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