third-party/mapbox/variant.hpp

//
// Copyright (c) MapBox All rights reserved.
// SPDX-License-Identifier: BSD-3-Clause
//

#ifndef MAPBOX_UTIL_VARIANT_HPP
#define MAPBOX_UTIL_VARIANT_HPP

#include <cassert>
#include <cstddef>   // size_t
#include <new>       // operator new
#include <stdexcept> // runtime_error
#include <string>
#include <tuple>
#include <type_traits>
#include <typeinfo>
#include <utility>
#include <functional>
#include <limits>

#include <mapbox/recursive_wrapper.hpp>
#include <mapbox/variant_visitor.hpp>

// clang-format off
// [[deprecated]] is only available in C++14, use this for the time being
#if __cplusplus <= 201103L
# ifdef __GNUC__
#  define MAPBOX_VARIANT_DEPRECATED __attribute__((deprecated))
# elif defined(_MSC_VER)
#  define MAPBOX_VARIANT_DEPRECATED __declspec(deprecated)
# else
#  define MAPBOX_VARIANT_DEPRECATED
# endif
#else
#  define MAPBOX_VARIANT_DEPRECATED [[deprecated]]
#endif


#ifdef _MSC_VER
// https://msdn.microsoft.com/en-us/library/bw1hbe6y.aspx
# ifdef NDEBUG
#  define VARIANT_INLINE __forceinline
# else
#  define VARIANT_INLINE //__declspec(noinline)
# endif
#else
# ifdef NDEBUG
#  define VARIANT_INLINE //inline __attribute__((always_inline))
# else
#  define VARIANT_INLINE __attribute__((noinline))
# endif
#endif
// clang-format on

// Exceptions
#if defined( __EXCEPTIONS) || defined( _MSC_VER)
#define HAS_EXCEPTIONS
#endif

#define VARIANT_MAJOR_VERSION 1
#define VARIANT_MINOR_VERSION 1
#define VARIANT_PATCH_VERSION 0

#define VARIANT_VERSION (VARIANT_MAJOR_VERSION * 100000) + (VARIANT_MINOR_VERSION * 100) + (VARIANT_PATCH_VERSION)

namespace mapbox {
namespace util {

// XXX This should derive from std::logic_error instead of std::runtime_error.
//     See https://github.com/mapbox/variant/issues/48 for details.
class bad_variant_access : public std::runtime_error
{

public:
    explicit bad_variant_access(const std::string& what_arg)
        : runtime_error(what_arg) {}

    explicit bad_variant_access(const char* what_arg)
        : runtime_error(what_arg) {}

}; // class bad_variant_access

#if !defined(MAPBOX_VARIANT_MINIMIZE_SIZE)
using type_index_t = unsigned int;
#else
#if defined(MAPBOX_VARIANT_OPTIMIZE_FOR_SPEED)
using type_index_t = std::uint_fast8_t;
#else
using type_index_t = std::uint_least8_t;
#endif
#endif

namespace detail {

static constexpr type_index_t invalid_value = type_index_t(-1);

template <typename T, typename... Types>
struct direct_type;

template <typename T, typename First, typename... Types>
struct direct_type<T, First, Types...>
{
    static constexpr type_index_t index = std::is_same<T, First>::value
        ? sizeof...(Types)
        : direct_type<T, Types...>::index;
};

template <typename T>
struct direct_type<T>
{
    static constexpr type_index_t index = invalid_value;
};

#if __cpp_lib_logical_traits >= 201510L

using std::conjunction;
using std::disjunction;

#else

template <typename...>
struct conjunction : std::true_type {};

template <typename B1>
struct conjunction<B1> : B1 {};

template <typename B1, typename B2>
struct conjunction<B1, B2> : std::conditional<B1::value, B2, B1>::type {};

template <typename B1, typename... Bs>
struct conjunction<B1, Bs...> : std::conditional<B1::value, conjunction<Bs...>, B1>::type {};

template <typename...>
struct disjunction : std::false_type {};

template <typename B1>
struct disjunction<B1> : B1 {};

template <typename B1, typename B2>
struct disjunction<B1, B2> : std::conditional<B1::value, B1, B2>::type {};

template <typename B1, typename... Bs>
struct disjunction<B1, Bs...> : std::conditional<B1::value, B1, disjunction<Bs...>>::type {};

