of the License, or (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
 *
 */

#ifndef COMMON_SINGLETON_H
#define COMMON_SINGLETON_H

#include "common/noncopyable.h"

namespace Common {

/**
 * Generic template base class for implementing the singleton design pattern.
 */
template<class T>
class Singleton : NonCopyable {
private:
	Singleton<T>(const Singleton<T> &);
	Singleton<T> &operator=(const Singleton<T> &);

	/**
	 * The default object factory used by the template class Singleton.
	 * By specialising this template function, one can make a singleton use a
	 * custom object factory. For example, to support encapsulation, your
	 * singleton class might be pure virtual (or "abstract" in Java terminology),
	 * and you specialise makeInstance to return an instance of a subclass.
	 */
	//template<class T>
#if defined(__WINS__)
//FIXME verify if __WINS__ needs this still
public:
#endif
	static T *makeInstance() {
		return new T();
	}

	static void destroyInstance() {
		delete _singletonRef();
		_singletonRef() = 0;
	}


public:
	static T& instance() {
		// TODO: We aren't thread safe. For now we ignore it since the
		// only thing using this singleton template is the config manager,
		// and that is first instantiated long before any threads.
		// TODO: We don't leak, but the destruction order is nevertheless
		// semi-random. If we use multiple singletons, the destruction
		// order might become an issue. There are various approaches
		// to solve that problem, but for now this is sufficient
		if (!_singletonRef())
			_singletonRef() = T::makeInstance();
		return *_singletonRef();
	}

	static void destroy() {
		T::destroyInstance();
	}
protected:
	Singleton<T>()		{ }
#ifdef __SYMBIAN32__
	virtual ~Singleton()	{ }
#else
	virtual ~Singleton<T>()	{ }
#endif

	typedef T	SingletonBaseType;

#if defined(__VMS) && !defined(__GNUC__)
	// See the port note below: this compiler cannot emit a static DATA member of
	// a class template, so the pointer lives inside a member FUNCTION instead.
	static T *&_singletonRef() {
		static T *ptr = 0;
		return ptr;
	}
#else
	static T *_singleton;

	// Uniform accessor so the code above (and config-manager.cpp) is spelled the
	// same on every platform; on a sane compiler it just names the data member.
	static T *&_singletonRef() {
		return _singleton;
	}
#endif
};

#if defined(__VMS) && !defined(__GNUC__)

// OpenVMS/__DECCXX PORT NOTE - Singleton storage
// ----------------------------------------------
// HP C++ on Alpha cannot cope with a STATIC DATA MEMBER of a class template.
// Both standard spellings fail, in opposite ways:
//
//   template<> T *Singleton<T>::_singleton = 0;   // what upstream's macro does
//       ...compiles with NO diagnostic but emits NOTHING, so every use became
//       %LINK-W-NUDFSYMS "Common::Singleton<X> ::_singleton" (all 12 at once).
//
//   template<class T> T *Singleton<T>::_singleton = 0;   // general template
//       ...CRASHES the back end while writing the symbol's name:
//       %GEM-F-ASSERTION in GEM_ST_GET_NAME <- GEM_OM_WRITE_GLOBALS <-
//       GEM_OM_MODULE_FINI (module finalization, emitting globals).
//
// What this compiler DOES handle reliably is template member FUNCTIONS - every
// Singleton<T>::instance() / destroy() instantiated and linked fine throughout.
// So the storage moves INTO a static member function as a function-local static:
// there is no template static data member anywhere, and the pointer lives in the
// one place the back end is happy to emit.  A function-local static of pointer
// type initialised to 0 is zero-initialised, so there is no dynamic-init guard
// and no construction-order change versus the original.
//
// _singletonRef() replaces every use of _singleton, including the three direct
// uses in common/config-manager.cpp (ConfigManager::defragment).  It returns a
// REFERENCE, so `delete _singletonRef();` and `_singletonRef() = p;` both work
// exactly as the plain member did.
#define DECLARE_SINGLETON(T) \
	template<class DECLARE_SINGLETON_unused> class Singleton_no_op_decl

#else

/**
 * Note that you need to use this macro from the Common namespace.
 *
 * This is because C++ requires initial explicit specialization
 * to be placed in the same namespace as the template.
 */
#define DECLARE_SINGLETON(T) \
	template<> T *Singleton<T>::_singleton = 0

#endif

} // End of namespace Common

#endif
