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Diffstat (limited to 'mfbt/Pair.h')
| -rw-r--r-- | mfbt/Pair.h | 219 |
1 files changed, 219 insertions, 0 deletions
diff --git a/mfbt/Pair.h b/mfbt/Pair.h new file mode 100644 index 000000000..ad7b86a29 --- /dev/null +++ b/mfbt/Pair.h @@ -0,0 +1,219 @@ +/* -*- 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 */ |