#endif

template <typename T, typename... Types>
struct convertible_type;

template <typename T, typename First, typename... Types>
struct convertible_type<T, First, Types...>
{
    static constexpr type_index_t index = std::is_convertible<T, First>::value
        ? disjunction<std::is_convertible<T, Types>...>::value ? invalid_value : sizeof...(Types)
        : convertible_type<T, Types...>::index;
};

template <typename T>
struct convertible_type<T>
{
    static constexpr type_index_t index = invalid_value;
};

template <typename T, typename... Types>
struct value_traits
{
    using value_type = typename std::remove_const<typename std::remove_reference<T>::type>::type;
    using value_type_wrapper = recursive_wrapper<value_type>;
    static constexpr type_index_t direct_index = direct_type<value_type, Types...>::index;
    static constexpr bool is_direct = direct_index != invalid_value;
    static constexpr type_index_t index_direct_or_wrapper = is_direct ? direct_index : direct_type<value_type_wrapper, Types...>::index;
    static constexpr bool is_direct_or_wrapper = index_direct_or_wrapper != invalid_value;
    static constexpr type_index_t index = is_direct_or_wrapper ? index_direct_or_wrapper : convertible_type<value_type, Types...>::index;
    static constexpr bool is_valid = index != invalid_value;
    static constexpr type_index_t tindex = is_valid ? sizeof...(Types)-index : 0;
    using target_type = typename std::tuple_element<tindex, std::tuple<void, Types...>>::type;
};

template <typename Src, typename Dest>
struct copy_cvref
{
    using type = Dest;
};

template <typename Src, typename Dest>
struct copy_cvref<Src const&, Dest>
{
    using type = Dest const&;
};

template <typename Src, typename Dest>
struct copy_cvref<Src&, Dest>
{
    using type = Dest&;
};

template <typename Src, typename Dest>
struct copy_cvref<Src&&, Dest>
{
    using type = Dest&&;
};

template <typename F, typename = void>
struct deduced_result_type
{};

template <typename F, typename... Args>
struct deduced_result_type<F(Args...), decltype((void)std::declval<F>()(std::declval<Args>()...))>
{
    using type = decltype(std::declval<F>()(std::declval<Args>()...));
};

template <typename F, typename = void>
struct visitor_result_type : deduced_result_type<F>
{};

// specialization for explicit result_type member in visitor class
template <typename F, typename... Args>
struct visitor_result_type<F(Args...), decltype((void)std::declval<typename std::decay<F>::type::result_type>())>
{
    using type = typename std::decay<F>::type::result_type;
};

template <typename F, typename T>
using result_of_unary_visit = typename visitor_result_type<F&&(T&&)>::type;

template <typename F, typename T>
using result_of_binary_visit = typename visitor_result_type<F&&(T&&, T&&)>::type;

template <type_index_t arg1, type_index_t... others>
struct static_max;

template <type_index_t arg>
struct static_max<arg>
{
    static const type_index_t value = arg;
};

template <type_index_t arg1, type_index_t arg2, type_index_t... others>
struct static_max<arg1, arg2, others...>
{
    static const type_index_t value = arg1 >= arg2 ? static_max<arg1, others...>::value : static_max<arg2, others...>::value;
};

template <typename... Types>
struct variant_helper;

template <typename T, typename... Types>
struct variant_helper<T, Types...>
{
    VARIANT_INLINE static void destroy(const type_index_t type_index, void* data)
    {
        if (type_index == sizeof...(Types))
        {
            reinterpret_cast<T*>(data)->~T();
        }
        else
        {
            variant_helper<Types...>::destroy(type_index, data);
        }
    }

    VARIANT_INLINE static void move(const type_index_t old_type_index, void* old_value, void* new_value)
    {
        if (old_type_index == sizeof...(Types))
        {
            new (new_value) T(std::move(*reinterpret_cast<T*>(old_value)));
        }
        else
        {
            variant_helper<Types...>::move(old_type_index, old_value, new_value);
        }
    }

