    // Copyright (C) 2015  Davis E. King (davis@dlib.net)
    // License: Boost Software License   See LICENSE.txt for the full license.
    #ifndef DLIB_DNn_CORE_H_
    #define DLIB_DNn_CORE_H_

    #include <iterator>
    #include <memory>
    #include <sstream>
    #include <utility>
    #include <tuple>
    #include <cmath>
    #include <vector>
    #include <type_traits>

    #include "core_abstract.h"
    #include "../cuda/tensor.h"
    #include "../cuda/tensor_tools.h"
    #include "../statistics.h"
    #include "../rand.h"
    #include "../algs.h"
    #include "../metaprogramming.h"
    #include "../utility.h"
    #include "../constexpr_if.h"

    #ifdef _MSC_VER
    // Tell Visual Studio not to recursively inline functions very much because otherwise it
    // takes hours to compile the DNN code sometimes.  It's crazy.  Hopefully we can remove
    // this some day when the visual studio compiler is more efficient.
    #pragma inline_depth(2)
    #endif

    namespace dlib
    {

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

        namespace impl
        {
            template<typename T>
            using has_get_learning_rate_multiplier = decltype(std::declval<T>().get_learning_rate_multiplier());

            template<typename T>
            using has_set_learning_rate_multiplier = decltype(std::declval<T>().set_learning_rate_multiplier(double{}));

            template<typename T>
            using has_get_bias_learning_rate_multiplier = decltype(std::declval<T>().get_bias_learning_rate_multiplier());

            template<typename T>
            using has_set_bias_learning_rate_multiplier = decltype(std::declval<T>().set_bias_learning_rate_multiplier(double{}));

            template<typename T>
            using has_get_weight_decay_multiplier = decltype(std::declval<T>().get_weight_decay_multiplier());

            template<typename T>
            using has_set_weight_decay_multiplier = decltype(std::declval<T>().set_weight_decay_multiplier(double{}));

            template<typename T>
            using has_get_bias_weight_decay_multiplier = decltype(std::declval<T>().get_bias_weight_decay_multiplier());

            template<typename T>
            using has_set_bias_weight_decay_multiplier = decltype(std::declval<T>().set_bias_weight_decay_multiplier(double{}));

            template<typename T>
            using has_disable_bias = decltype(std::declval<T>().disable_bias());

            template<typename T>
            using has_clean = decltype(std::declval<T>().clean());
        }

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

        template <typename T>
        double get_learning_rate_multiplier(const T& obj)
        {
            return switch_(bools(is_detected<impl::has_get_learning_rate_multiplier, T>{}),
                [&](true_t, auto _) { return _(obj).get_learning_rate_multiplier(); },
                [](auto...)         { return 1.0; }
            );
        }

        template <typename T>
        void set_learning_rate_multiplier(
            T& obj,
            double learning_rate_multiplier
        )
        {
            DLIB_CASSERT(learning_rate_multiplier >= 0);
            switch_(bools(is_detected<impl::has_set_learning_rate_multiplier, T>{}),
                [&](true_t, auto _) { _(obj).set_learning_rate_multiplier(learning_rate_multiplier); },
                [](auto...)         {/*no-op*/}
            );
        }

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

        template <typename T>
        double get_bias_learning_rate_multiplier(const T& obj)
        {
            return switch_(bools(is_detected<impl::has_get_bias_learning_rate_multiplier, T>{}),
                [&](true_t, auto _) { return _(obj).get_bias_learning_rate_multiplier(); },
                [](auto...)         { return 1.0; }
            );
        }

        template <typename T>
        void set_bias_learning_rate_multiplier(
            T& obj,
            double bias_learning_rate_multiplier
        )
        {
            DLIB_CASSERT(bias_learning_rate_multiplier >= 0);
            switch_(bools(is_detected<impl::has_set_bias_learning_rate_multiplier, T>{}),
                [&](true_t, auto _) { _(obj).set_bias_learning_rate_multiplier(bias_learning_rate_multiplier); },
                [](auto...)         {/*no-op*/}
            );
        }

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

        template <typename T>
        double get_weight_decay_multiplier(const T& obj)
        {
            return switch_(bools(is_detected<impl::has_get_weight_decay_multiplier, T>{}),
                [&](true_t, auto _) { return _(obj).get_weight_decay_multiplier(); },
                [](auto...)         { return 1.0; }
            );
        }

        template <typename T>
        void set_weight_decay_multiplier(
            T& obj,
            double weight_decay_multiplier
        )
        {
            DLIB_CASSERT(weight_decay_multiplier >= 0);
            switch_(bools(is_detected<impl::has_set_weight_decay_multiplier, T>{}),
                [&](true_t, auto _) { _(obj).set_weight_decay_multiplier(weight_decay_multiplier); },
                [](auto...)         {/*no-op*/}
            );
        }

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

        template <typename T>
        double get_bias_weight_decay_multiplier(const T& obj)
        {
            return switch_(bools(is_detected<impl::has_get_bias_weight_decay_multiplier, T>{}),
                [&](true_t, auto _) { return _(obj).get_bias_weight_decay_multiplier(); },
                [](auto...)         { return 1.0; }
            );
        }

        template <typename T>
        void set_bias_weight_decay_multiplier(
            T& obj,
            double bias_weight_decay_multiplier
        )
        {
            DLIB_CASSERT(bias_weight_decay_multiplier >= 0);
            switch_(bools(is_detected<impl::has_set_bias_weight_decay_multiplier, T>{}),
                [&](true_t, auto _) { _(obj).set_bias_weight_decay_multiplier(bias_weight_decay_multiplier); },
                [](auto...)         {/*no-op*/}
            );
        }

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

        template <typename T>
        void disable_bias(
            T& obj
        )
        {
            switch_(bools(is_detected<impl::has_disable_bias, T>{}),
                [&](true_t, auto _) { _(obj).disable_bias(); },
                [](auto...)         { /*no-op*/ }
            );
        }

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

        template <typename T>
        void call_clean_method_if_exists(T& obj)
        /*!
            ensures
                - calls obj.clean() if obj has a .clean() method.
        !*/
        {
            switch_(bools(is_detected<impl::has_clean, T>{}),
                [&](true_t, auto _) { _(obj).clean(); },
                [](auto...)         { /*no-op*/ }
            );
        }

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

        namespace impl
        {
            class repeat_input_layer
            {
                /*!
                    None of the declarations in this object are really used. The only reason it
                    exists is to allow the repeat object to use a special input layer in its
                    internal networks which will cause add_tag_layer objects that happen to be
                    right at the input to not create copies of their input tensors.  So
                    introducing the repeat_input_layer object allows us to optimize the
                    implementation of add_tag_layer for a special case that arises when it's
                    used in the context of the repeat layer.
                !*/
            public:
                typedef int input_type;

                template <typename forward_iterator>
                void to_tensor (
                    forward_iterator ,
                    forward_iterator ,
                    resizable_tensor&
                ) const
                {
                }

                friend void serialize(const repeat_input_layer&, std::ostream&){}
                friend void deserialize(repeat_input_layer&, std::istream&){}
                friend std::ostream& operator<<(std::ostream& out, const repeat_input_layer&) { return out; }
            };

            inline std::string tensor_to_str (
                const tensor& t,
                int& min_length
            )
            {
                if (t.size() == 0)
                    return "";

                std::ostringstream sout;
                sout << "output size=(num:"<<  t.num_samples() << ", ";
                sout << "k:" << t.k() << ",";
                while (sout.tellp() < 28) sout << " ";
                sout << "nr:" << t.nr() << ",";
                while (sout.tellp() < 28+8) sout << " ";
                sout << "nc:" << t.nc() << ")";
                while (sout.tellp() < min_length) sout << " ";
                min_length = sout.tellp();
                sout << "\t";
                return sout.str();
            }
        }

