Variant.h

Go to the documentation of this file.

#pragma once
#include <aws/common/assert.h>
#include <aws/crt/Utility.h>
 
#include <algorithm>
#include <type_traits>
#include <utility>
 
namespace Aws
{
    namespace Crt
    {
        namespace VariantDetail
        {
            template <typename T> constexpr const T &ConstExprMax(const T &a, const T &b)
            {
                return (a < b) ? b : a;
            }
 
            namespace ParameterPackSize
            {
                // Returns a max of sizeof(T) over all T in a template parameter pack
                template <typename Last> constexpr std::size_t GetMaxSizeOf(std::size_t curMax = 0)
                {
                    return ConstExprMax(curMax, sizeof(Last));
                }
 
                template <typename First, typename Second, typename... Rest>
                constexpr std::size_t GetMaxSizeOf(std::size_t curMax = 0)
                {
                    return ConstExprMax(curMax, GetMaxSizeOf<Second, Rest...>(ConstExprMax(curMax, sizeof(First))));
                }
 
                // some old gcc versions does not work with alignas(Ts..)
                template <typename Last> constexpr std::size_t AlignAsPack(std::size_t curMax = 0)
                {
                    return ConstExprMax(curMax, alignof(Last));
                }
 
                template <typename First, typename Second, typename... Rest>
                constexpr std::size_t AlignAsPack(std::size_t curMax = 0)
                {
                    return ConstExprMax(curMax, AlignAsPack<Second, Rest...>(ConstExprMax(curMax, alignof(First))));
                }
            } // namespace ParameterPackSize
 
            namespace Index
            {
                using VariantIndex = short;
 
                template <typename T, typename Last> constexpr VariantIndex GetIndexOf(VariantIndex curIndex = 0)
                {
                    return std::is_same<T, Last>::value ? curIndex : -1;
                }
 
                template <typename T, typename First, typename Second, typename... Rest>
                constexpr VariantIndex GetIndexOf(VariantIndex curIndex = 0)
                {
                    return std::is_same<T, First>::value ? curIndex : GetIndexOf<T, Second, Rest...>(++curIndex);
                }
            } // namespace Index
 
            namespace Checker
            {
                // Returns True if the template parameter pack Ts has a type T, i.e. ContainsType<T, Ts>() == true if T
                // is in the list of Ts
                template <typename T, typename Last> constexpr bool ContainsType()
                {
                    return std::is_same<T, Last>::value;
                }
 
                template <typename T, typename First, typename Second, typename... Rest> constexpr bool ContainsType()
                {
                    return std::is_same<T, First>::value || ContainsType<T, Second, Rest...>();
                }
 
                // a case when the template parameter pack is empty (i.e. Variant<>)
                template <typename T> constexpr bool ContainsType()
                {
                    return false;
                }
 
                template <typename T, typename... Ts> struct HasType
                {
                    static const bool value = ContainsType<T, Ts...>();
                };
            } // namespace Checker
#if defined(AWS_CRT_ENABLE_VARIANT_DEBUG)
            namespace VariantDebug
            {
                template <typename... Ts> class VariantDebugBrowser
                {
                  public:
                    VariantDebugBrowser(char *storage) { InitTuple<0, Ts...>(storage); }
                    std::tuple<typename std::add_pointer<Ts>::type...> as_tuple;
 
                  private:
                    template <IndexT Index, typename First, typename Second, typename... Rest>
                    void InitTuple(char *storage)
                    {
                        First *value = reinterpret_cast<First *>(storage);
                        std::get<Index>(as_tuple) = value;
                        InitTuple<Index + 1, Second, Rest...>(storage);
                    }
 
                    template <IndexT Index, typename Last> void InitTuple(char *storage)
                    {
                        Last *value = reinterpret_cast<Last *>(storage);
                        std::get<Index>(as_tuple) = value;
                    }
                };
            } // namespace VariantDebug
#endif /* defined(AWS_CRT_ENABLE_VARIANT_DEBUG) */
        } // namespace VariantDetail
 
        template <std::size_t Index, typename... Ts> class VariantAlternative;
 
        template <typename... Ts> class Variant
        {
          private:
            template <std::size_t Index> using ThisVariantAlternative = VariantAlternative<Index, Ts...>;
 
            template <typename OtherT>
            using EnableIfOtherIsThisVariantAlternative = typename std::
                enable_if<VariantDetail::Checker::HasType<typename std::decay<OtherT>::type, Ts...>::value, int>::type;
 
          public:
            using IndexT = VariantDetail::Index::VariantIndex;
            static constexpr std::size_t AlternativeCount = sizeof...(Ts);
 