    VARIANT_INLINE static void copy(const type_index_t old_type_index, const void* old_value, void* new_value)
    {
        if (old_type_index == sizeof...(Types))
        {
            new (new_value) T(*reinterpret_cast<const T*>(old_value));
        }
        else
        {
            variant_helper<Types...>::copy(old_type_index, old_value, new_value);
        }
    }
};

template <>
struct variant_helper<>
{
    VARIANT_INLINE static void destroy(const type_index_t, void*) {}
    VARIANT_INLINE static void move(const type_index_t, void*, void*) {}
    VARIANT_INLINE static void copy(const type_index_t, const void*, void*) {}
};

template <typename T>
struct unwrapper
{
    using value_type = T;

    template <typename V>
    static auto apply(typename std::remove_reference<V>::type& var)
        -> typename std::enable_if<std::is_lvalue_reference<V>::value,
                    decltype(var.template get_unchecked<T>())>::type
    {
        return var.template get_unchecked<T>();
    }

    template <typename V>
    static auto apply(typename std::remove_reference<V>::type& var)
        -> typename std::enable_if<!std::is_lvalue_reference<V>::value,
                    decltype(std::move(var.template get_unchecked<T>()))>::type
    {
        return std::move(var.template get_unchecked<T>());
    }
};

template <typename T>
struct unwrapper<recursive_wrapper<T>> : unwrapper<T>
{};

template <typename T>
struct unwrapper<std::reference_wrapper<T>> : unwrapper<T>
{};

template <typename R, typename... Types>
struct dispatcher;

template <typename R, typename T, typename... Types>
struct dispatcher<R, T, Types...>
{
    template <typename V, typename F>
    VARIANT_INLINE static R apply(V&& v, F&& f)
    {
        if (v.template is<T>())
        {
            return std::forward<F>(f)(unwrapper<T>::template apply<V>(v));
        }
        else
        {
            return dispatcher<R, Types...>::apply(std::forward<V>(v), std::forward<F>(f));
        }
    }
};

template <typename R, typename T>
struct dispatcher<R, T>
{
    template <typename V, typename F>
    VARIANT_INLINE static R apply(V&& v, F&& f)
    {
        return std::forward<F>(f)(unwrapper<T>::template apply<V>(v));
    }
};

template <typename R, typename T, typename... Types>
struct binary_dispatcher_rhs;

template <typename R, typename T0, typename T1, typename... Types>
struct binary_dispatcher_rhs<R, T0, T1, Types...>
{
    template <typename V, typename F>
    VARIANT_INLINE static R apply(V&& lhs, V&& rhs, F&& f)
    {
        if (rhs.template is<T1>()) // call binary functor
        {
            return std::forward<F>(f)(unwrapper<T0>::template apply<V>(lhs),
                                      unwrapper<T1>::template apply<V>(rhs));
        }
        else
        {
            return binary_dispatcher_rhs<R, T0, Types...>::apply(std::forward<V>(lhs),
                                                                 std::forward<V>(rhs),
                                                                 std::forward<F>(f));
        }
    }
};

template <typename R, typename T0, typename T1>
struct binary_dispatcher_rhs<R, T0, T1>
{
    template <typename V, typename F>
    VARIANT_INLINE static R apply(V&& lhs, V&& rhs, F&& f)
    {
        return std::forward<F>(f)(unwrapper<T0>::template apply<V>(lhs),
                                  unwrapper<T1>::template apply<V>(rhs));
    }
};

template <typename R, typename T, typename... Types>
struct binary_dispatcher_lhs;

template <typename R, typename T0, typename T1, typename... Types>
struct binary_dispatcher_lhs<R, T0, T1, Types...>
{
    template <typename V, typename F>
    VARIANT_INLINE static R apply(V&& lhs, V&& rhs, F&& f)
    {
        if (lhs.template is<T1>()) // call binary functor
        {
            return std::forward<F>(f)(unwrapper<T1>::template apply<V>(lhs),
                                      unwrapper<T0>::template apply<V>(rhs));
        }
        else
        {
            return binary_dispatcher_lhs<R, T0, Types...>::apply(std::forward<V>(lhs),
                                                                 std::forward<V>(rhs),
                                                                 std::forward<F>(f));
        }
    }
};

template <typename R, typename T0, typename T1>
struct binary_dispatcher_lhs<R, T0, T1>
{
    template <typename V, typename F>
    VARIANT_INLINE static R apply(V&& lhs, V&& rhs, F&& f)
    {
        return std::forward<F>(f)(unwrapper<T1>::template apply<V>(lhs),
                                  unwrapper<T0>::template apply<V>(rhs));
    }
};