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

        // Tell us if T is one of the special layer types (i.e. add_layer, repeat, add_tag_layer, or
        // add_skip_layer).
        template <typename T> struct is_nonloss_layer_type : std::false_type {};
        // Tell us if T is an instance of add_loss_layer.
        template <typename T> struct is_loss_layer_type : std::false_type {};
        // Tell us if T is an instance of add_layer
        template <typename T> struct is_add_layer : std::false_type {};

        namespace impl
        {
            template <typename Tuple, size_t... indices>
            auto tuple_subset(
                const Tuple& item,
                std::index_sequence<indices...>
            )
            {
                return std::make_tuple(std::get<indices>(item)...);
            }

            template <typename ...Types>
            auto basic_tuple_tail(
                const std::tuple<Types...>& item
            )
            {
                return tuple_subset(item, pop_front_t<index_sequence_for<Types...>>{});
            }

            template <typename T>
            auto tuple_flatten(const T& t)
            {
                return std::make_tuple(t);
            }

            template <size_t... I, typename... T>
            auto tuple_flatten(
                const std::tuple<T...>& item,
                std::index_sequence<I...>
            )
            {
                return std::tuple_cat(tuple_flatten(std::get<I>(item))...);
            }

            template <typename... T>
            auto tuple_flatten(const std::tuple<T...>& item)
            {
                return tuple_flatten(item, std::index_sequence_for<T...>{});
            }

            template <typename T>
            struct tuple_head_helper
            {
                typedef T type;
                static const type& get(const T& item)
                {
                    return item;
                }
            };

            template <typename T, typename... U>
            struct tuple_head_helper<std::tuple<T, U...>>
            {
                typedef typename tuple_head_helper<T>::type type;
                static const type& get(const std::tuple<T,U...>& item)
                {
                    return tuple_head_helper<T>::get(std::get<0>(item));
                }
            };

            template <typename T> struct alwaysbool { typedef bool type; };
            // one more structure for VS 2015 UP3 support workaround
            template <typename T> struct alwaysbool2 { typedef bool type; };

            resizable_tensor& rt();

            // The significance of a layer's backward method requiring forward's outputs is
            // that such as layer can't have an in-place layer stacked on top of it because
            // in-place layers overwrite the output of the layer they sit on top of.
            template <typename layer_type, typename SUBNET>
            constexpr auto backward_requires_forward_output(
                layer_type& layer,
                SUBNET& sub
            ) -> typename alwaysbool<decltype(layer.backward(rt(),rt(),sub,rt()))>::type
            {
                return true;
            }

            template <typename layer_type, typename SUBNET>
            constexpr auto backward_requires_forward_output(
                layer_type& layer,
                SUBNET& sub
            ) -> typename alwaysbool<decltype(layer.backward(rt(),sub,rt()))>::type
            {
                return false;
            }

            template <typename layer_type, typename SUBNET>
            constexpr auto backward_requires_forward_output(
                layer_type& layer,
                SUBNET& sub
            ) -> typename alwaysbool<decltype(layer.backward_inplace(rt(),rt(),sub.get_gradient_input(),rt()))>::type
            {
                return true;
            }

            template <typename layer_type, typename SUBNET>
            constexpr auto backward_requires_forward_output(
                layer_type& layer,
                SUBNET& sub
            ) -> typename alwaysbool<decltype(layer.backward_inplace(rt(),sub.get_gradient_input(),rt()))>::type
            {
                return false;
            }

            template <typename layer_type, typename SUBNET>
            constexpr auto has_inplace_backward(
                layer_type& layer,
                SUBNET& sub
            ) -> typename alwaysbool2<decltype(layer.backward(rt(),rt(),sub,rt()))>::type
            {
                return false;
            }

            template <typename layer_type, typename SUBNET>
            constexpr auto has_inplace_backward(
                layer_type& layer,
                SUBNET& sub
            ) -> typename alwaysbool2<decltype(layer.backward(rt(),sub,rt()))>::type
            {
                return false;
            }

            template <typename layer_type, typename SUBNET>
            constexpr auto has_inplace_backward(
                layer_type& layer,
                SUBNET& sub
            ) -> typename alwaysbool2<decltype(layer.backward_inplace(rt(),rt(),sub.get_gradient_input(),rt()))>::type
            {
                return true;
            }

            template <typename layer_type, typename SUBNET>
            constexpr auto has_inplace_backward(
                layer_type& layer,
                SUBNET& sub
            ) -> typename alwaysbool2<decltype(layer.backward_inplace(rt(),sub.get_gradient_input(),rt()))>::type
            {
                return true;
            }

            template <typename layer_type, typename SUBNET>
            constexpr auto is_inplace_layer(
                layer_type& layer,
                const SUBNET& sub
            ) -> typename alwaysbool2<decltype(layer.forward(sub,rt()))>::type
            {
                return false;
            }

            template <typename layer_type, typename SUBNET>
            constexpr auto is_inplace_layer(
                layer_type& layer,
                const SUBNET& sub
            ) -> typename alwaysbool<decltype(layer.forward_inplace(sub.get_output(),rt()))>::type
            {
                return true;
            }

            template <typename layer_type, typename SUBNET>
            auto call_layer_backward(
                layer_type& layer,
                const tensor& computed_output,
                const tensor& gradient_input,
                SUBNET& sub,
                tensor& params_grad
            ) -> decltype(layer.backward(computed_output,gradient_input,sub,params_grad))
            {
                layer.backward(computed_output,gradient_input,sub,params_grad);
            }

            template <typename layer_type, typename SUBNET>
            auto call_layer_backward(
                layer_type& layer,
                const tensor& ,
                const tensor& gradient_input,
                SUBNET& sub,
                tensor& params_grad
            ) -> decltype(layer.backward(gradient_input,sub,params_grad))
            {
                layer.backward(gradient_input,sub,params_grad);
            }

            template <typename layer_type, typename SUBNET>
            auto call_layer_backward(
                layer_type& layer,
                const tensor& computed_output,
                const tensor& gradient_input,
                SUBNET& sub,
                tensor& params_grad
            ) -> decltype(layer.backward_inplace(computed_output,gradient_input,sub.get_gradient_input(),params_grad))
            {
                layer.backward_inplace(computed_output,gradient_input,sub.get_gradient_input(),params_grad);
            }

            template <typename layer_type, typename SUBNET>
            auto call_layer_backward(
                layer_type& layer,
                const tensor& ,
                const tensor& gradient_input,
                SUBNET& sub,
                tensor& params_grad
            ) -> decltype(layer.backward_inplace(gradient_input,sub.get_gradient_input(),params_grad))
            {
                layer.backward_inplace(gradient_input,sub.get_gradient_input(),params_grad);
            }