            Variant()
            {
                using FirstAlternative = typename ThisVariantAlternative<0>::type;
                new (m_storage) FirstAlternative();
                m_index = 0;
            }
 
            Variant(const Variant &other)
            {
                AWS_FATAL_ASSERT(other.m_index != -1);
                m_index = other.m_index;
                VisitorUtil<0, Ts...>::VisitBinary(this, other, CopyMoveConstructor());
            }
 
            Variant(Variant &&other)
            {
                AWS_FATAL_ASSERT(other.m_index != -1);
                m_index = other.m_index;
                VisitorUtil<0, Ts...>::VisitBinary(this, std::move(other), CopyMoveConstructor());
            }
 
            template <typename T, EnableIfOtherIsThisVariantAlternative<T> = 1> Variant(const T &val)
            {
                static_assert(
                    VariantDetail::Checker::HasType<typename std::decay<T>::type, Ts...>::value,
                    "This variant does not have such alternative T.");
                static_assert(
                    sizeof(T) <= STORAGE_SIZE,
                    "Attempting to instantiate a Variant with a type bigger than all alternatives.");
 
                using PlainT = typename std::decay<T>::type;
                new (m_storage) PlainT(val);
                m_index = VariantDetail::Index::GetIndexOf<PlainT, Ts...>();
                AWS_ASSERT(m_index != -1);
            }
 
            template <typename T, EnableIfOtherIsThisVariantAlternative<T> = 1> Variant(T &&val)
            {
                static_assert(
                    VariantDetail::Checker::HasType<typename std::decay<T>::type, Ts...>::value,
                    "This variant does not have such alternative T.");
                static_assert(
                    sizeof(T) <= STORAGE_SIZE,
                    "Attempting to instantiate a Variant with a type bigger than all alternatives.");
 
                using PlainT = typename std::decay<T>::type;
                new (m_storage) PlainT(std::forward<T>(val));
                m_index = VariantDetail::Index::GetIndexOf<PlainT, Ts...>();
                AWS_ASSERT(m_index != -1);
            }
 
            // An overload to initialize with an Alternative T in-place
            template <typename T, typename... Args> explicit Variant(Aws::Crt::InPlaceTypeT<T>, Args &&...args)
            {
                static_assert(
                    VariantDetail::Checker::HasType<typename std::decay<T>::type, Ts...>::value,
                    "This variant does not have such alternative T.");
                static_assert(
                    sizeof(T) <= STORAGE_SIZE,
                    "Attempting to instantiate a Variant with a type bigger than all alternatives.");
 
                using PlainT = typename std::decay<T>::type;
                new (m_storage) PlainT(std::forward<Args>(args)...);
                m_index = VariantDetail::Index::GetIndexOf<PlainT, Ts...>();
                AWS_ASSERT(m_index != -1);
            }
 
            Variant &operator=(const Variant &other)
            {
                if (this != &other)
                {
                    AWS_FATAL_ASSERT(other.m_index != -1);
                    if (m_index != other.m_index)
                    {
                        Destroy();
                        m_index = other.m_index;
                        VisitorUtil<0, Ts...>::VisitBinary(this, other, CopyMoveConstructor());
                    }
                    else
                    {
                        VisitorUtil<0, Ts...>::VisitBinary(this, other, CopyMoveAssigner());
                    }
                }
                return *this;
            }
 
            Variant &operator=(Variant &&other)
            {
                if (this != &other)
                {
                    AWS_FATAL_ASSERT(other.m_index != -1);
                    if (m_index != other.m_index)
                    {
                        Destroy();
                        m_index = other.m_index;
                        VisitorUtil<0, Ts...>::VisitBinary(this, std::move(other), CopyMoveConstructor());
                    }
                    else
                    {
                        VisitorUtil<0, Ts...>::VisitBinary(this, std::move(other), CopyMoveAssigner());
                    };
                }
                return *this;
            }
 
