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diff --git a/dom/quota/StringifyUtils.h b/dom/quota/StringifyUtils.h
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+++ b/dom/quota/StringifyUtils.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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/. */
+
+#ifndef mozilla_dom_quota_stringifyutils_h__
+#define mozilla_dom_quota_stringifyutils_h__
+
+#include "mozilla/ThreadLocal.h"
+#include "nsTHashSet.h"
+#include "nsLiteralString.h"
+
+namespace mozilla {
+
+// Use these constants for common delimiters. Note that `Stringify` already
+// encloses each call to `DoStringify` with `kStringify[Start/End]Instance`.
+constexpr auto kStringifyDelimiter = "|"_ns;
+constexpr auto kStringifyStartSet = "["_ns;
+constexpr auto kStringifyEndSet = "]"_ns;
+constexpr auto kStringifyStartInstance = "{"_ns;
+constexpr auto kStringifyEndInstance = "}"_ns;
+
+// A Stringifyable class provides a method `Stringify` that returns a string
+// representation of the class content.
+//
+// It's content is just appended by the override of `DoStringify` but
+// `Stringifyable` ensures that we won't call `DoStringify` twice for the same
+// call and instance. A single `DoStringify` function thus needs not to be
+// aware of "should I `Stringify` the content of this member or not", it can
+// just do it always.
+//
+// Stringifyable does not bloat the object by additional members but uses
+// thread local memory only when needed.
+class Stringifyable {
+ public:
+  void Stringify(nsACString& aData);
+
+  static void InitTLS();
+
+ private:
+  virtual void DoStringify(nsACString& aData) = 0;
+
+  bool IsActive();
+  void SetActive(bool aIsActive);
+
+  static MOZ_THREAD_LOCAL(nsTHashSet<Stringifyable*>*)
+      sActiveStringifyableInstances;
+};
+
+}  // namespace mozilla
+
+#endif  // mozilla_dom_quota_stringifyutils_h__