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/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* 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 */
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