            /* emplace */
            template <typename T, typename... Args, EnableIfOtherIsThisVariantAlternative<T> = 1>
            T &emplace(Args &&...args)
            {
                static_assert(
                    VariantDetail::Checker::HasType<typename std::decay<T>::type, Ts...>::value,
                    "This variant does not have such alternative T.");
                static_assert(
                    sizeof(T) <= STORAGE_SIZE,
                    "Attempting to instantiate a Variant with a type bigger than all alternatives.");
 
                Destroy();
 
                using PlainT = typename std::decay<T>::type;
                new (m_storage) PlainT(std::forward<Args>(args)...);
                m_index = VariantDetail::Index::GetIndexOf<PlainT, Ts...>();
                AWS_ASSERT(m_index != -1);
 
                T *value = reinterpret_cast<T *>(m_storage);
                return *value;
            }
 
            template <std::size_t Index, typename... Args>
            auto emplace(Args &&...args) -> typename ThisVariantAlternative<Index>::type &
            {
                static_assert(Index < AlternativeCount, "Unknown alternative index to emplace");
                using AlternativeT = typename ThisVariantAlternative<Index>::type;
 
                return emplace<AlternativeT, Args...>(std::forward<Args>(args)...);
            }
 
            template <typename T, EnableIfOtherIsThisVariantAlternative<T> = 1> bool holds_alternative() const
            {
                AWS_ASSERT(m_index != -1);
                return m_index == VariantDetail::Index::GetIndexOf<T, Ts...>();
            }
 
            /* non-const get */
            template <typename T, EnableIfOtherIsThisVariantAlternative<T> = 1> T &get()
            {
                AWS_FATAL_ASSERT(holds_alternative<T>());
                T *value = reinterpret_cast<T *>(m_storage);
                return *value;
            }
 
            template <typename T, EnableIfOtherIsThisVariantAlternative<T> = 1> T *get_if()
            {
                if (holds_alternative<T>())
                {
                    T *value = reinterpret_cast<T *>(m_storage);
                    return value;
                }
                else
                {
                    return nullptr;
                }
            }
 
            template <std::size_t Index> auto get() -> typename ThisVariantAlternative<Index>::type &
            {
                static_assert(Index < AlternativeCount, "Unknown alternative index to get");
                AWS_FATAL_ASSERT(holds_alternative<Index>());
                using AlternativeT = typename ThisVariantAlternative<Index>::type;
                AlternativeT *ret = reinterpret_cast<AlternativeT *>(m_storage);
                return *ret;
            }
 
            /* const get */
            template <typename T, EnableIfOtherIsThisVariantAlternative<T> = 1> const T &get() const
            {
                AWS_FATAL_ASSERT(holds_alternative<T>());
                const T *value = reinterpret_cast<const T *>(m_storage);
                return *value;
            }
 
            template <typename T, EnableIfOtherIsThisVariantAlternative<T> = 1> const T *get_if() const
            {
                if (holds_alternative<T>())
                {
                    T *value = reinterpret_cast<T *>(m_storage);
                    return value;
                }
                else
                {
                    return nullptr;
                }
            }
 
            template <std::size_t Index> auto get() const -> const typename ThisVariantAlternative<Index>::type &
            {
                static_assert(Index < AlternativeCount, "Unknown alternative index to get");
                AWS_ASSERT(Index == m_index);
                using AlternativeT = typename ThisVariantAlternative<Index>::type;
                const AlternativeT *ret = reinterpret_cast<const AlternativeT *>(m_storage);
                return *ret;
            }
 
            /* This is just a templated way to say
             * "int*" for
             * a VariantAlternative<0, Variant<int, char, long>()>*/
            template <std::size_t Index>
            using RawAlternativePointerT =
                typename std::add_pointer<typename ThisVariantAlternative<Index>::type>::type;
 
            template <std::size_t Index> auto get_if() -> RawAlternativePointerT<Index>
            {
                static_assert(Index < AlternativeCount, "Unknown alternative index to get");
                if (holds_alternative<Index>())
                {
                    using AlternativePtrT = RawAlternativePointerT<Index>;
                    AlternativePtrT value = reinterpret_cast<AlternativePtrT>(m_storage);
                    return value;
                }
                else
                {
                    return nullptr;
                }
            }
 
            template <std::size_t Index>
            using ConstRawAlternativePointerT = typename std::add_pointer<
                typename std::add_const<typename ThisVariantAlternative<Index>::type>::type>::type;
 