            template <typename layer_type, typename SUBNET>
            auto call_layer_forward(
                layer_type& layer,
                const SUBNET& sub,
                tensor& /*data_output*/
            ) -> decltype(layer.forward(sub,rt()))
            {
                // This overload of call_layer_forward() is here because this template
                // naturally gets instantiated but only on code paths that never get executed.
                // So rather than writing a bunch of hard to read template magic around call
                // sites we just have this overload that doesn't do anything (and an assert to
                // make sure that's the case).
                DLIB_CASSERT(false, "This should never happen");
            }

            template <typename layer_type, typename SUBNET>
            auto call_layer_forward(
                layer_type& layer,
                const SUBNET& sub,
                resizable_tensor& data_output
            ) -> decltype(layer.forward(sub,data_output))
            {
                layer.forward(sub,data_output);
            }

            template <typename layer_type, typename SUBNET>
            auto call_layer_forward(
                layer_type& layer,
                const SUBNET& sub,
                tensor& data_output
            ) -> decltype(layer.forward_inplace(sub.get_output(),data_output))
            {
                layer.forward_inplace(sub.get_output(),data_output);
            }

            template <typename layer_type, typename SUBNET>
            auto call_layer_forward(
                layer_type& layer,
                const SUBNET& sub,
                resizable_tensor& data_output
            ) -> decltype(layer.forward_inplace(sub.get_output(),data_output))
            {
                if (!have_same_dimensions(data_output, sub.get_output()))
                    data_output.copy_size(sub.get_output());
                layer.forward_inplace(sub.get_output(),static_cast<tensor&>(data_output));
            }

        } // end namespace impl

        template <typename... T>
        auto tuple_head (
            const std::tuple<T...>& item
        )
        {
            return impl::tuple_head_helper<std::tuple<T...>>::get(item);
        }

        template <typename... T>
        auto tuple_tail(
            const std::tuple<T...>& item
        )
        {
            return impl::basic_tuple_tail(impl::tuple_flatten(item));
        }

        inline std::tuple<> tuple_tail(
            const std::tuple<>& item
        )
        {
            return item;
        }
    // ----------------------------------------------------------------------------------------

        template <typename T>
        class sstack
        {
        public:
            typedef T value_type;

            sstack() = delete;

            sstack (
                T* data_,
                size_t s
            ) : data(data_), mysize(s) {}

            const T& top() const
            {
                DLIB_CASSERT(size() != 0, "You can't call top() on an empty stack");
                return *data;
            }
            T& top()
            {
                DLIB_CASSERT(size() != 0, "You can't call top() on an empty stack");
                return *data;
            }

            size_t size() const { return mysize; }

            sstack pop(size_t num=1)
            {
                DLIB_CASSERT(num <= size(), "You can't pop more things from the stack than it has in it.");
                return sstack(data+num, mysize-num);
            }

        private:

            T* data;
            size_t mysize;
        };

        template <typename T>
        sstack<T> make_sstack(std::vector<T>& item)
        {
            return sstack<T>(item.data(), item.size());
        }

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

        namespace dimpl
        {
            template <typename T, bool is_first = true, typename enabled=void>
            class subnet_wrapper
            {
                /*!
                    WHAT THIS OBJECT REPRESENTS
                        This is a tool that makes an add_layer or add_loss_layer object
                        expose only the part of its interface defined by the SUBNET
                        type in layers_abstract.h.  This way, when we pass subnetwork
                        objects to the layer callbacks those callbacks won't be able to
                        interact with the subnetworks in a way other than specified
                        by the SUBNET interface spec.

                        We also allow the top layer of a subnet_wrapper stack to call the
                        private_get_output() and private_get_gradient_input() functions.  This
                        way, layers that have had their output/gradient overwritten by in-place
                        layers can only be accessed from the in-place layers that sit directly
                        on top of them since those in-place layers are the only layers that
                        know how to interact with them properly.
                !*/

            public:
                subnet_wrapper(const subnet_wrapper&) = delete;
                subnet_wrapper& operator=(const subnet_wrapper&) = delete;

                subnet_wrapper(T& l_, unsigned int sef) : l(l_),_sample_expansion_factor(sef) {}
                // Not much here because in this case T is one of the input layer types
                // that doesn't have anything in it.
                typedef T layer_details_type;
                typedef T input_layer_type;
                const layer_details_type& layer_details() const { return l; }
                const input_layer_type& input_layer() const { return l; }
                input_layer_type& input_layer() { return l; }
                unsigned int sample_expansion_factor() const { return _sample_expansion_factor; }
            private:
                T& l;
                unsigned int _sample_expansion_factor;
            };

            template <typename T>
            class subnet_wrapper<T,true, typename std::enable_if<is_nonloss_layer_type<T>::value>::type>
            {

            public:
                subnet_wrapper(const subnet_wrapper&) = delete;
                subnet_wrapper& operator=(const subnet_wrapper&) = delete;

                typedef T wrapped_type;
                const static size_t num_computational_layers = T::num_computational_layers;
                const static size_t num_layers = T::num_layers;
                typedef typename T::layer_details_type layer_details_type;
                typedef typename T::input_layer_type input_layer_type;

                subnet_wrapper(T& l_, unsigned int = 0) : l(l_),subnetwork(l.subnet(), l.sample_expansion_factor()) {}

                const tensor& get_output() const { return l.private_get_output(); }
                tensor& get_gradient_input() { return l.private_get_gradient_input(); }

                const layer_details_type& layer_details() const { return l.layer_details(); }

                const subnet_wrapper<typename T::subnet_type,false>& subnet() const { return subnetwork; }
                subnet_wrapper<typename T::subnet_type,false>& subnet() { return subnetwork; }
                unsigned int sample_expansion_factor() const { return l.sample_expansion_factor(); }

                const input_layer_type& input_layer() const { return l.input_layer(); }
                input_layer_type& input_layer() { return l.input_layer(); }

            private:
                T& l;
                subnet_wrapper<typename T::subnet_type,false> subnetwork;
            };

            template <typename T>
            class subnet_wrapper<T,false, typename std::enable_if<is_nonloss_layer_type<T>::value>::type>
            {

            public:
                subnet_wrapper(const subnet_wrapper&) = delete;
                subnet_wrapper& operator=(const subnet_wrapper&) = delete;

                typedef T wrapped_type;
                const static size_t num_computational_layers = T::num_computational_layers;
                const static size_t num_layers = T::num_layers;
                typedef typename T::layer_details_type layer_details_type;
                typedef typename T::input_layer_type input_layer_type;

                subnet_wrapper(T& l_, unsigned int = 0) : l(l_),subnetwork(l.subnet(), l.sample_expansion_factor()) {}

                const tensor& get_output() const { return l.get_output(); }
                tensor& get_gradient_input() { return l.get_gradient_input(); }

                const layer_details_type& layer_details() const { return l.layer_details(); }

                const subnet_wrapper<typename T::subnet_type,false>& subnet() const { return subnetwork; }
                subnet_wrapper<typename T::subnet_type,false>& subnet() { return subnetwork; }
                unsigned int sample_expansion_factor() const { return l.sample_expansion_factor(); }

                const input_layer_type& input_layer() const { return l.input_layer(); }
                input_layer_type& input_layer() { return l.input_layer(); }

            private:
                T& l;
                subnet_wrapper<typename T::subnet_type,false> subnetwork;
            };
        }

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

        enum class zero_gradients : uint8_t
        {
            no = 0,
            yes = 1
        };

