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/*
* Copyright 2003-2018 The Music Player Daemon Project
* http://www.musicpd.org
*
* 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 MPD_SAFE_SINGLETON_HXX
#define MPD_SAFE_SINGLETON_HXX
#include "Mutex.hxx"
/**
* This class manages at most one instance of a specific type. All
* instances of this class share the one object which gets deleted
* when the last instance of this class is destructed.
*
* This class is thread-safe, but the contained class may not be.
*/
template<typename T>
class SafeSingleton {
static Mutex mutex;
static unsigned ref;
static T *instance;
public:
template<typename... Args>
explicit SafeSingleton(Args&&... args) {
const std::lock_guard<Mutex> lock(mutex);
if (ref == 0)
instance = new T(std::forward<Args>(args)...);
/* increment after creating the instance; this way is
exception-safe, because we must not increment the
reference counter if we throw */
++ref;
}
~SafeSingleton() noexcept {
const std::lock_guard<Mutex> lock(mutex);
if (--ref > 0)
return;
delete std::exchange(instance, nullptr);
}
SafeSingleton(const SafeSingleton &) = delete;
SafeSingleton &operator=(const SafeSingleton &) = delete;
T *get() {
return instance;
}
T &operator*() noexcept {
return *instance;
}
T *operator->() noexcept {
return instance;
}
};
template<typename T>
Mutex SafeSingleton<T>::mutex;
template<typename T>
unsigned SafeSingleton<T>::ref;
template<typename T>
T *SafeSingleton<T>::instance;
#endif
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