1 files changed, 45 insertions, 0 deletions
diff --git a/dom/bindings/UnionMember.h b/dom/bindings/UnionMember.h
new file mode 100644
index 0000000000..2fba7910fa
--- /dev/null
+++ b/dom/bindings/UnionMember.h
@@ -0,0 +1,45 @@
+/* -*- 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/. */
+
+/* A class for holding the members of a union. */
+
+#ifndef mozilla_dom_UnionMember_h
+#define mozilla_dom_UnionMember_h
+
+#include "mozilla/Alignment.h"
+#include "mozilla/Attributes.h"
+#include <utility>
+
+namespace mozilla::dom {
+
+// The union type has an enum to keep track of which of its UnionMembers has
+// been constructed.
+template <class T>
+class UnionMember {
+  AlignedStorage2<T> mStorage;
+
+  // Copy construction can't be supported because C++ requires that any enclosed
+  // T be initialized in a way C++ knows about -- that is, by |new| or similar.
+  UnionMember(const UnionMember&) = delete;
+
+ public:
+  UnionMember() = default;
+  ~UnionMember() = default;
+
+  template <typename... Args>
+  T& SetValue(Args&&... args) {
+    new (mStorage.addr()) T(std::forward<Args>(args)...);
+    return *mStorage.addr();
+  }
+
+  T& Value() { return *mStorage.addr(); }
+  const T& Value() const { return *mStorage.addr(); }
+  void Destroy() { mStorage.addr()->~T(); }
+} MOZ_INHERIT_TYPE_ANNOTATIONS_FROM_TEMPLATE_ARGS;
+
+}  // namespace mozilla::dom
+
+#endif  // mozilla_dom_UnionMember_h