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2a0942be41
--HG-- extra : rebase_source : 668cd394806203ddfa34bd4f226335ff26c846b5
185 lines
4.8 KiB
C++
185 lines
4.8 KiB
C++
/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
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/* vim: set ts=8 sts=2 et sw=2 tw=80: */
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/* This Source Code Form is subject to the terms of the Mozilla Public
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* License, v. 2.0. If a copy of the MPL was not distributed with this
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* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
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/* A class holding a pair of objects that tries to conserve storage space. */
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#ifndef mozilla_Pair_h
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#define mozilla_Pair_h
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#include "mozilla/Attributes.h"
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#include "mozilla/Move.h"
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#include "mozilla/TypeTraits.h"
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namespace mozilla {
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namespace detail {
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enum StorageType { AsBase, AsMember };
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// Optimize storage using the Empty Base Optimization -- that empty base classes
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// don't take up space -- to optimize size when one or the other class is
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// stateless and can be used as a base class.
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//
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// The extra conditions on storage for B are necessary so that PairHelper won't
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// ambiguously inherit from either A or B, such that one or the other base class
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// would be inaccessible.
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template<typename A, typename B,
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detail::StorageType =
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IsEmpty<A>::value ? detail::AsBase : detail::AsMember,
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detail::StorageType =
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IsEmpty<B>::value && !IsBaseOf<A, B>::value && !IsBaseOf<B, A>::value
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? detail::AsBase
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: detail::AsMember>
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struct PairHelper;
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template<typename A, typename B>
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struct PairHelper<A, B, AsMember, AsMember>
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{
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protected:
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template<typename AArg, typename BArg>
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PairHelper(AArg&& aA, BArg&& aB)
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: mFirstA(Forward<AArg>(aA)),
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mSecondB(Forward<BArg>(aB))
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{}
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A& first() { return mFirstA; }
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const A& first() const { return mFirstA; }
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B& second() { return mSecondB; }
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const B& second() const { return mSecondB; }
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void swap(PairHelper& aOther)
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{
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Swap(mFirstA, aOther.mFirstA);
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Swap(mSecondB, aOther.mSecondB);
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}
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private:
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A mFirstA;
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B mSecondB;
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};
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template<typename A, typename B>
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struct PairHelper<A, B, AsMember, AsBase> : private B
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{
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protected:
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template<typename AArg, typename BArg>
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PairHelper(AArg&& aA, BArg&& aB)
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: B(Forward<BArg>(aB)),
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mFirstA(Forward<AArg>(aA))
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{}
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A& first() { return mFirstA; }
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const A& first() const { return mFirstA; }
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B& second() { return *this; }
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const B& second() const { return *this; }
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void swap(PairHelper& aOther)
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{
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Swap(mFirstA, aOther.mFirstA);
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Swap(static_cast<B&>(*this), static_cast<B&>(aOther));
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}
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private:
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A mFirstA;
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};
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template<typename A, typename B>
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struct PairHelper<A, B, AsBase, AsMember> : private A
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{
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protected:
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template<typename AArg, typename BArg>
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PairHelper(AArg&& aA, BArg&& aB)
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: A(Forward<AArg>(aA)),
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mSecondB(Forward<BArg>(aB))
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{}
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A& first() { return *this; }
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const A& first() const { return *this; }
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B& second() { return mSecondB; }
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const B& second() const { return mSecondB; }
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void swap(PairHelper& aOther)
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{
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Swap(static_cast<A&>(*this), static_cast<A&>(aOther));
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Swap(mSecondB, aOther.mSecondB);
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}
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private:
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B mSecondB;
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};
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template<typename A, typename B>
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struct PairHelper<A, B, AsBase, AsBase> : private A, private B
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{
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protected:
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template<typename AArg, typename BArg>
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PairHelper(AArg&& aA, BArg&& aB)
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: A(Forward<AArg>(aA)),
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B(Forward<BArg>(aB))
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{}
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A& first() { return static_cast<A&>(*this); }
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const A& first() const { return static_cast<A&>(*this); }
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B& second() { return static_cast<B&>(*this); }
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const B& second() const { return static_cast<B&>(*this); }
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void swap(PairHelper& aOther)
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{
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Swap(static_cast<A&>(*this), static_cast<A&>(aOther));
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Swap(static_cast<B&>(*this), static_cast<B&>(aOther));
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}
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};
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} // namespace detail
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/**
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* Pair is the logical concatenation of an instance of A with an instance B.
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* Space is conserved when possible. Neither A nor B may be a final class.
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*
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* It's typically clearer to have individual A and B member fields. Except if
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* you want the space-conserving qualities of Pair, you're probably better off
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* not using this!
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*
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* No guarantees are provided about the memory layout of A and B, the order of
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* initialization or destruction of A and B, and so on. (This is approximately
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* required to optimize space usage.) The first/second names are merely
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* conceptual!
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*/
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template<typename A, typename B>
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struct Pair
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: private detail::PairHelper<A, B>
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{
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typedef typename detail::PairHelper<A, B> Base;
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public:
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template<typename AArg, typename BArg>
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Pair(AArg&& aA, BArg&& aB)
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: Base(Forward<AArg>(aA), Forward<BArg>(aB))
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{}
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/** The A instance. */
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using Base::first;
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/** The B instance. */
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using Base::second;
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/** Swap this pair with another pair. */
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void swap(Pair& aOther) { Base::swap(aOther); }
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private:
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Pair(const Pair&) MOZ_DELETE;
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};
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template<typename A, class B>
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void
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Swap(Pair<A, B>& aX, Pair<A, B>& aY)
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{
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aX.swap(aY);
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}
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} // namespace mozilla
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#endif /* mozilla_Pair_h */
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