template <typename R, typename... Types>
struct binary_dispatcher;

template <typename R, typename T, typename... Types>
struct binary_dispatcher<R, T, Types...>
{
    template <typename V, typename F>
    VARIANT_INLINE static R apply(V&& v0, V&& v1, F&& f)
    {
        if (v0.template is<T>())
        {
            if (v1.template is<T>())
            {
                return std::forward<F>(f)(unwrapper<T>::template apply<V>(v0),
                                          unwrapper<T>::template apply<V>(v1)); // call binary functor
            }
            else
            {
                return binary_dispatcher_rhs<R, T, Types...>::apply(std::forward<V>(v0),
                                                                    std::forward<V>(v1),
                                                                    std::forward<F>(f));
            }
        }
        else if (v1.template is<T>())
        {
            return binary_dispatcher_lhs<R, T, Types...>::apply(std::forward<V>(v0),
                                                                std::forward<V>(v1),
                                                                std::forward<F>(f));
        }
        return binary_dispatcher<R, Types...>::apply(std::forward<V>(v0),
                                                     std::forward<V>(v1),
                                                     std::forward<F>(f));
    }
};

template <typename R, typename T>
struct binary_dispatcher<R, T>
{
    template <typename V, typename F>
    VARIANT_INLINE static R apply(V&& v0, V&& v1, F&& f)
    {
        return std::forward<F>(f)(unwrapper<T>::template apply<V>(v0),
                                  unwrapper<T>::template apply<V>(v1)); // call binary functor
    }
};

// comparator functors
struct equal_comp
{
    template <typename T>
    bool operator()(T const& lhs, T const& rhs) const
    {
        return lhs == rhs;
    }
};

struct less_comp
{
    template <typename T>
    bool operator()(T const& lhs, T const& rhs) const
    {
        return lhs < rhs;
    }
};

template <typename Variant, typename Comp>
class comparer
{
public:
    explicit comparer(Variant const& lhs) noexcept
        : lhs_(lhs) {}
    comparer& operator=(comparer const&) = delete;
    // visitor
    template <typename T>
    bool operator()(T const& rhs_content) const
    {
        T const& lhs_content = lhs_.template get_unchecked<T>();
        return Comp()(lhs_content, rhs_content);
    }

private:
    Variant const& lhs_;
};

// hashing visitor
struct hasher
{
    template <typename T>
    std::size_t operator()(const T& hashable) const
    {
        return std::hash<T>{}(hashable);
    }
};

} // namespace detail

struct no_init {};

template <typename... Types>
class variant
{
    static_assert(sizeof...(Types) > 0, "Template parameter type list of variant can not be empty.");
    static_assert(!detail::disjunction<std::is_reference<Types>...>::value, "Variant can not hold reference types. Maybe use std::reference_wrapper?");
    static_assert(!detail::disjunction<std::is_array<Types>...>::value, "Variant can not hold array types.");
    static_assert(sizeof...(Types) < std::numeric_limits<type_index_t>::max(), "Internal index type must be able to accommodate all alternatives.");
private:
    static const std::size_t data_size = detail::static_max<sizeof(Types)...>::value;
    static const std::size_t data_align = detail::static_max<alignof(Types)...>::value;
public:
    struct adapted_variant_tag;
    using types = std::tuple<Types...>;
private:
    using first_type = typename std::tuple_element<0, types>::type;
    using unwrap_first_type = typename detail::unwrapper<first_type>::value_type;
    using data_type = typename std::aligned_storage<data_size, data_align>::type;
    using helper_type = detail::variant_helper<Types...>;

    template <typename V, typename T = unwrap_first_type>
        using alternative_ref = typename detail::copy_cvref<V, T>::type;

    type_index_t type_index;
#ifdef __clang_analyzer__
    data_type data {};
#else
    data_type data;
#endif

public:
    VARIANT_INLINE variant() noexcept(std::is_nothrow_default_constructible<first_type>::value)
        : type_index(sizeof...(Types)-1)
    {
        static_assert(std::is_default_constructible<first_type>::value, "First type in variant must be default constructible to allow default construction of variant.");
        new (&data) first_type();
    }

