thread/SafeSingleton: new thread-safe utility class

Makefile.am

@@ -470,6 +470,7 @@ libutil_a_SOURCES = \
 # Multi-threading library
 
 libthread_a_SOURCES = \
+	src/thread/SafeSingleton.hxx \
 	src/thread/Util.cxx src/thread/Util.hxx \
 	src/thread/Name.hxx \
 	src/thread/Slack.hxx \

src/thread/SafeSingleton.hxx

@@ -0,0 +1,82 @@
+/*
+ * Copyright 2003-2017 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 "check.h"
+#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:
+	SafeSingleton() {
+		const std::lock_guard<Mutex> lock(mutex);
+
+		if (ref == 0)
+			instance = new T;
+
+		/* 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);
+	}
+
+	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