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

        template <typename LAYER_DETAILS, typename SUBNET, typename enabled = void>
        class add_layer;

        template <typename LAYER_DETAILS, typename SUBNET, typename enabled>
        void serialize(const add_layer<LAYER_DETAILS,SUBNET,enabled>& item, std::ostream& out);
        template <typename LAYER_DETAILS, typename SUBNET, typename enabled>
        void deserialize(add_layer<LAYER_DETAILS,SUBNET,enabled>& item, std::istream& in);

        template <typename T, typename U>
        struct is_nonloss_layer_type<add_layer<T,U>> : std::true_type {};

        template <typename LAYER_DETAILS, typename SUBNET>
        class add_layer<LAYER_DETAILS,SUBNET,
                typename std::enable_if<is_nonloss_layer_type<SUBNET>::value>::type>
        {
        public:
            typedef LAYER_DETAILS layer_details_type;
            typedef SUBNET subnet_type;
            typedef typename subnet_type::input_layer_type input_layer_type;
            typedef typename subnet_type::input_type input_type;
            const static size_t num_layers = subnet_type::num_layers + 1;
            const static size_t num_computational_layers = subnet_type::num_computational_layers + 1;

            add_layer(
            ):
                subnetwork(new subnet_type()),
                this_layer_setup_called(false),
                gradient_input_is_stale(true),
                get_output_and_gradient_input_disabled(false)
            {
                if (this_layer_operates_inplace())
                    subnetwork->disable_output_and_gradient_getters();
            }

            add_layer(const add_layer& item)
            {
                details = item.details;
                subnetwork.reset(new subnet_type(*item.subnetwork));
                this_layer_setup_called = item.this_layer_setup_called;
                gradient_input_is_stale = item.gradient_input_is_stale;
                get_output_and_gradient_input_disabled = item.get_output_and_gradient_input_disabled;
                x_grad = item.x_grad;
                cached_output = item.cached_output;
                params_grad = item.params_grad;
                temp_tensor = item.temp_tensor;
            }
            add_layer& operator=(const add_layer& item) { add_layer(item).swap(*this); return *this;}
            add_layer(add_layer&& item) : add_layer() { swap(item); }
            add_layer& operator=(add_layer&& item) { swap(item); return *this; }

            template <typename T, typename U, typename E>
            friend class add_layer;
            template <typename T, bool is_first, typename E>
            friend class dimpl::subnet_wrapper;
            template <unsigned long T, typename U, typename E>
            friend class add_tag_layer;
            template <template<typename> class T, typename U>
            friend class add_skip_layer;
            template <size_t N, template<typename> class L, typename S>
            friend class repeat;

            // Allow copying networks from one to another as long as their corresponding
            // layers can be constructed from each other.
            template <typename T, typename U, typename E>
            add_layer(
                const add_layer<T,U,E>& item
            ) :
                details(item.layer_details()),
                subnetwork(new subnet_type(item.subnet())),
                this_layer_setup_called(item.this_layer_setup_called),
                gradient_input_is_stale(item.gradient_input_is_stale),
                get_output_and_gradient_input_disabled(item.get_output_and_gradient_input_disabled),
                x_grad(item.x_grad),
                cached_output(item.cached_output)
            {
                if (this_layer_operates_inplace())
                    subnetwork->disable_output_and_gradient_getters();
            }

            template <typename ...T>
            add_layer(
                const LAYER_DETAILS& layer_det,
                T&& ...args
            ) :
                details(layer_det),
                subnetwork(new subnet_type(std::forward<T>(args)...)),
                this_layer_setup_called(false),
                gradient_input_is_stale(true),
                get_output_and_gradient_input_disabled(false)
            {
                if (this_layer_operates_inplace())
                    subnetwork->disable_output_and_gradient_getters();
            }

            template <typename T, typename ...U>
            struct disable_forwarding_constr
            {
                const static bool value = std::is_constructible<LAYER_DETAILS,T>::value;
            };
            template <typename ...T, typename ...U>
            struct disable_forwarding_constr<std::tuple<T...>,U...>
            {
                const static bool value = disable_forwarding_constr<typename std::remove_reference<T>::type...>::value;
            };
            template <typename T, typename ...U>
            struct disable_forwarding_constr<std::tuple<T>,U...>
            {
                const static bool value = disable_forwarding_constr<typename std::remove_reference<T>::type>::value;
            };
            template <typename ...U>
            struct disable_forwarding_constr<std::tuple<>,U...>
            {
                const static bool value = true;
            };
            template <typename ...T>
            struct disable_forwarding_constr<add_layer<T...>>
            {
                const static bool value = true;
            };

            template <
                typename ...T,
                typename = typename std::enable_if<!disable_forwarding_constr<typename std::remove_reference<T>::type...>::value>::type
                >
            add_layer(
                T&& ...args
            ) :
                subnetwork(new subnet_type(std::forward<T>(args)...)),
                this_layer_setup_called(false),
                gradient_input_is_stale(true),
                get_output_and_gradient_input_disabled(false)
            {
                if (this_layer_operates_inplace())
                    subnetwork->disable_output_and_gradient_getters();
            }

            template <typename ...T>
            add_layer(
                LAYER_DETAILS&& layer_det,
                T&& ...args
            ) :
                details(std::move(layer_det)),
                subnetwork(new subnet_type(std::forward<T>(args)...)),
                this_layer_setup_called(false),
                gradient_input_is_stale(true),
                get_output_and_gradient_input_disabled(false)
            {
                if (this_layer_operates_inplace())
                    subnetwork->disable_output_and_gradient_getters();
            }

            template <typename ...T, typename LD, typename ...U>
            add_layer(
                const std::tuple<LD,U...>& layer_det,
                T&& ...args
            ) :
                details(tuple_head(layer_det)),
                subnetwork(new subnet_type(tuple_tail(layer_det),std::forward<T>(args)...)),
                this_layer_setup_called(false),
                gradient_input_is_stale(true),
                get_output_and_gradient_input_disabled(false)
            {
                if (this_layer_operates_inplace())
                    subnetwork->disable_output_and_gradient_getters();
            }

            template <typename ...T, typename LD, typename ...U>
            add_layer(
                std::tuple<>,
                const std::tuple<LD,U...>& layer_det,
                T&& ...args
            ) : add_layer(layer_det,args...) { }

            add_layer (
                std::tuple<>
            ) : add_layer() {}

            template <typename ...T>
            add_layer(
                std::tuple<>,
                LAYER_DETAILS&& layer_det,
                T&& ...args
            ) : add_layer(layer_det, args...) { }

            template <typename forward_iterator>
            void to_tensor (
                forward_iterator ibegin,
                forward_iterator iend,
                resizable_tensor& data
            ) const
            {
                subnetwork->to_tensor(ibegin,iend,data);
            }

            template <typename forward_iterator>
            const tensor& operator() (
                forward_iterator ibegin,
                forward_iterator iend
            )
            {
                to_tensor(ibegin,iend,temp_tensor);
                return forward(temp_tensor);
            }

            const tensor& operator() (const input_type& x)
            {
                return (*this)(&x, &x+1);
            }

            const tensor& forward(const tensor& x)
            {
                subnetwork->forward(x);
                const dimpl::subnet_wrapper<subnet_type> wsub(*subnetwork);
                if (!this_layer_setup_called)
                {
                    details.setup(wsub);
                    this_layer_setup_called = true;
                }
                if (this_layer_operates_inplace())
                    impl::call_layer_forward(details, wsub, private_get_output());
                else
                    impl::call_layer_forward(details, wsub, cached_output);