    VARIANT_INLINE variant(no_init) noexcept
        : type_index(detail::invalid_value) {}

    // http://isocpp.org/blog/2012/11/universal-references-in-c11-scott-meyers
    template <typename T, typename Traits = detail::value_traits<T, Types...>,
              typename Enable = typename std::enable_if<Traits::is_valid && !std::is_same<variant<Types...>, typename Traits::value_type>::value>::type >
    VARIANT_INLINE variant(T&& val) noexcept(std::is_nothrow_constructible<typename Traits::target_type, T&&>::value)
        : type_index(Traits::index)
    {
        new (&data) typename Traits::target_type(std::forward<T>(val));
    }

    VARIANT_INLINE variant(variant<Types...> const& old)
        : type_index(old.type_index)
    {
        helper_type::copy(old.type_index, &old.data, &data);
    }

    VARIANT_INLINE variant(variant<Types...>&& old)
        noexcept(detail::conjunction<std::is_nothrow_move_constructible<Types>...>::value)
        : type_index(old.type_index)
    {
        helper_type::move(old.type_index, &old.data, &data);
    }

private:
    VARIANT_INLINE void copy_assign(variant<Types...> const& rhs)
    {
        helper_type::destroy(type_index, &data);
        type_index = detail::invalid_value;
        helper_type::copy(rhs.type_index, &rhs.data, &data);
        type_index = rhs.type_index;
    }

    VARIANT_INLINE void move_assign(variant<Types...>&& rhs)
    {
        helper_type::destroy(type_index, &data);
        type_index = detail::invalid_value;
        helper_type::move(rhs.type_index, &rhs.data, &data);
        type_index = rhs.type_index;
    }

public:
    VARIANT_INLINE variant<Types...>& operator=(variant<Types...>&& other)
        // note we check for nothrow-constructible, not nothrow-assignable, since
        // move_assign uses move-construction via placement new.
        noexcept(detail::conjunction<std::is_nothrow_move_constructible<Types>...>::value)
    {
        if (this == &other) { // playing safe in release mode, hit assertion in debug.
            assert(false);
            return *this;
        }
        move_assign(std::move(other));
        return *this;
    }

    VARIANT_INLINE variant<Types...>& operator=(variant<Types...> const& other)
    {
        if (this != &other)
            copy_assign(other);
        return *this;
    }

    // conversions
    // move-assign
    template <typename T, typename Traits = detail::value_traits<T, Types...>,
              typename Enable = typename std::enable_if<Traits::is_valid && !std::is_same<variant<Types...>, typename Traits::value_type>::value>::type >
    VARIANT_INLINE variant<Types...>& operator=(T&& rhs)
        // not that we check is_nothrow_constructible<T>, not is_nothrow_move_assignable<T>,
        // since we construct a temporary
        noexcept(std::is_nothrow_constructible<typename Traits::target_type, T&&>::value
                 && std::is_nothrow_move_assignable<variant<Types...>>::value)
    {
        variant<Types...> temp(std::forward<T>(rhs));
        move_assign(std::move(temp));
        return *this;
    }

    // copy-assign
    template <typename T>
    VARIANT_INLINE variant<Types...>& operator=(T const& rhs)
    {
        variant<Types...> temp(rhs);
        copy_assign(temp);
        return *this;
    }

    template <typename T, typename std::enable_if<
                          (detail::direct_type<T, Types...>::index != detail::invalid_value)>::type* = nullptr>
    VARIANT_INLINE bool is() const
    {
        return type_index == detail::direct_type<T, Types...>::index;
    }

    template <typename T,typename std::enable_if<
                         (detail::direct_type<recursive_wrapper<T>, Types...>::index != detail::invalid_value)>::type* = nullptr>
    VARIANT_INLINE bool is() const
    {
        return type_index == detail::direct_type<recursive_wrapper<T>, Types...>::index;
    }

    VARIANT_INLINE bool valid() const
    {
        return type_index != detail::invalid_value;
    }

    template <typename T, typename... Args>
    VARIANT_INLINE void set(Args&&... args)
    {
        helper_type::destroy(type_index, &data);
        type_index = detail::invalid_value;
        new (&data) T(std::forward<Args>(args)...);
        type_index = detail::direct_type<T, Types...>::index;
    }

