70 lines
1.4 KiB
C++
70 lines
1.4 KiB
C++
// SPDX-License-Identifier: GPL-2.0-or-later
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// Copyright The Music Player Daemon Project
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#ifndef MPD_SAFE_SINGLETON_HXX
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#define MPD_SAFE_SINGLETON_HXX
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#include "Mutex.hxx"
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/**
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* This class manages at most one instance of a specific type. All
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* instances of this class share the one object which gets deleted
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* when the last instance of this class is destructed.
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*
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* This class is thread-safe, but the contained class may not be.
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*/
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template<typename T>
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class SafeSingleton {
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static Mutex mutex;
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static unsigned ref;
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static T *instance;
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public:
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template<typename... Args>
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explicit SafeSingleton(Args&&... args) {
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const std::scoped_lock<Mutex> lock(mutex);
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if (ref == 0)
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instance = new T(std::forward<Args>(args)...);
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/* increment after creating the instance; this way is
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exception-safe, because we must not increment the
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reference counter if we throw */
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++ref;
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}
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~SafeSingleton() noexcept {
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const std::scoped_lock<Mutex> lock(mutex);
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if (--ref > 0)
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return;
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delete std::exchange(instance, nullptr);
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}
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SafeSingleton(const SafeSingleton &) = delete;
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SafeSingleton &operator=(const SafeSingleton &) = delete;
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T *get() {
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return instance;
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}
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T &operator*() noexcept {
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return *instance;
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}
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T *operator->() noexcept {
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return instance;
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}
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};
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template<typename T>
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Mutex SafeSingleton<T>::mutex;
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template<typename T>
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unsigned SafeSingleton<T>::ref;
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template<typename T>
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T *SafeSingleton<T>::instance;
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#endif
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