                gradient_input_is_stale = true;
                return private_get_output();
            }

        private:
            tensor& private_get_output() const
            {
                if (const_cast<add_layer&>(*this).this_layer_operates_inplace())
                    return subnetwork->private_get_output();
                else
                    return const_cast<resizable_tensor&>(cached_output);
            }
            tensor& private_get_gradient_input()
            {
                if (this_layer_operates_inplace())
                {
                    return subnetwork->private_get_gradient_input();
                }
                else
                {
                    if (gradient_input_is_stale)
                    {
                        gradient_input_is_stale = false;
                        x_grad.copy_size(private_get_output());
                        x_grad = 0;
                    }
                    return x_grad;
                }
            }
            void disable_output_and_gradient_getters (
            ) { get_output_and_gradient_input_disabled = true; }
        public:
            const tensor& get_output() const
            {
                if (get_output_and_gradient_input_disabled)
                    throw dlib::error("Accessing this layer's get_output() is disabled because an in-place layer has been stacked on top of it.");
                return private_get_output();
            }
            tensor& get_gradient_input()
            {
                if (get_output_and_gradient_input_disabled)
                    throw dlib::error("Accessing this layer's get_gradient_input() is disabled because an in-place layer has been stacked on top of it.");
                return private_get_gradient_input();
            }

            const tensor& get_final_data_gradient(
            ) const { return subnetwork->get_final_data_gradient(); }

            void back_propagate_error(
                const tensor& x,
                zero_gradients zero_grads = zero_gradients::yes
            )
            {
                back_propagate_error(x, private_get_gradient_input(), zero_grads);
            }
            void back_propagate_error(
                const tensor& x,
                const tensor& gradient_input,
                zero_gradients zero_grads = zero_gradients::yes
            )
            {
                dimpl::subnet_wrapper<subnet_type> wsub(*subnetwork);
                params_grad.copy_size(details.get_layer_params());
                impl::call_layer_backward(details, private_get_output(),
                    gradient_input, wsub, static_cast<tensor&>(params_grad));

                subnetwork->back_propagate_error(x, zero_grads);

                // zero out get_gradient_input()
                gradient_input_is_stale = zero_grads == zero_gradients::yes;
            }

            template <typename solver_type>
            void update_parameters(sstack<solver_type> solvers, double learning_rate)
            {
                DLIB_CASSERT(solvers.size()>=num_computational_layers);
                // Don't try to adjust the parameters if this layer doesn't have any or the
                // learning rate is disabled for this layer.
                if (params_grad.size() != 0 && get_learning_rate_multiplier(details) != 0)
                {
                    const tensor& step = solvers.top()(learning_rate, details, static_cast<const tensor&>(params_grad));
                    tt::add(details.get_layer_params(), details.get_layer_params(), step);
                }
                subnetwork->update_parameters(solvers.pop(), learning_rate);
            }

            template <typename solver_type>
            void update_parameters(std::vector<solver_type>& solvers, double learning_rate)
            {
                update_parameters(make_sstack(solvers), learning_rate);
            }

            const tensor& get_parameter_gradient(
            ) const { return params_grad; }

            tensor& get_parameter_gradient (
            ) { return params_grad; }

            const subnet_type& subnet() const { return *subnetwork; }
            subnet_type& subnet() { return *subnetwork; }

            const input_layer_type& input_layer() const { return subnet().input_layer(); }
            input_layer_type& input_layer() { return subnet().input_layer(); }

            const layer_details_type& layer_details() const { return details; }
            layer_details_type& layer_details() { return details; }

            unsigned int sample_expansion_factor() const { return subnet().sample_expansion_factor(); }

            void set_gradient_inputs_to_zero()
            {
                gradient_input_is_stale = true;
                subnetwork->set_gradient_inputs_to_zero();
            }

            void clean()
            {
                x_grad.clear();
                cached_output.clear();
                params_grad.clear();
                temp_tensor.clear();
                gradient_input_is_stale = true;
                subnetwork->clean();
                call_clean_method_if_exists(details);
            }

            friend void serialize(const add_layer& item, std::ostream& out)
            {
                int version = 2;
                serialize(version, out);
                serialize(*item.subnetwork, out);
                serialize(item.details, out);
                serialize(item.this_layer_setup_called, out);
                serialize(item.gradient_input_is_stale, out);
                serialize(item.get_output_and_gradient_input_disabled, out);
                serialize(item.x_grad, out);
                serialize(item.cached_output, out);
                serialize(item.params_grad, out);
            }

            friend void deserialize(add_layer& item, std::istream& in)
            {
                int version = 0;
                deserialize(version, in);
                if (!(1 <= version && version <= 2))
                    throw serialization_error("Unexpected version found while deserializing dlib::add_layer.");
                deserialize(*item.subnetwork, in);
                deserialize(item.details, in);
                deserialize(item.this_layer_setup_called, in);
                deserialize(item.gradient_input_is_stale, in);
                deserialize(item.get_output_and_gradient_input_disabled, in);
                deserialize(item.x_grad, in);
                deserialize(item.cached_output, in);
                if (version == 2)
                    deserialize(item.params_grad, in);
            }

            friend std::ostream& operator<< (std::ostream& out, const add_layer& item)
            {
                int min_length = 0;
                item.print(out, 0, min_length);
                return out;
            }

            void print (std::ostream& out, unsigned long idx, int& min_length) const
            {
                out << "layer<" << idx << ">\t" << impl::tensor_to_str(private_get_output(), min_length) << layer_details() << "\n";
                subnet().print(out, idx+1, min_length);
            }

        private:

            bool this_layer_operates_inplace(
            )
            {
                // This layer can run in-place if it's an in-place capable layer and also if
                // the layer it's on top of doesn't need its own output tensor (since in-place
                // layers overwrite that tensor)
                return impl::is_inplace_layer(details, *subnetwork) && !subnetwork->this_layer_requires_forward_output();
            }
            bool this_layer_requires_forward_output(
            )
            {
                return impl::backward_requires_forward_output(details, *subnetwork);
            }

            void swap(add_layer& item)
            {
                std::swap(subnetwork,item.subnetwork);
                std::swap(details, item.details);
                std::swap(this_layer_setup_called, item.this_layer_setup_called);
                std::swap(gradient_input_is_stale, item.gradient_input_is_stale);
                std::swap(get_output_and_gradient_input_disabled, item.get_output_and_gradient_input_disabled);
                std::swap(x_grad, item.x_grad);
                std::swap(cached_output, item.cached_output);
                std::swap(params_grad, item.params_grad);
            }

            LAYER_DETAILS details;
            std::unique_ptr<subnet_type> subnetwork;
            bool this_layer_setup_called;
            bool gradient_input_is_stale;
            bool get_output_and_gradient_input_disabled;
            // Note that if this_layer_operates_inplace()==true then x_grad and cached_output
            // are not used at all.  Instead, this layer uses these variables from the lower
            // layer.
            resizable_tensor x_grad;
            resizable_tensor cached_output;

            resizable_tensor params_grad;

            // temp_tensor doesn't logically contribute to the state of this object.
            // It is here only to prevent it from being reallocated over and over.
            resizable_tensor temp_tensor;

