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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 holding a pair of objects that tries to conserve storage space. */
#ifndef mozilla_Pair_h
#define mozilla_Pair_h
#include "mozilla/Attributes.h"
#include "mozilla/Move.h"
#include "mozilla/TypeTraits.h"
namespace mozilla {
namespace detail {
enum StorageType { AsBase, AsMember };
// Optimize storage using the Empty Base Optimization -- that empty base classes
// don't take up space -- to optimize size when one or the other class is
// stateless and can be used as a base class.
//
// The extra conditions on storage for B are necessary so that PairHelper won't
// ambiguously inherit from either A or B, such that one or the other base class
// would be inaccessible.
template<typename A, typename B,
detail::StorageType =
IsEmpty<A>::value ? detail::AsBase : detail::AsMember,
detail::StorageType =
IsEmpty<B>::value && !IsBaseOf<A, B>::value && !IsBaseOf<B, A>::value
? detail::AsBase
: detail::AsMember>
struct PairHelper;
template<typename A, typename B>
struct PairHelper<A, B, AsMember, AsMember>
{
protected:
template<typename AArg, typename BArg>
PairHelper(AArg&& aA, BArg&& aB)
: mFirstA(Forward<AArg>(aA)),
mSecondB(Forward<BArg>(aB))
{}
A& first() { return mFirstA; }
const A& first() const { return mFirstA; }
B& second() { return mSecondB; }
const B& second() const { return mSecondB; }
void swap(PairHelper& aOther)
{
Swap(mFirstA, aOther.mFirstA);
Swap(mSecondB, aOther.mSecondB);
}
private:
A mFirstA;
B mSecondB;
};
template<typename A, typename B>
struct PairHelper<A, B, AsMember, AsBase> : private B
{
protected:
template<typename AArg, typename BArg>
PairHelper(AArg&& aA, BArg&& aB)
: B(Forward<BArg>(aB)),
mFirstA(Forward<AArg>(aA))
{}
A& first() { return mFirstA; }
const A& first() const { return mFirstA; }
B& second() { return *this; }
const B& second() const { return *this; }
void swap(PairHelper& aOther)
{
Swap(mFirstA, aOther.mFirstA);
Swap(static_cast<B&>(*this), static_cast<B&>(aOther));
}
private:
A mFirstA;
};
template<typename A, typename B>
struct PairHelper<A, B, AsBase, AsMember> : private A
{
protected:
template<typename AArg, typename BArg>
PairHelper(AArg&& aA, BArg&& aB)
: A(Forward<AArg>(aA)),
mSecondB(Forward<BArg>(aB))
{}
A& first() { return *this; }
const A& first() const { return *this; }
B& second() { return mSecondB; }
const B& second() const { return mSecondB; }
void swap(PairHelper& aOther)
{
Swap(static_cast<A&>(*this), static_cast<A&>(aOther));
Swap(mSecondB, aOther.mSecondB);
}
private:
B mSecondB;
};
template<typename A, typename B>
struct PairHelper<A, B, AsBase, AsBase> : private A, private B
{
protected:
template<typename AArg, typename BArg>
PairHelper(AArg&& aA, BArg&& aB)
: A(Forward<AArg>(aA)),
B(Forward<BArg>(aB))
{}
A& first() { return static_cast<A&>(*this); }
const A& first() const { return static_cast<A&>(*this); }
B& second() { return static_cast<B&>(*this); }
const B& second() const { return static_cast<B&>(*this); }
void swap(PairHelper& aOther)
{
Swap(static_cast<A&>(*this), static_cast<A&>(aOther));
Swap(static_cast<B&>(*this), static_cast<B&>(aOther));
}
};
} // namespace detail
/**
* Pair is the logical concatenation of an instance of A with an instance B.
* Space is conserved when possible. Neither A nor B may be a final class.
*
* It's typically clearer to have individual A and B member fields. Except if
* you want the space-conserving qualities of Pair, you're probably better off
* not using this!
*
* No guarantees are provided about the memory layout of A and B, the order of
* initialization or destruction of A and B, and so on. (This is approximately
* required to optimize space usage.) The first/second names are merely
* conceptual!
*/
template<typename A, typename B>
struct Pair
: private detail::PairHelper<A, B>
{
typedef typename detail::PairHelper<A, B> Base;
public:
template<typename AArg, typename BArg>
Pair(AArg&& aA, BArg&& aB)
: Base(Forward<AArg>(aA), Forward<BArg>(aB))
{}
Pair(Pair&& aOther)
: Base(Move(aOther.first()), Move(aOther.second()))
{ }
Pair(const Pair& aOther) = default;
Pair& operator=(Pair&& aOther)
{
MOZ_ASSERT(this != &aOther, "Self-moves are prohibited");
first() = Move(aOther.first());
second() = Move(aOther.second());
return *this;
}
Pair& operator=(const Pair& aOther) = default;
/** The A instance. */
using Base::first;
/** The B instance. */
using Base::second;
/** Swap this pair with another pair. */
void swap(Pair& aOther) { Base::swap(aOther); }
};
template<typename A, class B>
void
Swap(Pair<A, B>& aX, Pair<A, B>& aY)
{
aX.swap(aY);
}
/**
* MakePair allows you to construct a Pair instance using type inference. A call
* like this:
*
* MakePair(Foo(), Bar())
*
* will return a Pair<Foo, Bar>.
*/
template<typename A, typename B>
Pair<typename RemoveCV<typename RemoveReference<A>::Type>::Type,
typename RemoveCV<typename RemoveReference<B>::Type>::Type>
MakePair(A&& aA, B&& aB)
{
return
Pair<typename RemoveCV<typename RemoveReference<A>::Type>::Type,
typename RemoveCV<typename RemoveReference<B>::Type>::Type>(
Forward<A>(aA),
Forward<B>(aB));
}
} // namespace mozilla
#endif /* mozilla_Pair_h */
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