    // 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_

    #include "../algs.h"
    #include "../noncopyable.h"

    namespace dlib
    {

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

        template <
            typename T1,
            typename T2 = _void,  // _void indicates parameter not used.
            typename T3 = _void,
            typename T4 = _void,
            typename T5 = _void,
            typename T6 = _void,
            typename T7 = _void,
            typename T8 = _void,
            typename T9 = _void,
            typename T10 = _void,
            typename T11 = _void,
            typename T12 = _void,
            typename T13 = _void,
            typename T14 = _void,
            typename T15 = _void,
            typename T16 = _void,
            typename T17 = _void,
            typename T18 = _void,
            typename T19 = _void,
            typename T20 = _void
            >
        class type_safe_union : noncopyable
        {
            /*!
                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
            !*/

        public:

            typedef T1 type1;
            typedef T2 type2;
            typedef T3 type3;
            typedef T4 type4;
            typedef T5 type5;
            typedef T6 type6;
            typedef T7 type7;
            typedef T8 type8;
            typedef T9 type9;
            typedef T10 type10;
            typedef T11 type11;
            typedef T12 type12;
            typedef T13 type13;
            typedef T14 type14;
            typedef T15 type15;
            typedef T16 type16;
            typedef T17 type17;
            typedef T18 type18;
            typedef T19 type19;
            typedef T20 type20;

            type_safe_union(
            );
            /*!
                ensures
                    - this object is properly initialized
            !*/

            template <typename T>
            type_safe_union (
                const T& item
            );
            /*!
                requires
                    - T must be one of the types given to this object's template arguments
                ensures
                    - this object is properly initialized
                    - #get<T>() == item
                      (i.e. this object will contain a copy of item)
            !*/

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

            template <typename T>
            static 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.
                       (e.g. if T is the same type as T3 then this function returns 3)
                   - else
                      - returns -1
            !*/

            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
            !*/

            template <typename T>
            void apply_to_contents (
                T& obj
            );
            /*!
                requires
                    - obj is a function object capable of operating on all the types contained
                      in this type_safe_union.  I.e.  obj(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 obj(this->get<U>())
                        - The object returned by this->get<U>() will be non-const
            !*/

            template <typename T>
            void apply_to_contents (
                const T& obj
            );
            /*!
                requires
                    - obj is a function object capable of operating on all the types contained
                      in this type_safe_union.  I.e.  obj(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 obj(this->get<U>())
                        - The object returned by this->get<U>() will be non-const
            !*/

            template <typename T>
            void apply_to_contents (
                T& obj
            ) const;
            /*!
                requires
                    - obj is a function object capable of operating on all the types contained
                      in this type_safe_union.  I.e.  obj(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 obj(this->get<U>())
                        - The object returned by this->get<U>() will be const
            !*/

            template <typename T>
            void apply_to_contents (
                const T& obj
            ) const;
            /*!
                requires
                    - obj is a function object capable of operating on all the types contained
                      in this type_safe_union.  I.e.  obj(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 obj(this->get<U>())
                        - The object returned 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>
            type_safe_union& operator= (
                const T& item
            );
            /*!
                requires
                    - T must be one of the types given to this object's template arguments
                ensures
                    - #get<T>() == item
                      (i.e. this object will contain a copy of item)
                    - returns *this
            !*/

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

        };

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

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

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

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

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

    }

    #endif // DLIB_TYPE_SAFE_UNION_KERNEl_ABSTRACT_