    // Copyright (C) 2003  Davis E. King (davis@dlib.net)
    // License: Boost Software License   See LICENSE.txt for the full license.
    #undef DLIB_QUEUE_KERNEl_ABSTRACT_
    #ifdef DLIB_QUEUE_KERNEl_ABSTRACT_

    #include "../interfaces/enumerable.h"
    #include "../interfaces/remover.h"
    #include "../serialize.h"
    #include "../algs.h"

    namespace dlib
    {

        template <
            typename T,
            typename mem_manager = default_memory_manager
            >
        class queue : public enumerable<T>,
                      public remover<T>
        {

            /*!
                REQUIREMENTS ON T
                    T must be swappable by a global swap()
                    T must have a default constructor

                REQUIREMENTS ON mem_manager
                    must be an implementation of memory_manager/memory_manager_kernel_abstract.h or
                    must be an implementation of memory_manager_global/memory_manager_global_kernel_abstract.h or
                    must be an implementation of memory_manager_stateless/memory_manager_stateless_kernel_abstract.h
                    mem_manager::type can be set to anything.

                POINTERS AND REFERENCES TO INTERNAL DATA
                    swap() and current() functions do not invalidate pointers or
                    references to internal data.
                    All other functions have no such guarantee.

                INITIAL VALUE
                    size() == 0

                ENUMERATION ORDER
                    The enumerator will iterate over the elements in the queue in the
                    same order they would be removed by repeated calls to dequeue().
                    (e.g. current() would be the first element enumerated)

                WHAT THIS OBJECT REPRESENTS
                    This is a first in first out queue containing items of type T

                    e.g. if the queue is {b,c,d,e} and then 'a' is enqueued
                    the queue becomes {a,b,c,d,e} and then calling dequeue takes e out
                    making the queue {a,b,c,d}

                    Also note that unless specified otherwise, no member functions
                    of this object throw exceptions.
            !*/

            public:

                typedef T type;
                typedef mem_manager mem_manager_type;

                queue (
                );
                /*!
                    ensures
                        - #*this is properly initialized
                    throws
                        - std::bad_alloc or any exception thrown by T's constructor
                !*/

                virtual ~queue (
                );
                /*!
                    ensures
                        - all memory associated with *this has been released
                !*/

                void clear(
                );
                /*!
                    ensures
                        - #*this has its initial value
                    throws
                        - std::bad_alloc or any exception thrown by T's constructor
                            if this exception is thrown then *this is unusable
                            until clear() is called and succeeds
                !*/

                void enqueue (
                    T& item
                );
                /*!
                    ensures
                        - item is now at the left end of #*this
                        - #item has an initial value for its type
                        - #size() == size() + 1
                        - #at_start() == true
                    throws
                        - std::bad_alloc or any exception thrown by T's constructor
                            if enqueue() throws then it has no effect
                !*/

                void dequeue (
                    T& item
                );
                /*!
                    requires
                        - size() != 0
                    ensures
                        - #size() == size() - 1
                        - the far right element of *this has been removed and swapped
                          into #item
                        - #at_start() == true
                !*/

                void cat (
                    queue& item
                );
                /*!
                    ensures
                        - item has been concatenated onto the left end of *this.
                          i.e. item.current() is attached onto the left end of *this and
                          the left most element in item will also be the left most item
                          in #*this
                        - #size() == size() + item.size()
                        - #item has its initial value
                        - #at_start() == true
                    throws
                        - std::bad_alloc or any exception thrown by T's constructor
                            if cat() throws then the state of #item and *this is undefined
                            until clear() is successfully called on them.
                !*/

                T& current (
                );
                /*!
                    requires
                        - size() != 0
                    ensures
                        - returns a const reference to the next element to be dequeued.
                          i.e.  the right most element.
                !*/

                const T& current (
                ) const;
                /*!
                    requires
                        - size() != 0
                    ensures
                        - returns a non-const reference to the next element to be dequeued.
                          i.e.  the right most element.
                !*/

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

            private:

                // restricted functions
                queue(queue&);        // copy constructor
                queue& operator=(queue&);    // assignment operator

        };

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

        template <
            typename T,
            typename mem_manager
            >
        void deserialize (
            queue<T,mem_manager>& item,
            std::istream& in
        );
        /*!
            provides deserialization support
        !*/
    }

    #endif // DLIB_QUEUE_KERNEl_ABSTRACT_