    // get_unchecked<T>()
    template <typename T, typename std::enable_if<
                          (detail::direct_type<T, Types...>::index != detail::invalid_value)>::type* = nullptr>
    VARIANT_INLINE T& get_unchecked()
    {
        return *reinterpret_cast<T*>(&data);
    }

#ifdef HAS_EXCEPTIONS
    // get<T>()
    template <typename T, typename std::enable_if<
                          (detail::direct_type<T, Types...>::index != detail::invalid_value)>::type* = nullptr>
    VARIANT_INLINE T& get()
    {
        if (type_index == detail::direct_type<T, Types...>::index)
        {
            return *reinterpret_cast<T*>(&data);
        }
        else
        {
            throw bad_variant_access("in get<T>()");
        }
    }
#endif

    template <typename T, typename std::enable_if<
                          (detail::direct_type<T, Types...>::index != detail::invalid_value)>::type* = nullptr>
    VARIANT_INLINE T const& get_unchecked() const
    {
        return *reinterpret_cast<T const*>(&data);
    }

#ifdef HAS_EXCEPTIONS
    template <typename T, typename std::enable_if<
                          (detail::direct_type<T, Types...>::index != detail::invalid_value)>::type* = nullptr>
    VARIANT_INLINE T const& get() const
    {
        if (type_index == detail::direct_type<T, Types...>::index)
        {
            return *reinterpret_cast<T const*>(&data);
        }
        else
        {
            throw bad_variant_access("in get<T>()");
        }
    }
#endif

    // get_unchecked<T>() - T stored as recursive_wrapper<T>
    template <typename T, typename std::enable_if<
                          (detail::direct_type<recursive_wrapper<T>, Types...>::index != detail::invalid_value)>::type* = nullptr>
    VARIANT_INLINE T& get_unchecked()
    {
        return (*reinterpret_cast<recursive_wrapper<T>*>(&data)).get();
    }

#ifdef HAS_EXCEPTIONS
    // get<T>() - T stored as recursive_wrapper<T>
    template <typename T, typename std::enable_if<
                          (detail::direct_type<recursive_wrapper<T>, Types...>::index != detail::invalid_value)>::type* = nullptr>
    VARIANT_INLINE T& get()
    {
        if (type_index == detail::direct_type<recursive_wrapper<T>, Types...>::index)
        {
            return (*reinterpret_cast<recursive_wrapper<T>*>(&data)).get();
        }
        else
        {
            throw bad_variant_access("in get<T>()");
        }
    }
#endif

    template <typename T, typename std::enable_if<
                          (detail::direct_type<recursive_wrapper<T>, Types...>::index != detail::invalid_value)>::type* = nullptr>
    VARIANT_INLINE T const& get_unchecked() const
    {
        return (*reinterpret_cast<recursive_wrapper<T> const*>(&data)).get();
    }

#ifdef HAS_EXCEPTIONS
    template <typename T, typename std::enable_if<
                          (detail::direct_type<recursive_wrapper<T>, Types...>::index != detail::invalid_value)>::type* = nullptr>
    VARIANT_INLINE T const& get() const
    {
        if (type_index == detail::direct_type<recursive_wrapper<T>, Types...>::index)
        {
            return (*reinterpret_cast<recursive_wrapper<T> const*>(&data)).get();
        }
        else
        {
            throw bad_variant_access("in get<T>()");
        }
    }
#endif

    // get_unchecked<T>() - T stored as std::reference_wrapper<T>
    template <typename T, typename std::enable_if<
                          (detail::direct_type<std::reference_wrapper<T>, Types...>::index != detail::invalid_value)>::type* = nullptr>
    VARIANT_INLINE T& get_unchecked()
    {
        return (*reinterpret_cast<std::reference_wrapper<T>*>(&data)).get();
    }

#ifdef HAS_EXCEPTIONS
    // get<T>() - T stored as std::reference_wrapper<T>
    template <typename T, typename std::enable_if<
                          (detail::direct_type<std::reference_wrapper<T>, Types...>::index != detail::invalid_value)>::type* = nullptr>
    VARIANT_INLINE T& get()
    {
        if (type_index == detail::direct_type<std::reference_wrapper<T>, Types...>::index)
        {
            return (*reinterpret_cast<std::reference_wrapper<T>*>(&data)).get();
        }
        else
        {
            throw bad_variant_access("in get<T>()");
        }
    }
#endif

    template <typename T, typename std::enable_if<
                          (detail::direct_type<std::reference_wrapper<T const>, Types...>::index != detail::invalid_value)>::type* = nullptr>
    VARIANT_INLINE T const& get_unchecked() const
    {
        return (*reinterpret_cast<std::reference_wrapper<T const> const*>(&data)).get();
    }

