    // Copyright (C) 2009  Davis E. King (davis@dlib.net)
    // License: Boost Software License   See LICENSE.txt for the full license.
    #undef DLIB_TYPE_SAFE_UNION_KERNEl_ABSTRACT_
    #ifdef DLIB_TYPE_SAFE_UNION_KERNEl_ABSTRACT_

    namespace dlib
    {

    // ----------------------------------------------------------------------------------------

        class bad_type_safe_union_cast : public std::bad_cast
        {
            /*!
                This is the exception object thrown by type_safe_union::cast_to() if the
                type_safe_union does not contain the type of object being requested.
            !*/
        };

    // ----------------------------------------------------------------------------------------

        template<typename T>
        struct in_place_tag { using type = T; };
        /*!
            This is an empty class type used as a special disambiguation tag to be
            passed as the first argument to the constructor of type_safe_union that performs
            in-place construction of an object.

            Here is an example of its usage:

                struct A
                {
                    int i = 0;
                    int j = 0;

                    A(int i_, int j_) : i(i_), j(j_) {}
                };

                using tsu = type_safe_union<A,std::string>;
                tsu a(in_place_tag<A>{}, 0, 1); // a now contains an object of type A

            It is also used with type_safe_union::for_each_type() to disambiguate types.
        !*/

    // ----------------------------------------------------------------------------------------

        template<typename TSU>
        struct type_safe_union_size
        {
            static constexpr size_t value = The number of types in the TSU.
        };
        /*!
            requires
                - TSU must be of type type_safe_union<Types...> with possible cv qualification
            ensures
                - value contains the number of types in TSU, i.e. sizeof...(Types...)
        !*/

    // ----------------------------------------------------------------------------------------

        template <size_t I, typename TSU>
        struct type_safe_union_alternative;
        /*!
            requires
                - TSU is a type_safe_union
            ensures
                - type_safe_union_alternative<I, TSU>::type is the Ith type in the TSU.
                - TSU::get_type_id<typename type_safe_union_alternative<I, TSU>::type>() == I
        !*/

        template<size_t I, typename TSU>
        using type_safe_union_alternative_t = type_safe_union_alternative<I,TSU>::type;
        /*!
            ensures
                - provides template alias for type_safe_union_alternative
        !*/

    // ----------------------------------------------------------------------------------------

        template <typename... Types>
        class type_safe_union
        {
            /*!
                REQUIREMENTS ON ALL TEMPLATE ARGUMENTS
                    All template arguments must be default constructable and have
                    a global swap.

                INITIAL VALUE
                    - is_empty() == true
                    - contains<U>() == false, for all possible values of U

                WHAT THIS OBJECT REPRESENTS
                    This object is a type safe analogue of the classic C union object.
                    The type_safe_union, unlike a union, can contain non-POD types such
                    as std::string.

                    For example:
                        union my_union
                        {
                            int a;
                            std::string b;   // Error, std::string isn't a POD
                        };

                        type_safe_union<int,std::string> my_type_safe_union;  // No error

                    Finally, note that if the constructor of one of the Types throws when the type
                    safe union is constructing it then the type safe union will be left in an empty
                    state.  I.e. this->empty() == true.
            !*/

        public:

            type_safe_union(
            ) = default;
            /*!
                ensures
                    - #is_empty() == true
            !*/

            type_safe_union (
                const type_safe_union& item
            )
            /*!
                ensures
                    - copy constructs *this from item
            !*/

            type_safe_union& operator=(
                const type_safe_union& item
            );
            /*!
                ensures
                    - copy assigns *this from item
            !*/

            type_safe_union (
                type_safe_union&& item
            );
            /*!
                ensures
                    - move constructs *this from item.
            !*/

            type_safe_union& operator= (
                type_safe_union&& item
            );
            /*!
                ensures
                    - move assigns *this from item.
            !*/

