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/**
* @file llsingleton.h
*
* $LicenseInfo:firstyear=2002&license=viewerlgpl$
* Second Life Viewer Source Code
* Copyright (C) 2010, Linden Research, Inc.
*
* This library is free software; you can redistribute it and/or
* modify it under the terms of the GNU Lesser General Public
* License as published by the Free Software Foundation;
* version 2.1 of the License only.
*
* This library 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
* Lesser General Public License for more details.
*
* You should have received a copy of the GNU Lesser General Public
* License along with this library; if not, write to the Free Software
* Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
*
* Linden Research, Inc., 945 Battery Street, San Francisco, CA 94111 USA
* $/LicenseInfo$
*/
#ifndef LLSINGLETON_H
#define LLSINGLETON_H
#include "llerror.h" // *TODO: eliminate this
#include <typeinfo>
#include <boost/noncopyable.hpp>
// LLSingleton implements the getInstance() method part of the Singleton
// pattern. It can't make the derived class constructors protected, though, so
// you have to do that yourself.
//
// There are two ways to use LLSingleton. The first way is to inherit from it
// while using the typename that you'd like to be static as the template
// parameter, like so:
//
// class Foo: public LLSingleton<Foo>{};
//
// Foo& instance = Foo::instance();
//
// The second way is to use the singleton class directly, without inheritance:
//
// typedef LLSingleton<Foo> FooSingleton;
//
// Foo& instance = FooSingleton::instance();
//
// In this case, the class being managed as a singleton needs to provide an
// initSingleton() method since the LLSingleton virtual method won't be
// available
//
// As currently written, it is not thread-safe.
template <typename DERIVED_TYPE>
class LLSingleton : private boost::noncopyable
{
private:
typedef enum e_init_state
{
UNINITIALIZED,
CONSTRUCTING,
INITIALIZING,
INITIALIZED,
DELETED
} EInitState;
static DERIVED_TYPE* constructSingleton()
{
return new DERIVED_TYPE();
}
// stores pointer to singleton instance
struct SingletonLifetimeManager
{
SingletonLifetimeManager()
{
construct();
}
static void construct()
{
sData.mInitState = CONSTRUCTING;
sData.mInstance = constructSingleton();
sData.mInitState = INITIALIZING;
}
~SingletonLifetimeManager()
{
if (sData.mInitState != DELETED)
{
deleteSingleton();
}
}
};
public:
virtual ~LLSingleton()
{
sData.mInstance = NULL;
sData.mInitState = DELETED;
}
/**
* @brief Immediately delete the singleton.
*
* A subsequent call to LLProxy::getInstance() will construct a new
* instance of the class.
*
* LLSingletons are normally destroyed after main() has exited and the C++
* runtime is cleaning up statically-constructed objects. Some classes
* derived from LLSingleton have objects that are part of a runtime system
* that is terminated before main() exits. Calling the destructor of those
* objects after the termination of their respective systems can cause
* crashes and other problems during termination of the project. Using this
* method to destroy the singleton early can prevent these crashes.
*
* An example where this is needed is for a LLSingleton that has an APR
* object as a member that makes APR calls on destruction. The APR system is
* shut down explicitly before main() exits. This causes a crash on exit.
* Using this method before the call to apr_terminate() and NOT calling
* getInstance() again will prevent the crash.
*/
static void deleteSingleton()
{
delete sData.mInstance;
sData.mInstance = NULL;
sData.mInitState = DELETED;
}
static DERIVED_TYPE* getInstance()
{
static SingletonLifetimeManager sLifeTimeMgr;
switch (sData.mInitState)
{
case UNINITIALIZED:
// should never be uninitialized at this point
llassert(false);
return NULL;
case CONSTRUCTING:
llerrs << "Tried to access singleton " << typeid(DERIVED_TYPE).name() << " from singleton constructor!" << llendl;
return NULL;
case INITIALIZING:
// go ahead and flag ourselves as initialized so we can be reentrant during initialization
sData.mInitState = INITIALIZED;
// initialize singleton after constructing it so that it can reference other singletons which in turn depend on it,
// thus breaking cyclic dependencies
sData.mInstance->initSingleton();
return sData.mInstance;
case INITIALIZED:
return sData.mInstance;
case DELETED:
llwarns << "Trying to access deleted singleton " << typeid(DERIVED_TYPE).name() << " creating new instance" << llendl;
SingletonLifetimeManager::construct();
// same as first time construction
sData.mInitState = INITIALIZED;
sData.mInstance->initSingleton();
return sData.mInstance;
}
return NULL;
}
static DERIVED_TYPE* getIfExists()
{
return sData.mInstance;
}
// Reference version of getInstance()
// Preferred over getInstance() as it disallows checking for NULL
static DERIVED_TYPE& instance()
{
return *getInstance();
}
// Has this singleton been created uet?
// Use this to avoid accessing singletons before the can safely be constructed
static bool instanceExists()
{
return sData.mInitState == INITIALIZED;
}
// Has this singleton already been deleted?
// Use this to avoid accessing singletons from a static object's destructor
static bool destroyed()
{
return sData.mInitState == DELETED;
}
private:
virtual void initSingleton() {}
struct SingletonData
{
// explicitly has a default constructor so that member variables are zero initialized in BSS
// and only changed by singleton logic, not constructor running during startup
EInitState mInitState;
DERIVED_TYPE* mInstance;
};
static SingletonData sData;
};
template<typename T>
typename LLSingleton<T>::SingletonData LLSingleton<T>::sData;
#endif
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