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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
/* A class holding a pair of objects that tries to conserve storage space. */
#ifndef mozilla_CompactPair_h
#define mozilla_CompactPair_h
#include <type_traits>
#include <tuple>
#include <utility>
#include "mozilla/Attributes.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 CompactPairHelper
// 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 =
std::is_empty_v<A> ? detail::AsBase : detail::AsMember,
detail::StorageType = std::is_empty_v<B> &&
!std::is_base_of<A, B>::value &&
!std::is_base_of<B, A>::value
? detail::AsBase
: detail::AsMember>
struct CompactPairHelper;
template <typename A, typename B>
struct CompactPairHelper<A, B, AsMember, AsMember> {
protected:
template <typename... AArgs, std::size_t... AIndexes, typename... BArgs,
std::size_t... BIndexes>
constexpr CompactPairHelper(std::tuple<AArgs...>& aATuple,
std::tuple<BArgs...>& aBTuple,
std::index_sequence<AIndexes...>,
std::index_sequence<BIndexes...>)
: mFirstA(std::forward<AArgs>(std::get<AIndexes>(aATuple))...),
mSecondB(std::forward<BArgs>(std::get<BIndexes>(aBTuple))...) {}
public:
template <typename AArg, typename BArg>
constexpr CompactPairHelper(AArg&& aA, BArg&& aB)
: mFirstA(std::forward<AArg>(aA)), mSecondB(std::forward<BArg>(aB)) {}
constexpr A& first() { return mFirstA; }
constexpr const A& first() const { return mFirstA; }
constexpr B& second() { return mSecondB; }
constexpr const B& second() const { return mSecondB; }
void swap(CompactPairHelper& aOther) {
std::swap(mFirstA, aOther.mFirstA);
std::swap(mSecondB, aOther.mSecondB);
}
private:
A mFirstA;
B mSecondB;
};
template <typename A, typename B>
struct CompactPairHelper<A, B, AsMember, AsBase> : private B {
protected:
template <typename... AArgs, std::size_t... AIndexes, typename... BArgs,
std::size_t... BIndexes>
constexpr CompactPairHelper(std::tuple<AArgs...>& aATuple,
std::tuple<BArgs...>& aBTuple,
std::index_sequence<AIndexes...>,
std::index_sequence<BIndexes...>)
: B(std::forward<BArgs>(std::get<BIndexes>(aBTuple))...),
mFirstA(std::forward<AArgs>(std::get<AIndexes>(aATuple))...) {}
public:
template <typename AArg, typename BArg>
constexpr CompactPairHelper(AArg&& aA, BArg&& aB)
: B(std::forward<BArg>(aB)), mFirstA(std::forward<AArg>(aA)) {}
constexpr A& first() { return mFirstA; }
constexpr const A& first() const { return mFirstA; }
constexpr B& second() { return *this; }
constexpr const B& second() const { return *this; }
void swap(CompactPairHelper& aOther) {
std::swap(mFirstA, aOther.mFirstA);
std::swap(static_cast<B&>(*this), static_cast<B&>(aOther));
}
private:
A mFirstA;
};
template <typename A, typename B>
struct CompactPairHelper<A, B, AsBase, AsMember> : private A {
protected:
template <typename... AArgs, std::size_t... AIndexes, typename... BArgs,
std::size_t... BIndexes>
constexpr CompactPairHelper(std::tuple<AArgs...>& aATuple,
std::tuple<BArgs...>& aBTuple,
std::index_sequence<AIndexes...>,
std::index_sequence<BIndexes...>)
: A(std::forward<AArgs>(std::get<AIndexes>(aATuple))...),
mSecondB(std::forward<BArgs>(std::get<BIndexes>(aBTuple))...) {}
public:
template <typename AArg, typename BArg>
constexpr CompactPairHelper(AArg&& aA, BArg&& aB)
: A(std::forward<AArg>(aA)), mSecondB(std::forward<BArg>(aB)) {}
constexpr A& first() { return *this; }
constexpr const A& first() const { return *this; }