            template <std::size_t Index> auto get_if() const -> ConstRawAlternativePointerT<Index>
            {
                static_assert(Index < AlternativeCount, "Unknown alternative index to get");
                if (holds_alternative<Index>())
                {
                    using AlternativePtrT = ConstRawAlternativePointerT<Index>;
                    AlternativePtrT value = reinterpret_cast<AlternativePtrT>(m_storage);
                    return value;
                }
                else
                {
                    return nullptr;
                }
            }
 
            std::size_t index() const { return m_index; }
 
            ~Variant() { Destroy(); }
 
            template <typename VisitorT> void Visit(VisitorT &&visitor)
            {
                return VisitorUtil<0, Ts...>::Visit(this, std::forward<VisitorT>(visitor));
            }
 
          private:
            static constexpr std::size_t STORAGE_SIZE = VariantDetail::ParameterPackSize::GetMaxSizeOf<Ts...>();
 
            alignas(VariantDetail::ParameterPackSize::AlignAsPack<Ts...>()) char m_storage[STORAGE_SIZE];
            IndexT m_index = -1;
#if defined(AWS_CRT_ENABLE_VARIANT_DEBUG)
            VariantDetail::VariantDebug::VariantDebugBrowser<Ts...> browser = m_storage;
#endif /* defined(AWS_CRT_ENABLE_VARIANT_DEBUG) */
 
            template <size_t Index> constexpr bool holds_alternative() const { return Index == m_index; }
 
            struct Destroyer
            {
                template <typename AlternativeT> void operator()(AlternativeT &&value) const
                {
                    (void)value;
                    using PlaintT = typename std::remove_reference<AlternativeT>::type;
                    value.~PlaintT();
                }
            };
 
            void Destroy()
            {
                AWS_FATAL_ASSERT(m_index != -1);
                Visit(Destroyer());
 
                m_index = -1;
            }
 
            struct CopyMoveConstructor
            {
                template <typename AlternativeT> void operator()(AlternativeT &&value, AlternativeT &&other) const
                {
                    using PlaintT = typename std::remove_reference<AlternativeT>::type;
                    new (&value) PlaintT(std::move<AlternativeT>(other));
                }
 
                template <typename AlternativeT, typename ConstAlternativeT>
                void operator()(AlternativeT &&value, ConstAlternativeT &other) const
                {
                    using PlaintT = typename std::remove_reference<AlternativeT>::type;
                    using PlaintOtherT =
                        typename std::remove_const<typename std::remove_reference<AlternativeT>::type>::type;
                    static_assert(std::is_same<PlaintT, PlaintOtherT>::value, "Incompatible types");
 
                    new (&value) PlaintT(other);
                }
            };
 
            struct CopyMoveAssigner
            {
                template <typename AlternativeT> void operator()(AlternativeT &&value, AlternativeT &&other) const
                {
                    value = std::move(other);
                }
 
                template <typename AlternativeT, typename ConstAlternativeT>
                void operator()(AlternativeT &&value, ConstAlternativeT &other) const
                {
                    using PlaintT = typename std::remove_reference<AlternativeT>::type;
                    using PlaintOtherT =
                        typename std::remove_const<typename std::remove_reference<AlternativeT>::type>::type;
                    static_assert(std::is_same<PlaintT, PlaintOtherT>::value, "Incompatible types");
 
                    value = other;
                }
            };
 
            template <IndexT Index, typename... Args> struct VisitorUtil;
 
            template <IndexT Index, typename First, typename Second, typename... Rest>
            struct VisitorUtil<Index, First, Second, Rest...>
            {
                template <typename VisitorStruct> static void Visit(Variant *pThis, VisitorStruct &&visitor)
                {
                    static_assert(Index < AlternativeCount, "Attempting to visit unknown Index Type");
 
                    if (Index == pThis->m_index)
                    {
                        using AlternativeT = typename ThisVariantAlternative<Index>::type;
                        AlternativeT *value = reinterpret_cast<AlternativeT *>(pThis->m_storage);
                        visitor(*value);
                    }
                    else
                    {
                        VisitorUtil<static_cast<IndexT>(Index + 1), Second, Rest...>::Visit(
                            pThis, std::forward<VisitorStruct>(visitor));
                    }
                }
 
