From c9a35e104bd107dad027bcb35041d9835ddb2fae Mon Sep 17 00:00:00 2001
From: Andreas Kling <kling@serenityos.org>
Date: Thu, 9 Dec 2021 10:02:49 +0100
Subject: [PATCH] AK: Add RefCountForwarder<T>

This is a convenience template that implements reference count
forwarding. This means that an object forwards ref() and unref() to
another object.

We can use this when two ref-counted objects need to keep each other
alive. This situation poses two problems:

- Using 2x RefPtr would cause a ref cycle and leak both objects.
- Using 2x WeakPtr would allow one of them to be destroyed early.

With RefCountForwarder, only one of the objects has a ref count. The
object with the ref count points to the forwarding object by using a
non-counting smart pointer (OwnPtr or NonnullOwnPtr). Thus, both objects
are kept alive by the same ref count, and they can safely point to each
other without worrying about disjoint lifetimes.
---
 AK/RefCountForwarder.h | 32 ++++++++++++++++++++++++++++++++
 1 file changed, 32 insertions(+)
 create mode 100644 AK/RefCountForwarder.h

diff --git a/AK/RefCountForwarder.h b/AK/RefCountForwarder.h
new file mode 100644
index 00000000000..c52360747ff
--- /dev/null
+++ b/AK/RefCountForwarder.h
@@ -0,0 +1,32 @@
+/*
+ * Copyright (c) 2021, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+namespace AK {
+
+template<typename T>
+class RefCountForwarder {
+public:
+    void ref() { m_ref_count_target.ref(); }
+    void unref() { m_ref_count_target.unref(); }
+
+    T& ref_count_target() { return m_ref_count_target; }
+    T const& ref_count_target() const { return m_ref_count_target; }
+
+protected:
+    RefCountForwarder(T& target)
+        : m_ref_count_target(target)
+    {
+    }
+
+private:
+    T& m_ref_count_target;
+};
+
+}
+
+using AK::RefCountForwarder;