constexpr B& second() { return mSecondB; }
constexpr const B& second() const { return mSecondB; }
void swap(CompactPairHelper& aOther) {
std::swap(static_cast<A&>(*this), static_cast<A&>(aOther));
std::swap(mSecondB, aOther.mSecondB);
}
private:
B mSecondB;
};
template <typename A, typename B>
struct CompactPairHelper<A, B, AsBase, AsBase> : private A, private B {
protected:
template <typename... AArgs, std::size_t... AIndexes, typename... BArgs,
std::size_t... BIndexes>
constexpr CompactPairHelper(std::tuple<AArgs...>& aATuple,
std::tuple<BArgs...>& aBTuple,
std::index_sequence<AIndexes...>,
std::index_sequence<BIndexes...>)
: A(std::forward<AArgs>(std::get<AIndexes>(aATuple))...),
B(std::forward<BArgs>(std::get<BIndexes>(aBTuple))...) {}
public:
template <typename AArg, typename BArg>
constexpr CompactPairHelper(AArg&& aA, BArg&& aB)
: A(std::forward<AArg>(aA)), B(std::forward<BArg>(aB)) {}
constexpr A& first() { return static_cast<A&>(*this); }
constexpr const A& first() const { return static_cast<A&>(*this); }
constexpr B& second() { return static_cast<B&>(*this); }
constexpr const B& second() const { return static_cast<B&>(*this); }
void swap(CompactPairHelper& aOther) {
std::swap(static_cast<A&>(*this), static_cast<A&>(aOther));
std::swap(static_cast<B&>(*this), static_cast<B&>(aOther));
}
};
} // namespace detail
/**
* CompactPair 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.
*
* In general if space conservation is not critical is preferred to use
* std::pair.
*
* It's typically clearer to have individual A and B member fields. Except if
* you want the space-conserving qualities of CompactPair, 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 CompactPair : private detail::CompactPairHelper<A, B> {
typedef typename detail::CompactPairHelper<A, B> Base;
using Base::Base;
template <typename... AArgs, typename... BArgs>
constexpr CompactPair(std::piecewise_construct_t, std::tuple<AArgs...> aFirst,
std::tuple<BArgs...> aSecond)
: Base(aFirst, aSecond, std::index_sequence_for<AArgs...>(),
std::index_sequence_for<BArgs...>()) {}
CompactPair(CompactPair&& aOther) = default;
CompactPair(const CompactPair& aOther) = default;
CompactPair& operator=(CompactPair&& aOther) = default;
CompactPair& operator=(const CompactPair& aOther) = default;
/** The A instance. */
using Base::first;
/** The B instance. */
using Base::second;
/** Swap this pair with another pair. */
void swap(CompactPair& aOther) { Base::swap(aOther); }
};
/**
* MakeCompactPair allows you to construct a CompactPair instance using type
* inference. A call like this:
*
* MakeCompactPair(Foo(), Bar())
*
* will return a CompactPair<Foo, Bar>.
*/
template <typename A, typename B>
CompactPair<std::remove_cv_t<std::remove_reference_t<A>>,
std::remove_cv_t<std::remove_reference_t<B>>>
MakeCompactPair(A&& aA, B&& aB) {
return CompactPair<std::remove_cv_t<std::remove_reference_t<A>>,
std::remove_cv_t<std::remove_reference_t<B>>>(
std::forward<A>(aA), std::forward<B>(aB));
}
/**
* CompactPair equality comparison
*/
template <typename A, typename B>
bool operator==(const CompactPair<A, B>& aLhs, const CompactPair<A, B>& aRhs) {
return aLhs.first() == aRhs.first() && aLhs.second() == aRhs.second();
}
} // namespace mozilla
namespace std {
template <typename A, class B>
void swap(mozilla::CompactPair<A, B>& aX, mozilla::CompactPair<A, B>& aY) {
aX.swap(aY);
}
} // namespace std
#endif /* mozilla_CompactPair_h */