#ifdef HAS_EXCEPTIONS
    template <typename T, typename std::enable_if<
                          (detail::direct_type<std::reference_wrapper<T const>, Types...>::index != detail::invalid_value)>::type* = nullptr>
    VARIANT_INLINE T const& get() const
    {
        if (type_index == detail::direct_type<std::reference_wrapper<T const>, Types...>::index)
        {
            return (*reinterpret_cast<std::reference_wrapper<T const> const*>(&data)).get();
        }
        else
        {
            throw bad_variant_access("in get<T>()");
        }
    }
#endif

    // This function is deprecated because it returns an internal index field.
    // Use which() instead.
    MAPBOX_VARIANT_DEPRECATED VARIANT_INLINE type_index_t get_type_index() const
    {
        return type_index;
    }

    VARIANT_INLINE int which() const noexcept
    {
        return static_cast<int>(sizeof...(Types) - type_index - 1);
    }

    template <typename T, typename std::enable_if<
                          (detail::direct_type<T, Types...>::index != detail::invalid_value)>::type* = nullptr>
    VARIANT_INLINE static constexpr int which() noexcept
    {
        return static_cast<int>(sizeof...(Types)-detail::direct_type<T, Types...>::index - 1);
    }

    // visitor
    // unary
    template <typename F, typename V, typename T0 = alternative_ref<V>,
              typename R = detail::result_of_unary_visit<F, T0>>
    VARIANT_INLINE static R visit(V&& v, F&& f)
    {
        return detail::dispatcher<R, Types...>::apply(std::forward<V>(v), std::forward<F>(f));
    }

    // binary
    template <typename F, typename V, typename T0 = alternative_ref<V>,
              typename R = detail::result_of_binary_visit<F, T0>>
    VARIANT_INLINE static R binary_visit(V&& v0, V&& v1, F&& f)
    {
        return detail::binary_dispatcher<R, Types...>::apply(std::forward<V>(v0),
                                                             std::forward<V>(v1),
                                                             std::forward<F>(f));
    }

    // match
    // unary
    template <typename... Fs>
    auto VARIANT_INLINE match(Fs&&... fs) const&
        -> decltype(variant::visit(*this, ::mapbox::util::make_visitor(std::forward<Fs>(fs)...)))
    {
        return variant::visit(*this, ::mapbox::util::make_visitor(std::forward<Fs>(fs)...));
    }
    // non-const
    template <typename... Fs>
    auto VARIANT_INLINE match(Fs&&... fs) &
        -> decltype(variant::visit(*this, ::mapbox::util::make_visitor(std::forward<Fs>(fs)...)))
    {
        return variant::visit(*this, ::mapbox::util::make_visitor(std::forward<Fs>(fs)...));
    }
    template <typename... Fs>
    auto VARIANT_INLINE match(Fs&&... fs) &&
        -> decltype(variant::visit(std::move(*this), ::mapbox::util::make_visitor(std::forward<Fs>(fs)...)))
    {
        return variant::visit(std::move(*this), ::mapbox::util::make_visitor(std::forward<Fs>(fs)...));
    }

    ~variant() noexcept // no-throw destructor
    {
        helper_type::destroy(type_index, &data);
    }

    // comparison operators
    // equality
    VARIANT_INLINE bool operator==(variant const& rhs) const
    {
        assert(valid() && rhs.valid());
        if (this->which() != rhs.which())
        {
            return false;
        }
        detail::comparer<variant, detail::equal_comp> visitor(*this);
        return visit(rhs, visitor);
    }

    VARIANT_INLINE bool operator!=(variant const& rhs) const
    {
        return !(*this == rhs);
    }