                template <typename VisitorStruct>
                static void VisitBinary(Variant<Ts...> *pThis, Variant<Ts...> &&other, VisitorStruct &&visitor)
                {
                    static_assert(Index < AlternativeCount, "Attempting to visit unknown Index Type");
 
                    if (Index == pThis->m_index)
                    {
                        using AlternativeT = typename ThisVariantAlternative<Index>::type;
                        AlternativeT *value = reinterpret_cast<AlternativeT *>(pThis->m_storage);
                        visitor(*value, other.get<AlternativeT>());
                    }
                    else
                    {
                        VisitorUtil<static_cast<IndexT>(Index + 1), Second, Rest...>::VisitBinary(
                            pThis, std::forward<Variant<Ts...>>(other), std::forward<VisitorStruct>(visitor));
                    }
                }
 
                template <typename VisitorStruct>
                static void VisitBinary(Variant<Ts...> *pThis, const Variant<Ts...> &other, VisitorStruct &&visitor)
                {
                    static_assert(Index < AlternativeCount, "Attempting to visit unknown Index Type");
 
                    if (Index == pThis->m_index)
                    {
                        using AlternativeT = typename ThisVariantAlternative<Index>::type;
                        AlternativeT *value = reinterpret_cast<AlternativeT *>(pThis->m_storage);
                        const AlternativeT &otherValue = other.get<AlternativeT>();
                        visitor(*value, otherValue);
                    }
                    else
                    {
                        VisitorUtil<static_cast<IndexT>(Index + 1), Second, Rest...>::VisitBinary(
                            pThis, other, std::forward<VisitorStruct>(visitor));
                    }
                }
            };
 
            template <IndexT Index, typename Last> struct VisitorUtil<Index, Last>
            {
                template <typename VisitorStruct> static void Visit(Variant *pThis, VisitorStruct &&visitor)
                {
                    static_assert(Index < AlternativeCount, "Attempting to visit unknown Index Type");
 
                    if (Index == pThis->m_index)
                    {
                        using AlternativeT = typename ThisVariantAlternative<Index>::type;
                        AlternativeT *value = reinterpret_cast<AlternativeT *>(pThis->m_storage);
                        visitor(*value);
                    }
                    else
                    {
                        AWS_FATAL_ASSERT(!"Unknown variant alternative to visit!");
                    }
                }
 
                template <typename VisitorStruct>
                static void VisitBinary(Variant<Ts...> *pThis, Variant<Ts...> &&other, VisitorStruct &&visitor)
                {
                    static_assert(Index < AlternativeCount, "Attempting to visit unknown Index Type");
 
                    if (Index == pThis->m_index)
                    {
                        using AlternativeT = typename ThisVariantAlternative<Index>::type;
                        AlternativeT *value = reinterpret_cast<AlternativeT *>(pThis->m_storage);
                        visitor(*value, other.get<AlternativeT>());
                    }
                    else
                    {
                        AWS_FATAL_ASSERT(!"Unknown variant alternative to visit!");
                    }
                }
 
                template <typename VisitorStruct>
                static void VisitBinary(Variant<Ts...> *pThis, const Variant<Ts...> &other, VisitorStruct &&visitor)
                {
                    static_assert(Index < AlternativeCount, "Attempting to visit unknown Index Type");
 
                    if (Index == pThis->m_index)
                    {
                        using AlternativeT = typename ThisVariantAlternative<Index>::type;
                        AlternativeT *value = reinterpret_cast<AlternativeT *>(pThis->m_storage);
                        const AlternativeT &otherValue = other.get<AlternativeT>();
                        visitor(*value, otherValue);
                    }
                    else
                    {
                        AWS_FATAL_ASSERT(!"Unknown variant alternative to visit!");
                    }
                }
            };
        };
 
        /* Helper template to get an actual type from an Index */
        template <std::size_t Index, typename... Ts> class VariantAlternative
        {
          public:
            // uses std::tuple as a helper struct to provide index-based access of a parameter pack
            using type = typename std::tuple_element<Index, std::tuple<Ts...>>::type;
 
            VariantAlternative(const Variant<Ts...> &) {}
 
            VariantAlternative(const Variant<Ts...> *) {}
        };
 
        template <typename T> class VariantSize
        {
            constexpr static const std::size_t Value = T::AlternativeCount;
        };
    } // namespace Crt
} // namespace Aws