--- /dev/null
+/***************************************************************************
+ * Copyright (C) 2005-2018 by the Quassel Project *
+ * devel@quassel-irc.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) version 3. *
+ * *
+ * 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. *
+ ***************************************************************************/
+
+#pragma once
+
+#include <QtGlobal>
+
+/**
+ * Mixin class for "pseudo" singletons.
+ *
+ * Classes inheriting from this mixin become "pseudo" singletons that can still be constructed
+ * and destroyed in a controlled manner, but also gain an instance() method for static access.
+ * This is very similar to the behavior of e.g. QCoreApplication.
+ *
+ * The mixin protects against multiple instantiation, use-before-instantiation and use-after-destruction
+ * by aborting the program. This is intended to find lifetime issues during development; abort()
+ * produces a backtrace that makes it easy to find the culprit.
+ *
+ * The Curiously Recurring Template Pattern (CRTP) is used for the mixin to be able to provide a
+ * correctly typed instance pointer.
+ */
+template<typename T>
+class Singleton
+{
+public:
+ /**
+ * Constructs the mixin.
+ *
+ * The constructor can only be called once; subsequent invocations abort the program.
+ *
+ * @param instance Pointer to the instance being created, i.e. the 'this' pointer of the parent class
+ */
+ Singleton(T *instance)
+ {
+ if (_destroyed) {
+ qFatal("Trying to reinstantiate a destroyed singleton, this must not happen!");
+ abort(); // This produces a backtrace, which is highly useful for finding the culprit
+ }
+ if (_instance) {
+ qFatal("Trying to reinstantiate a singleton that is already instantiated, this must not happen!");
+ abort();
+ }
+ _instance = instance;
+ }
+
+ // Satisfy Rule of Five
+ Singleton(const Singleton &) = delete;
+ Singleton(Singleton &&) = delete;
+ Singleton &operator=(const Singleton &) = delete;
+ Singleton &operator=(Singleton &&) = delete;
+
+ /**
+ * Destructor.
+ *
+ * Sets the instance pointer to null and flags the destruction, so a subsequent reinstantiation will fail.
+ */
+ ~Singleton()
+ {
+ _instance = nullptr;
+ _destroyed = true;
+ }
+
+ /**
+ * Accesses the instance pointer.
+ *
+ * If the singleton hasn't been instantiated yet, the program is aborted. No lazy instantiation takes place,
+ * because the singleton's lifetime shall be explicitly controlled.
+ *
+ * @returns A pointer to the instance
+ */
+ static T *instance()
+ {
+ if (_instance) {
+ return _instance;
+ }
+ qFatal("Trying to access a singleton that has not been instantiated yet");
+ abort();
+ }
+
+private:
+ static T *_instance;
+ static bool _destroyed;
+
+};
+
+template<typename T>
+T *Singleton<T>::_instance{nullptr};
+
+template<typename T>
+bool Singleton<T>::_destroyed{false};