tor-browser

MaybeOneOf.h (4868B)

---

/* -*- 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 <algorithm>

#include "mozilla/Assertions.h"
#include "mozilla/OperatorNewExtensions.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 = std::max(alignof(T1), alignof(T2));
 static constexpr size_t StorageSize = std::max(sizeof(T1), sizeof(T2));

 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();
   }
 }

 template <typename Func>
 constexpr auto mapNonEmpty(Func&& aFunc) const {
   MOZ_ASSERT(!empty());
   if (state == SomeT1) {
     return std::forward<Func>(aFunc)(as<T1>());
   }
   return std::forward<Func>(aFunc)(as<T2>());
 }
 template <typename Func>
 constexpr auto mapNonEmpty(Func&& aFunc) {
   MOZ_ASSERT(!empty());
   if (state == SomeT1) {
     return std::forward<Func>(aFunc)(as<T1>());
   }
   return std::forward<Func>(aFunc)(as<T2>());
 }

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 */