    // less than
    VARIANT_INLINE bool operator<(variant const& rhs) const
    {
        assert(valid() && rhs.valid());
        if (this->which() != rhs.which())
        {
            return this->which() < rhs.which();
        }
        detail::comparer<variant, detail::less_comp> visitor(*this);
        return visit(rhs, visitor);
    }
    VARIANT_INLINE bool operator>(variant const& rhs) const
    {
        return rhs < *this;
    }
    VARIANT_INLINE bool operator<=(variant const& rhs) const
    {
        return !(*this > rhs);
    }
    VARIANT_INLINE bool operator>=(variant const& rhs) const
    {
        return !(*this < rhs);
    }
};

// unary visitor interface
template <typename F, typename V>
auto VARIANT_INLINE apply_visitor(F&& f, V&& v)
    -> decltype(v.visit(std::forward<V>(v), std::forward<F>(f)))
{
    return v.visit(std::forward<V>(v), std::forward<F>(f));
}

// binary visitor interface
template <typename F, typename V>
auto VARIANT_INLINE apply_visitor(F&& f, V&& v0, V&& v1)
    -> decltype(v0.binary_visit(std::forward<V>(v0), std::forward<V>(v1), std::forward<F>(f)))
{
    return v0.binary_visit(std::forward<V>(v0), std::forward<V>(v1), std::forward<F>(f));
}

// getter interface

#ifdef HAS_EXCEPTIONS
template <typename ResultType, typename T>
auto get(T& var)->decltype(var.template get<ResultType>())
{
    return var.template get<ResultType>();
}
#endif

template <typename ResultType, typename T>
ResultType& get_unchecked(T& var)
{
    return var.template get_unchecked<ResultType>();
}

#ifdef HAS_EXCEPTIONS
template <typename ResultType, typename T>
auto get(T const& var)->decltype(var.template get<ResultType>())
{
    return var.template get<ResultType>();
}
#endif

template <typename ResultType, typename T>
ResultType const& get_unchecked(T const& var)
{
    return var.template get_unchecked<ResultType>();
}
// variant_size
template <typename T>
struct variant_size;

//variable templates is c++14
//template <typename T>
//constexpr std::size_t variant_size_v = variant_size<T>::value;

template <typename T>
struct variant_size<const T>
    : variant_size<T> {};

template <typename T>
struct variant_size<volatile T>
    : variant_size<T> {};

template <typename T>
struct variant_size<const volatile T>
    : variant_size<T> {};

template <typename... Types>
struct variant_size<variant<Types...>>
    : std::integral_constant<std::size_t, sizeof...(Types)> {};

// variant_alternative
template <std::size_t Index, typename T>
struct variant_alternative;

#if defined(__clang__)
#if __has_builtin(__type_pack_element)
#define has_type_pack_element
#endif
#endif

#if defined(has_type_pack_element)
template <std::size_t Index, typename ...Types>
struct variant_alternative<Index, variant<Types...>>
{
    static_assert(sizeof...(Types) > Index , "Index out of range");
    using type = __type_pack_element<Index, Types...>;
};
#else
template <std::size_t Index, typename First, typename...Types>
struct variant_alternative<Index, variant<First, Types...>>
    : variant_alternative<Index - 1, variant<Types...>>
{
    static_assert(sizeof...(Types) > Index -1 , "Index out of range");
};

template <typename First, typename...Types>
struct variant_alternative<0, variant<First, Types...>>
{
    using type = First;
};

#endif

template <size_t Index, typename T>
using variant_alternative_t = typename variant_alternative<Index, T>::type;

template <size_t Index, typename T>
struct variant_alternative<Index, const T>
    : std::add_const<variant_alternative<Index, T>> {};

template <size_t Index, typename T>
struct variant_alternative<Index, volatile T>
    : std::add_volatile<variant_alternative<Index, T>> {};

template <size_t Index, typename T>
struct variant_alternative<Index, const volatile T>
    : std::add_cv<variant_alternative<Index, T>> {};

} // namespace util
} // namespace mapbox

// hashable iff underlying types are hashable
namespace std {
template <typename... Types>
struct hash< ::mapbox::util::variant<Types...>> {
    std::size_t operator()(const ::mapbox::util::variant<Types...>& v) const noexcept
    {
        return ::mapbox::util::apply_visitor(::mapbox::util::detail::hasher{}, v);
    }
};

}

#endif // MAPBOX_UTIL_VARIANT_HPP