        };

        template <typename T, typename U, typename E>
        struct is_add_layer<add_layer<T,U,E>> : std::true_type {};
        template <typename T, typename U, typename E>
        struct is_add_layer<const add_layer<T,U,E>> : std::true_type {};
        template <typename T, typename U, typename E>
        struct is_add_layer<add_layer<T,U,E>&> : std::true_type {};
        template <typename T, typename U, typename E>
        struct is_add_layer<const add_layer<T,U,E>&> : std::true_type {};

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

    // This version of add_layer handles the special case where the subnetwork being given is
    // just an input layer object.
        template <typename LAYER_DETAILS, typename INPUT_LAYER, typename enabled>
        class add_layer
        {
        public:
            typedef LAYER_DETAILS layer_details_type;
            typedef INPUT_LAYER subnet_type;
            typedef INPUT_LAYER input_layer_type;
            typedef typename INPUT_LAYER::input_type input_type;
            const static size_t num_layers = 2;
            const static size_t num_computational_layers = 1;

            add_layer(
            ):
                this_layer_setup_called(false),
                gradient_input_is_stale(true),
                get_output_and_gradient_input_disabled(false),
                _sample_expansion_factor(0)
            {}

            add_layer(const add_layer&) = default;
            add_layer(add_layer&& item) : add_layer() { swap(item); }
            add_layer& operator=(const add_layer&) = default;
            add_layer& operator=(add_layer&& item) { swap(item); return *this; }

            template <typename T, typename U, typename E>
            friend class add_layer;
            template <typename T, bool is_first, typename E>
            friend class dimpl::subnet_wrapper;
            template <unsigned long T, typename U, typename E>
            friend class add_tag_layer;
            template <template<typename> class T, typename U>
            friend class add_skip_layer;
            template <size_t N, template<typename> class L, typename S>
            friend class repeat;

            // Allow copying networks from one to another as long as their corresponding
            // layers can be constructed from each other.
            template <typename T, typename U, typename E>
            add_layer(
                const add_layer<T,U,E>& item
            ):
                input_layer_(item.subnet()),
                details(item.layer_details()),
                this_layer_setup_called(item.this_layer_setup_called),
                gradient_input_is_stale(item.gradient_input_is_stale),
                get_output_and_gradient_input_disabled(false),
                _sample_expansion_factor(item._sample_expansion_factor),
                x_grad(item.x_grad),
                cached_output(item.cached_output),
                grad_final(item.grad_final)
            {
            }

            add_layer(
                const LAYER_DETAILS& layer_det
            ) :
                details(layer_det),
                this_layer_setup_called(false),
                gradient_input_is_stale(true),
                get_output_and_gradient_input_disabled(false),
                _sample_expansion_factor(0)
            {}

            add_layer(
                const INPUT_LAYER& il
            ) :
                input_layer_(il),
                this_layer_setup_called(false),
                gradient_input_is_stale(true),
                get_output_and_gradient_input_disabled(false),
                _sample_expansion_factor(0)
            {}

            add_layer(
                LAYER_DETAILS&& layer_det
            ) :
                details(std::move(layer_det)),
                this_layer_setup_called(false),
                gradient_input_is_stale(true),
                get_output_and_gradient_input_disabled(false),
                _sample_expansion_factor(0)
            {}

            add_layer(
                LAYER_DETAILS layer_det,
                INPUT_LAYER il
            ) :
                details(std::move(layer_det)),
                input_layer_(std::move(il)),
                this_layer_setup_called(false),
                gradient_input_is_stale(true),
                get_output_and_gradient_input_disabled(false),
                _sample_expansion_factor(0)
            {}

            add_layer(
                std::tuple<>,
                const LAYER_DETAILS& layer_det
            ) : add_layer(layer_det) {}

            add_layer(
                std::tuple<>,
                LAYER_DETAILS&& layer_det
            ) : add_layer(layer_det) {}

            add_layer(
                std::tuple<>,
                LAYER_DETAILS layer_det,
                INPUT_LAYER il
            ) : add_layer(layer_det,il) {}

            add_layer(
                const std::tuple<LAYER_DETAILS>& layer_det
            ) : add_layer(tuple_head(layer_det)) {}

            add_layer(
                const std::tuple<LAYER_DETAILS>& layer_det,
                INPUT_LAYER il
            ) : add_layer(tuple_head(layer_det),il) {}

            template <typename forward_iterator>
            void to_tensor (
                forward_iterator ibegin,
                forward_iterator iend,
                resizable_tensor& data
            ) const
            {
                input_layer_.to_tensor(ibegin, iend, data);
                // make sure the input layer's to_tensor() function is implemented properly.
                DLIB_CASSERT(data.num_samples() >= std::distance(ibegin,iend),
                "The input layer can't produce fewer output tensors than there are inputs.");
                DLIB_CASSERT(data.num_samples()%std::distance(ibegin,iend) == 0,
                "The number of tensors produced by the input layer must be an integer multiple of the number of input objects.");

                _sample_expansion_factor = data.num_samples()/std::distance(ibegin,iend);
                data.async_copy_to_device();
            }

            template <typename forward_iterator>
            const tensor& operator() (
                forward_iterator ibegin,
                forward_iterator iend
            )
            {
                to_tensor(ibegin,iend,temp_tensor);
                return forward(temp_tensor);
            }

            const tensor& operator() (const input_type& x)
            {
                return (*this)(&x, &x+1);
            }

            const tensor& forward (const tensor& x)
            {
                DLIB_CASSERT(sample_expansion_factor() != 0, "You must call to_tensor() before this function can be used.");
                DLIB_CASSERT(x.num_samples()%sample_expansion_factor() == 0);
                subnet_wrapper wsub(x, grad_final, _sample_expansion_factor);
                if (!this_layer_setup_called)
                {
                    details.setup(wsub);
                    this_layer_setup_called = true;
                }
                impl::call_layer_forward(details, wsub, cached_output);
                gradient_input_is_stale = true;
                return private_get_output();
            }

        private:
            tensor& private_get_output() const { return const_cast<resizable_tensor&>(cached_output); }
            tensor& private_get_gradient_input()
            {
                if (gradient_input_is_stale)
                {
                    gradient_input_is_stale = false;
                    x_grad.copy_size(private_get_output());
                    x_grad = 0;
                }
                return x_grad;
            }
            void disable_output_and_gradient_getters (
            ) { get_output_and_gradient_input_disabled = true; }
        public:
            const tensor& get_output() const
            {
                if (get_output_and_gradient_input_disabled)
                    throw dlib::error("Accessing this layer's get_output() is disabled because an in-place layer has been stacked on top of it.");
                return private_get_output();
            }
            tensor& get_gradient_input()
            {
                if (get_output_and_gradient_input_disabled)
                    throw dlib::error("Accessing this layer's get_gradient_input() is disabled because an in-place layer has been stacked on top of it.");
                return private_get_gradient_input();
            }

            const tensor& get_final_data_gradient(
            ) const { return grad_final; }

            void back_propagate_error(
                const tensor& x,
                zero_gradients zero_grads = zero_gradients::yes
            )
            {
                back_propagate_error(x, private_get_gradient_input(), zero_grads);
            }
            void back_propagate_error(
                const tensor& x,
                const tensor& gradient_input,
                zero_gradients zero_grads = zero_gradients::yes
            )
            {
                // make sure grad_final is initialized to 0
                if (!have_same_dimensions(x, grad_final))
                    grad_final.copy_size(x);
                grad_final = 0;

                subnet_wrapper wsub(x, grad_final, _sample_expansion_factor);
                params_grad.copy_size(details.get_layer_params());
                impl::call_layer_backward(details, private_get_output(),
                    gradient_input, wsub, static_cast<tensor&>(params_grad));