            template <
                typename T
            >
            type_safe_union (
                T&& item
            );
            /*!
                requires
                    - std::decay_t<T> must be one of the types given to this object's template arguments
                ensures
                    - constructs *this from item using perfect forwarding (converting constructor)
                    - #get<T>() == std::forward<T>(item)
                      (i.e. this object will either contain a copy of item or will have moved item into *this
                       depending on the reference type)
            !*/

            template <
                typename T
            >
            type_safe_union& operator= (
                T&& item
            );
            /*!
                requires
                    - std::decay_t<T> must be one of the types given to this object's template arguments
                ensures
                    - assigns *this from item using perfect forwarding (converting assignment)
                    - #get<T> == std::forward<T>(item)
                      (i.e. this object will either contain a copy of item or will have moved item into *this
                       depending on the reference type)
            !*/

            template <
                typename T,
                typename... Args
            >
            type_safe_union (
                in_place_tag<T>,
                Args&&... args
            );
            /*!
                requires
                    - T must be one of the types given to this object's template arguments
                ensures
                    - constructs *this with type T using constructor-arguments args...
                      (i.e. efficiently performs *this = T(args...))
            !*/

            ~type_safe_union(
            );
            /*!
                ensures
                    - all resources associated with this object have been freed
            !*/

            void clear();
            /*!
                ensures
                    - all resources associated with this object have been freed
                    - #is_empty() == true
            !*/

            template <
                typename T,
                typename... Args
            >
            void emplace(
                Args&&... args
            );
            /*!
                requires
                    - T must be one of the types given to this object's template arguments
                ensures
                    - re-assigns *this with type T using constructor-arguments args...
                      (i.e. efficiently performs *this = T(args...))
            !*/

            template <typename T>
            static constexpr int get_type_id (
            );
            /*!
               ensures
                    - if (T is the same type as one of the template arguments) then
                        - returns a number indicating which template argument it is. In particular,
                          if it's the first template argument it returns 1, if the second then 2, and so on.
                    - else if (T is in_place_tag<U>) then
                        - equivalent to returning get_type_id<U>())
                    - else
                        - returns -1
            !*/

            template <typename T>
            static constexpr int get_type_id (
                in_place_tag<T>
            );
            /*!
               ensures
                    - returns get_type_id<T>()
                    - This is useful when using for_each_type() with a generic lambda. For example:
                      for_each_type([](auto tag, auto&& me) {
                        printf("ID %i\n", me.get_type_id(tag));
                      }, item);
            !*/

            template <typename T>
            bool contains (
            ) const;
            /*!
                ensures
                    - if (this type_safe_union currently contains an object of type T) then
                        - returns true
                    - else
                        - returns false
            !*/

            bool is_empty (
            ) const;
            /*!
                ensures
                    - if (this type_safe_union currently contains any object at all) then
                        - returns true
                    - else
                        - returns false
            !*/

            int get_current_type_id(
            ) const;
            /*!
                ensures
                    - if (is_empty()) then
                        - returns 0
                    - else
                        - Returns the type id of the currently held type.  This is the same as
                          get_type_id<WhateverTypeIsCurrentlyHeld>().  Therefore, if the current type is
                          the first template argument it returns 1, if it's the second then 2, and so on.
            !*/

            template <typename F>
            void apply_to_contents(
                F&& f
            );
            /*!
                requires
                    - f is a callable object capable of operating on all the types contained
                      in this type_safe_union.  I.e.  std::forward<F>(f)(this->get<U>()) must be a valid
                      expression for all the possible U types.
                ensures
                    - if (is_empty() == false) then
                        - Let U denote the type of object currently contained in this type_safe_union
                        - calls std::forward<F>(f)(this->get<U>())
                        - The object passed to f() (i.e. by this->get<U>()) will be non-const.
            !*/

            template <typename F>
            void apply_to_contents(
                F&& f
            ) const;
            /*!
                requires
                    - f is a callable object capable of operating on all the types contained
                      in this type_safe_union.  I.e.  std::forward<F>(f)(this->get<U>()) must be a valid
                      expression for all the possible U types.
                ensures
                    - if (is_empty() == false) then
                        - Let U denote the type of object currently contained in this type_safe_union
                        - calls std::forward<F>(f)(this->get<U>())
                        - The object passed to f() (i.e. by this->get<U>()) will be const.
            !*/

