Implement LazyInit<T>

lib/base/CMakeLists.txt

@@ -35,6 +35,7 @@ set(base_SOURCES
   function.cpp function.hpp function-ti.hpp function-script.cpp functionwrapper.hpp
   initialize.cpp initialize.hpp
   json.cpp json.hpp json-script.cpp
+  lazy-init.hpp
   library.cpp library.hpp
   loader.cpp loader.hpp
   logger.cpp logger.hpp logger-ti.hpp

lib/base/lazy-init.hpp

@@ -0,0 +1,89 @@
+/******************************************************************************
+ * Icinga 2                                                                   *
+ * Copyright (C) 2012-2018 Icinga Development Team (https://icinga.com/)      *
+ *                                                                            *
+ * 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 St, Fifth Floor, Boston, MA 02110-1301, USA.             *
+ ******************************************************************************/
+
+#ifndef LAZY_INIT
+#define LAZY_INIT
+
+#include <atomic>
+#include <functional>
+#include <mutex>
+#include <utility>
+
+namespace icinga
+{
+
+/**
+ * Lazy object initialization abstraction inspired from
+ * <https://docs.microsoft.com/en-us/dotnet/api/system.lazy-1?view=netframework-4.7.2>.
+ *
+ * @ingroup base
+ */
+template<class T>
+class LazyInit
+{
+public:
+	inline
+	LazyInit(std::function<T()> initializer = []() { return T(); }) : m_Initializer(std::move(initializer))
+	{
+		m_Underlying.store(nullptr, std::memory_order_release);
+	}
+
+	LazyInit(const LazyInit&) = delete;
+	LazyInit(LazyInit&&) = delete;
+	LazyInit& operator=(const LazyInit&) = delete;
+	LazyInit& operator=(LazyInit&&) = delete;
+
+	inline
+	~LazyInit()
+	{
+		auto ptr (m_Underlying.load(std::memory_order_acquire));
+
+		if (ptr != nullptr) {
+			delete ptr;
+		}
+	}
+
+	inline
+	T& Get()
+	{
+		auto ptr (m_Underlying.load(std::memory_order_acquire));
+
+		if (ptr == nullptr) {
+			std::unique_lock<std::mutex> lock (m_Mutex);
+
+			ptr = m_Underlying.load(std::memory_order_acquire);
+
+			if (ptr == nullptr) {
+				ptr = new T(m_Initializer());
+				m_Underlying.store(ptr, std::memory_order_release);
+			}
+		}
+
+		return *ptr;
+	}
+
+private:
+	std::function<T()> m_Initializer;
+	std::mutex m_Mutex;
+	std::atomic<T*> m_Underlying;
+};
+
+}
+
+#endif /* LAZY_INIT */