1 files changed, 92 insertions, 0 deletions
diff --git a/mfbt/MaybeStorageBase.h b/mfbt/MaybeStorageBase.h
new file mode 100644
index 0000000000..2732d78d05
--- /dev/null
+++ b/mfbt/MaybeStorageBase.h
@@ -0,0 +1,92 @@
+/* -*- Mode: C++; tab-width: 2; 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/. */
+
+/* Internal storage class used e.g. by Maybe and Result. This file doesn't
+ * contain any public declarations. */
+
+#ifndef mfbt_MaybeStorageBase_h
+#define mfbt_MaybeStorageBase_h
+
+#include <type_traits>
+#include <utility>
+
+namespace mozilla::detail {
+
+template <typename T>
+constexpr bool IsTriviallyDestructibleAndCopyable =
+    std::is_trivially_destructible_v<T> &&
+    (std::is_trivially_copy_constructible_v<T> ||
+     !std::is_copy_constructible_v<T>);
+
+template <typename T, bool TriviallyDestructibleAndCopyable =
+                          IsTriviallyDestructibleAndCopyable<T>>
+struct MaybeStorageBase;
+
+template <typename T>
+struct MaybeStorageBase<T, false> {
+ protected:
+  using NonConstT = std::remove_const_t<T>;
+
+  union Union {
+    Union() {}
+    explicit Union(const T& aVal) : val{aVal} {}
+    template <typename U,
+              typename = std::enable_if_t<std::is_move_constructible_v<U>>>
+    explicit Union(U&& aVal) : val{std::forward<U>(aVal)} {}
+    template <typename... Args>
+    explicit Union(std::in_place_t, Args&&... aArgs)
+        : val{std::forward<Args>(aArgs)...} {}
+
+    ~Union() {}
+
+    NonConstT val;
+  } mStorage;
+
+ public:
+  MaybeStorageBase() = default;
+  explicit MaybeStorageBase(const T& aVal) : mStorage{aVal} {}
+  explicit MaybeStorageBase(T&& aVal) : mStorage{std::move(aVal)} {}
+  template <typename... Args>
+  explicit MaybeStorageBase(std::in_place_t, Args&&... aArgs)
+      : mStorage{std::in_place, std::forward<Args>(aArgs)...} {}
+
+  const T* addr() const { return &mStorage.val; }
+  T* addr() { return &mStorage.val; }
+};
+
+template <typename T>
+struct MaybeStorageBase<T, true> {
+ protected:
+  using NonConstT = std::remove_const_t<T>;
+
+  union Union {
+    constexpr Union() : dummy() {}
+    constexpr explicit Union(const T& aVal) : val{aVal} {}
+    constexpr explicit Union(T&& aVal) : val{std::move(aVal)} {}
+    template <typename... Args>
+    constexpr explicit Union(std::in_place_t, Args&&... aArgs)
+        : val{std::forward<Args>(aArgs)...} {}
+
+    NonConstT val;
+    char dummy;
+  } mStorage;
+
+ public:
+  constexpr MaybeStorageBase() = default;
+  constexpr explicit MaybeStorageBase(const T& aVal) : mStorage{aVal} {}
+  constexpr explicit MaybeStorageBase(T&& aVal) : mStorage{std::move(aVal)} {}
+
+  template <typename... Args>
+  constexpr explicit MaybeStorageBase(std::in_place_t, Args&&... aArgs)
+      : mStorage{std::in_place, std::forward<Args>(aArgs)...} {}
+
+  constexpr const T* addr() const { return &mStorage.val; }
+  constexpr T* addr() { return &mStorage.val; }
+};
+
+}  // namespace mozilla::detail
+
+#endif