            template <typename T>
            T& get(
            );
            /*!
                requires
                    - T must be one of the types given to this object's template arguments
                ensures
                    - #is_empty() == false
                    - #contains<T>() == true
                    - if (contains<T>() == true)
                        - returns a non-const reference to the object contained in this type_safe_union.
                    - else
                        - Constructs an object of type T inside *this
                        - Any previous object stored in this type_safe_union is destructed and its
                          state is lost.
                        - returns a non-const reference to the newly created T object.
            !*/

            template <typename T>
            T& get(
                const in_place_tag<T>& tag
            );
            /*!
                ensures
                    - equivalent to calling get<T>()
            !*/

            template <typename T>
            const T& cast_to (
            ) const;
            /*!
                requires
                    - T must be one of the types given to this object's template arguments
                ensures
                    - if (contains<T>() == true) then
                        - returns a const reference to the object contained in this type_safe_union.
                    - else
                        - throws bad_type_safe_union_cast
            !*/

            template <typename T>
            T& cast_to (
            );
            /*!
                requires
                    - T must be one of the types given to this object's template arguments
                ensures
                    - if (contains<T>() == true) then
                        - returns a non-const reference to the object contained in this type_safe_union.
                    - else
                        - throws bad_type_safe_union_cast
            !*/

            void swap (
                type_safe_union& item
            );
            /*!
                ensures
                    - swaps *this and item
            !*/
        };

    // ----------------------------------------------------------------------------------------

        template<typename... Types>
        inline void swap (
            type_safe_union<Types...>& a,
            type_safe_union<Types...>& b
        ) { a.swap(b); }
        /*!
            provides a global swap function
        !*/

    // ----------------------------------------------------------------------------------------

        template<
            typename TSU,
            typename F
        >
        void for_each_type(
            F&& f,
            TSU&& tsu
        );
        /*!
            requires
                - tsu is an object of type type_safe_union<Types...> for some types Types...
                - f is a callable object such that the following expression is valid for
                  all types U in Types...:
                    std::forward<F>(f)(in_place_tag<U>{}, std::forward<TSU>(tsu))
            ensures
                - This function iterates over all types U in Types... and calls:
                    std::forward<F>(f)(in_place_tag<U>{}, std::forward<TSU>(tsu))
        !*/

    // ----------------------------------------------------------------------------------------

        template<
            typename F,
            typename TSU
        >
        auto visit(
            F&& f,
            TSU&& tsu
        );
        /*!
            requires
                - tsu is an object of type type_safe_union<Types...> for some types Types...
                - f is a callable object capable of operating on all the types contained
                  in tsu.  I.e.  std::forward<F>(f)(this->get<U>()) must be a valid
                  expression for all the possible U types.
            ensures
                - if (tsu.is_empty() == false) then
                    - Let U denote the type of object currently contained in tsu.
                    - returns std::forward<F>(f)(this->get<U>())
                    - The object passed to f() (i.e. by this->get<U>()) will have the same reference
                      type as TSU.
        !*/

    // ----------------------------------------------------------------------------------------

        template<typename... Types>
        void serialize (
            const type_safe_union<Types...>& item,
            std::ostream& out
        );
        /*!
            provides serialization support

            Note that type_safe_union objects are serialized as follows:
             - if (item.is_empty()) then
                - perform: serialize(0, out)
             - else
                - perform: serialize(item.get_type_id<type_of_object_in_item>(), out);
                           serialize(item.get<type_of_object_in_item>(), out);
        !*/

        template<typename... Types>
        void deserialize (
            type_safe_union<Types...>& item,
            std::istream& in
        );
        /*!
            provides deserialization support
        !*/

    // ----------------------------------------------------------------------------------------
    }

    #endif // DLIB_TYPE_SAFE_UNION_KERNEl_ABSTRACT_