1 files changed, 143 insertions, 0 deletions
diff --git a/mfbt/MaybeOneOf.h b/mfbt/MaybeOneOf.h
new file mode 100644
index 000000000..9c38ff8b0
--- /dev/null
+++ b/mfbt/MaybeOneOf.h
@@ -0,0 +1,143 @@
+/* -*- 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_MaybeOneOf_h
+#define mozilla_MaybeOneOf_h
+
+#include "mozilla/Alignment.h"
+#include "mozilla/Assertions.h"
+#include "mozilla/Move.h"
+#include "mozilla/TemplateLib.h"
+
+#include <new>    // For placement new
+
+namespace mozilla {
+
+/*
+ * MaybeOneOf<T1, T2> is like Maybe, but it supports constructing either T1
+ * or T2. When a MaybeOneOf<T1, T2> is constructed, it is |empty()|, i.e.,
+ * no value has been constructed and no destructor will be called when the
+ * MaybeOneOf<T1, T2> is destroyed. Upon calling |construct<T1>()| or
+ * |construct<T2>()|, a T1 or T2 object will be constructed with the given
+ * arguments and that object will be destroyed when the owning MaybeOneOf is
+ * destroyed.
+ */
+template<class T1, class T2>
+class MaybeOneOf
+{
+  AlignedStorage<tl::Max<sizeof(T1), sizeof(T2)>::value> storage;
+
+  enum State { None, SomeT1, SomeT2 } state;
+  template <class T, class Ignored = void> struct Type2State {};
+
+  template <class T>
+  T& as()
+  {
+    MOZ_ASSERT(state == Type2State<T>::result);
+    return *(T*)storage.addr();
+  }
+
+  template <class T>
+  const T& as() const
+  {
+    MOZ_ASSERT(state == Type2State<T>::result);
+    return *(T*)storage.addr();
+  }
+
+public:
+  MaybeOneOf() : state(None) {}
+  ~MaybeOneOf() { destroyIfConstructed(); }
+
+  MaybeOneOf(MaybeOneOf&& rhs)
+    : state(None)
+  {
+    if (!rhs.empty()) {
+      if (rhs.constructed<T1>()) {
+        construct<T1>(Move(rhs.as<T1>()));
+        rhs.as<T1>().~T1();
+      } else {
+        construct<T2>(Move(rhs.as<T2>()));
+        rhs.as<T2>().~T2();
+      }
+      rhs.state = None;
+    }
+  }
+
+  MaybeOneOf &operator=(MaybeOneOf&& rhs)
+  {
+    MOZ_ASSERT(this != &rhs, "Self-move is prohibited");
+    this->~MaybeOneOf();
+    new(this) MaybeOneOf(Move(rhs));
+    return *this;
+  }
+
+  bool empty() const { return state == None; }
+
+  template <class T>
+  bool constructed() const { return state == Type2State<T>::result; }
+
+  template <class T, class... Args>
+  void construct(Args&&... aArgs)
+  {
+    MOZ_ASSERT(state == None);
+    state = Type2State<T>::result;
+    ::new (storage.addr()) T(Forward<Args>(aArgs)...);
+  }
+
+  template <class T>
+  T& ref()
+  {
+    return as<T>();
+  }
+
+  template <class T>
+  const T& ref() const
+  {
+    return as<T>();
+  }
+
+  void destroy()
+  {
+    MOZ_ASSERT(state == SomeT1 || state == SomeT2);
+    if (state == SomeT1) {
+      as<T1>().~T1();
+    } else if (state == SomeT2) {
+      as<T2>().~T2();
+    }
+    state = None;
+  }
+
+  void destroyIfConstructed()
+  {
+    if (!empty()) {
+      destroy();
+    }
+  }
+
+private:
+  MaybeOneOf(const MaybeOneOf& aOther) = delete;
+  const MaybeOneOf& operator=(const MaybeOneOf& aOther) = delete;
+};
+
+template <class T1, class T2>
+template <class Ignored>
+struct MaybeOneOf<T1, T2>::Type2State<T1, Ignored>
+{
+  typedef MaybeOneOf<T1, T2> Enclosing;
+  static const typename Enclosing::State result = Enclosing::SomeT1;
+};
+
+template <class T1, class T2>
+template <class Ignored>
+struct MaybeOneOf<T1, T2>::Type2State<T2, Ignored>
+{
+  typedef MaybeOneOf<T1, T2> Enclosing;
+  static const typename Enclosing::State result = Enclosing::SomeT2;
+};
+
+} // namespace mozilla
+
+#endif /* mozilla_MaybeOneOf_h */