text: /* -*- 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 storing one of two optional value types that supports in-place lazy
   * construction.
   */
  
  #ifndef mozilla_MaybeOneOf_h
  #define mozilla_MaybeOneOf_h
  
  #include <stddef.h> // for size_t
  
  #include <new> // for placement new
  #include <utility>
  
  #include "./Assertions.h"
  #include "./OperatorNewExtensions.h"
  #include "./TemplateLib.h"
  
  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.
   *
   * Because MaybeOneOf must be aligned suitable to hold any value stored within
   * it, and because |alignas| requirements don't affect platform ABI with respect
   * to how parameters are laid out in memory, MaybeOneOf can't be used as the
   * type of a function parameter.  Pass MaybeOneOf to functions by pointer or
   * reference instead.
   */
  template <class T1, class T2> class MOZ_NON_PARAM MaybeOneOf {
    static constexpr size_t StorageAlignment =
        tl::Max<alignof(T1), alignof(T2)>::value;
    static constexpr size_t StorageSize = tl::Max<sizeof(T1), sizeof(T2)>::value;
  
    alignas(StorageAlignment) unsigned char storage[StorageSize];
  
    // GCC fails due to -Werror=strict-aliasing if |storage| is directly cast to
    // T*.  Indirecting through these functions addresses the problem.
    void *data() { return storage; }
    const void *data() const { return 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 *static_cast<T *>(data());
    }
  
    template <class T> const T &as() const {
      MOZ_ASSERT(state == Type2State<T>::result);
      return *static_cast<const T *>(data());
    }
  
  public:
    MaybeOneOf() : state(None) {}
    ~MaybeOneOf() { destroyIfConstructed(); }
  
    MaybeOneOf(MaybeOneOf &&rhs) : state(None) {
      if (!rhs.empty()) {
        if (rhs.constructed<T1>()) {
          construct<T1>(std::move(rhs.as<T1>()));
          rhs.as<T1>().~T1();
        } else {
          construct<T2>(std::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(std::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 (KnownNotNull, data()) T(std::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 */
  