mfbt/MaybeOneOf.h
author Bryce Van Dyk <bvandyk@mozilla.com>
Mon, 27 Aug 2018 16:25:54 +0000
changeset 488616 259675bd67f96acf1ae0f2ed6115de4331a94d32
parent 475188 bb85c5ee5afc151be0d07ecc48318dc69cfef446
child 505383 6f3709b3878117466168c40affa7bca0b60cf75b
permissions -rw-r--r--
Bug 1486502 - Add widevine CDM headers to third party paths, clang-format ignore. r=sylvestre We wish to keep the widevine headers in the same formatting as upstream to ease comparison and as we do not modify these files. This patch adds the existing headers, as well as another we anticipate pulling down for our next bump (content_decryption_module_proxy.h) to the ignored paths. These files are ignored individually rather than the whole directory they're in, as we also have Mozilla code in that dir. Differential Revision: https://phabricator.services.mozilla.com/D4347

/* -*- 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 "mozilla/Assertions.h"
#include "mozilla/Move.h"
#include "mozilla/OperatorNewExtensions.h"
#include "mozilla/TemplateLib.h"

#include <new> // for placement new
#include <stddef.h> // for size_t

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 */