                // zero out get_gradient_input()
                gradient_input_is_stale = zero_grads == zero_gradients::yes;
            }

            template <typename solver_type>
            void update_parameters(sstack<solver_type> solvers, double learning_rate)
            {
                DLIB_CASSERT(solvers.size()>=num_computational_layers);
                // Don't try to adjust the parameters if this layer doesn't have any or the
                // learning rate is disabled for this layer.
                if (params_grad.size() != 0 && get_learning_rate_multiplier(details) != 0)
                {
                    const tensor& step = solvers.top()(learning_rate, details, static_cast<const tensor&>(params_grad));
                    tt::add(details.get_layer_params(), details.get_layer_params(), step);
                }
            }

            template <typename solver_type>
            void update_parameters(std::vector<solver_type>& solvers, double learning_rate)
            {
                update_parameters(make_sstack(solvers), learning_rate);
            }

            const tensor& get_parameter_gradient(
            ) const { return params_grad; }

            tensor& get_parameter_gradient (
            )  { return params_grad; }

            const subnet_type& subnet() const { return input_layer_; }
            subnet_type& subnet() { return input_layer_; }

            const subnet_type& input_layer() const { return input_layer_; }
            subnet_type& input_layer() { return input_layer_; }

            const layer_details_type& layer_details() const { return details; }
            layer_details_type& layer_details() { return details; }

            unsigned int sample_expansion_factor() const { return _sample_expansion_factor; }

            void set_gradient_inputs_to_zero()
            {
                gradient_input_is_stale = true;
            }

            void clean()
            {
                x_grad.clear();
                grad_final.clear();
                cached_output.clear();
                params_grad.clear();
                temp_tensor.clear();
                gradient_input_is_stale = true;
                call_clean_method_if_exists(details);
            }

            friend void serialize(const add_layer& item, std::ostream& out)
            {
                int version = 3;
                serialize(version, out);
                serialize(item.input_layer_, out);
                serialize(item.details, out);
                serialize(item.this_layer_setup_called, out);
                serialize(item.gradient_input_is_stale, out);
                serialize(item.get_output_and_gradient_input_disabled, out);
                serialize(item.x_grad, out);
                serialize(item.cached_output, out);
                serialize(item.grad_final, out);
                serialize(item._sample_expansion_factor, out);
            }

            friend void deserialize(add_layer& item, std::istream& in)
            {
                int version = 0;
                deserialize(version, in);
                if (!(2 <= version && version <= 3))
                    throw serialization_error("Unexpected version found while deserializing dlib::add_layer.");
                deserialize(item.input_layer_, in);
                deserialize(item.details, in);
                deserialize(item.this_layer_setup_called, in);
                deserialize(item.gradient_input_is_stale, in);
                deserialize(item.get_output_and_gradient_input_disabled, in);
                deserialize(item.x_grad, in);
                deserialize(item.cached_output, in);
                deserialize(item.grad_final, in);
                if (version >= 3)
                    deserialize(item._sample_expansion_factor, in);
                else
                    item._sample_expansion_factor = 1; // all layer types set this to 1 in older dlib versions, so that's what we put here.
            }

            friend std::ostream& operator<< (std::ostream& out, const add_layer& item)
            {
                int min_length = 0;
                item.print(out, 0, min_length);
                return out;
            }

            void print (std::ostream& out, unsigned long idx, int& min_length) const
            {
                out << "layer<" << idx << ">\t" << impl::tensor_to_str(private_get_output(), min_length) << layer_details() << "\n";

                // Don't print the repeat_input_layer since it doesn't exist from the user's
                // point of view.  It's just an artifact of how repeat<> works.
                if (!std::is_same<subnet_type, impl::repeat_input_layer>::value)
                    out << "layer<" << idx+1 << ">\t" << subnet() << "\n";
            }

        private:

            bool this_layer_requires_forward_output(
            )
            {
                subnet_wrapper wsub(grad_final, grad_final, _sample_expansion_factor);
                return impl::backward_requires_forward_output(details, wsub);
            }

            class subnet_wrapper
            {
            public:
                subnet_wrapper(const tensor& x_, resizable_tensor& grad_final_, unsigned int sef) :
                    x(x_), grad_final(grad_final_), _sample_expansion_factor(sef) {}

                subnet_wrapper(const subnet_wrapper&) = delete;
                subnet_wrapper& operator=(const subnet_wrapper&) = delete;

                unsigned int sample_expansion_factor() const { return _sample_expansion_factor;}
                const tensor& get_output() const { return x; }
                tensor& get_gradient_input()
                {
                    if (!have_same_dimensions(x, grad_final))
                    {
                        grad_final.copy_size(x);
                        grad_final = 0;
                    }
                    return grad_final;
                }

            private:
                const tensor& x;
                resizable_tensor& grad_final;
                unsigned int _sample_expansion_factor;
            };

            void swap(add_layer& item)
            {
                std::swap(input_layer_, item.input_layer_);
                std::swap(details, item.details);
                std::swap(this_layer_setup_called, item.this_layer_setup_called);
                std::swap(gradient_input_is_stale, item.gradient_input_is_stale);
                std::swap(get_output_and_gradient_input_disabled, item.get_output_and_gradient_input_disabled);
                std::swap(x_grad, item.x_grad);
                std::swap(cached_output, item.cached_output);
                std::swap(grad_final, item.grad_final);
                std::swap(_sample_expansion_factor, item._sample_expansion_factor);
            }

            subnet_type input_layer_;
            LAYER_DETAILS details;
            bool this_layer_setup_called;
            bool gradient_input_is_stale;
            bool get_output_and_gradient_input_disabled;
            mutable unsigned int _sample_expansion_factor;
            resizable_tensor x_grad;
            resizable_tensor cached_output;
            resizable_tensor grad_final;

            // The following 2 objects don't logically contribute to the state of this class.
            // They are only here to prevent them from being reallocated over and over in
            // member functions.
            resizable_tensor params_grad;
            resizable_tensor temp_tensor;
        };

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

        template <unsigned long ID, typename SUBNET, typename enabled=void>
        class add_tag_layer;

        template <template<typename SUBNET> class tag>
        struct tag_id
        {
            const static unsigned long id = tag<impl::repeat_input_layer>::id;
        };

        template <unsigned long ID, typename SUBNET>
        class add_tag_layer<ID,SUBNET,
                typename std::enable_if<is_nonloss_layer_type<SUBNET>::value>::type>
        {
        public:
            typedef SUBNET subnet_type;
            typedef typename subnet_type::input_type input_type;
            typedef typename subnet_type::input_layer_type input_layer_type;
            typedef int layer_details_type; // not really used anywhere, but required by subnet_wrapper.
            const static size_t num_layers = subnet_type::num_layers + 1;
            const static size_t num_computational_layers = subnet_type::num_computational_layers;
            const static unsigned long id = ID;

            add_tag_layer() {};
            add_tag_layer(const add_tag_layer&) = default;
            add_tag_layer(add_tag_layer&&) = default;
            add_tag_layer& operator=(add_tag_layer&&) = default;
            add_tag_layer& operator=(const add_tag_layer&) = default;

