move doubleCheckedInit() to a comphelper header file

For further reuse in more source files.

Change-Id: I2fcbb98a81725e14d6d433f62622d2c48d146de1
Reviewed-on: https://gerrit.libreoffice.org/55763
Tested-by: Jenkins
Reviewed-by: Michael Meeks <michael.meeks@collabora.com>

1 files changed, 64 insertions, 0 deletions

diff --git a/include/comphelper/doublecheckedinit.hxx b/include/comphelper/doublecheckedinit.hxx
new file mode 100644
index 000000000000..200a9c88c42b
--- /dev/null
+++ b/include/comphelper/doublecheckedinit.hxx
@@ -0,0 +1,64 @@
+/* -*- Mode: C++; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 4 -*- */
+/*
+ * This file is part of the LibreOffice project.
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/.
+ *
+ * This file incorporates work covered by the following license notice:
+ *
+ *   Licensed to the Apache Software Foundation (ASF) under one or more
+ *   contributor license agreements. See the NOTICE file distributed
+ *   with this work for additional information regarding copyright
+ *   ownership. The ASF licenses this file to you under the Apache
+ *   License, Version 2.0 (the "License"); you may not use this file
+ *   except in compliance with the License. You may obtain a copy of
+ *   the License at http://www.apache.org/licenses/LICENSE-2.0 .
+ */
+
+#ifndef INCLUDED_COMPHELPER_DOUBLECHECKEDINIT_HXX
+#define INCLUDED_COMPHELPER_DOUBLECHECKEDINIT_HXX
+
+#include <osl/getglobalmutex.hxx>
+
+#include <atomic>
+// HACK: <atomic> includes <stdbool.h>, which in some Clang versions does '#define bool bool',
+// which confuses clang plugins.
+#undef bool
+#include <functional>
+
+namespace comphelper
+{
+/**
+ * Thread-safe singleton creation.
+ *
+ * It is normally sufficient to create singletons using static variables in a function.
+ * This function is only for use cases that have a more complex lifetime of the object,
+ * such as when the object may require specific cleanup or may be created more times
+ * (e.g. when there is a "singleton" per each instance of another object).
+ */
+template <typename Type, typename Function = std::function<Type*()>,
+          typename Guard = osl::MutexGuard, typename GuardCtor = osl::GetGlobalMutex>
+static inline Type* doubleCheckedInit(std::atomic<Type*>& pointer, Function function,
+                                      GuardCtor guardCtor = osl::GetGlobalMutex())
+{
+    Type* p = pointer.load(std::memory_order_acquire);
+    if (!p)
+    {
+        Guard guard(guardCtor());
+        p = pointer.load(std::memory_order_relaxed);
+        if (!p)
+        {
+            p = function();
+            pointer.store(p, std::memory_order_release);
+        }
+    }
+    return p;
+}
+
+} // namespace
+
+#endif // INCLUDED_COMPHELPER_DOUBLECHECKEDINIT_HXX
+
+/* vim:set shiftwidth=4 softtabstop=4 expandtab: */