            template <typename T>
            add_tag_layer(
                const add_tag_layer<ID,T>& item
            ) : subnetwork(item.subnet())
            {}

            template <typename ...T>
            add_tag_layer(
                T ...args
            ) :
                subnetwork(std::move(args)...)
            {
            }

            template <typename forward_iterator>
            void to_tensor (
                forward_iterator ibegin,
                forward_iterator iend,
                resizable_tensor& data
            ) const
            {
                subnetwork.to_tensor(ibegin,iend,data);
            }

            template <typename forward_iterator>
            const tensor& operator() (
                forward_iterator ibegin,
                forward_iterator iend
            )
            {
                return subnetwork(ibegin,iend);
            }

            const tensor& operator() (const input_type& x)
            {
                return subnetwork(x);
            }

            const tensor& forward(const tensor& x)
            {
                return subnetwork.forward(x);
            }

            const tensor& get_output() const { return subnetwork.get_output(); }

            tensor& get_gradient_input()
            {
                return subnetwork.get_gradient_input();
            }

            const tensor& get_final_data_gradient(
            ) const { return subnetwork.get_final_data_gradient(); }

            void back_propagate_error(
                const tensor& x,
                zero_gradients zero_grads = zero_gradients::yes
            )
            {
                subnetwork.back_propagate_error(x, zero_grads);
            }
            void back_propagate_error(
                const tensor& x,
                const tensor& gradient_input,
                zero_gradients zero_grads = zero_gradients::yes
            )
            {
                subnetwork.back_propagate_error(x,gradient_input, zero_grads);
            }

            template <typename solver_type>
            void update_parameters(sstack<solver_type> solvers, double learning_rate)
            {
                subnetwork.update_parameters(solvers, learning_rate);
            }

            template <typename solver_type>
            void update_parameters(std::vector<solver_type>& solvers, double learning_rate)
            {
                update_parameters(make_sstack(solvers), learning_rate);
            }

            const tensor& get_parameter_gradient(
            ) const { return params_grad; }

            tensor& get_parameter_gradient (
            ) { return params_grad; }

            const subnet_type& subnet() const { return subnetwork; }
            subnet_type& subnet() { return subnetwork; }

            const input_layer_type& input_layer() const { return subnet().input_layer(); }
            input_layer_type& input_layer() { return subnet().input_layer(); }

            unsigned int sample_expansion_factor() const { return subnet().sample_expansion_factor(); }

            void set_gradient_inputs_to_zero()
            {
                subnetwork.set_gradient_inputs_to_zero();
            }

            void clean()
            {
                subnetwork.clean();
            }

            friend void serialize(const add_tag_layer& item, std::ostream& out)
            {
                int version = 1;
                serialize(version, out);
                serialize(item.subnetwork, out);
            }

            friend void deserialize(add_tag_layer& item, std::istream& in)
            {
                int version = 0;
                deserialize(version, in);
                if (version != 1)
                    throw serialization_error("Unexpected version found while deserializing dlib::add_tag_layer.");
                deserialize(item.subnetwork, in);
            }

            friend std::ostream& operator<< (std::ostream& out, const add_tag_layer& item)
            {
                int min_length = 0;
                item.print(out, 0, min_length);
                return out;
            }

            void print (std::ostream& out, unsigned long idx, int& min_length) const
            {
                out << "layer<" << idx << ">\t" << impl::tensor_to_str(private_get_output(), min_length) << "tag" << ID << "\n";
                subnet().print(out, idx+1, min_length);
            }

        private:

            template <typename T, typename U, typename E>
            friend class add_layer;
            template <typename T, bool is_first, typename E>
            friend class dimpl::subnet_wrapper;
            template <unsigned long T, typename U, typename E>
            friend class add_tag_layer;
            template <template<typename> class T, typename U>
            friend class add_skip_layer;
            template <size_t N, template<typename> class L, typename S>
            friend class repeat;

            // You wouldn't put a tag on a layer if you didn't want to access its forward
            // outputs.  So this is always true.
            bool this_layer_requires_forward_output(
            ) { return true; }

            void disable_output_and_gradient_getters (
            )
            {
                // This should never happen because only inplace layers call
                // disable_output_and_gradient_getters(), however, putting a tag layer right
                // before an inplace layer basically means you don't want the following layer
                // to operate in place.  So the inplace layer should turn itself into an
                // out-of-place layer and not call disable_output_and_gradient_getters().
                DLIB_CASSERT(false,"This should never happen");
            }

            tensor& private_get_output() const
            { return subnetwork.private_get_output(); }
            tensor& private_get_gradient_input()
            { return subnetwork.private_get_gradient_input(); }

            subnet_type subnetwork;

            // This member doesn't logically contribute to the state of the object since it is
            // always empty. It's just here so we can have the get_parameter_gradient() methods
            // which have to return something.  So they return this empty tensor.
            resizable_tensor params_grad;
        };

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

        template <typename ...T>
        struct decorator_repeat_group
        {
            decorator_repeat_group(
                T&& ...args
            ) : data(std::forward<T>(args)...) {}

            std::tuple<T...> data;
        };
        template <typename ...T>
        decorator_repeat_group<T...> repeat_group (
            T&& ...args
        )
        {
            return decorator_repeat_group<T...>(std::forward<T>(args)...);
        }

        template <
            size_t num,
            template<typename> class REPEATED_LAYER,
            typename SUBNET
            >
        class repeat
        {
            static_assert(num > 0, "You can't have a layer repeated 0 times.");
        public:
            typedef SUBNET subnet_type;
            typedef typename SUBNET::input_type input_type;
            typedef typename subnet_type::input_layer_type input_layer_type;
            typedef int layer_details_type; // not really used anywhere, but required by subnet_wrapper.
            const static size_t comp_layers_in_each_group = (REPEATED_LAYER<SUBNET>::num_computational_layers-SUBNET::num_computational_layers);
            const static size_t comp_layers_in_repeated_group = comp_layers_in_each_group*num;
            const static size_t num_computational_layers = comp_layers_in_repeated_group + SUBNET::num_computational_layers;

            const static size_t layers_in_each_group = (REPEATED_LAYER<SUBNET>::num_layers-SUBNET::num_layers);
            const static size_t layers_in_repeated_group = layers_in_each_group*num;
            const static size_t num_layers = subnet_type::num_layers + layers_in_repeated_group;

            typedef REPEATED_LAYER<impl::repeat_input_layer> repeated_layer_type;

            repeat(
            ) :
                details(num)
            {
            }

            size_t num_repetitions (
            ) const { return num; }

            const repeated_layer_type& get_repeated_layer (
                size_t i
            ) const
            {
                DLIB_CASSERT(i < num_repetitions());
                return details[i];
            }

            repeated_layer_type& get_repeated_layer (
                size_t i
            )
            {
                DLIB_CASSERT(i < num_repetitions());
                return details[i];
            }

            repeat(const repeat&) = default;
            repeat(repeat&&) = default;
            repeat& operator=(repeat&&) = default;
            repeat& operator=(const repeat&) = default;

            template <template<typename> class T, typename U>
            repeat(
                const repeat<num,T,U>& item
            ) :
                subnetwork(item.subnetwork)
            {
                for (auto&& d : item.details)
                    details.emplace_back(d);
            }

            template <typename T, typename ...U>
            repeat(
                T arg1,
                U ...args2
            ):
                details(num, std::move(arg1)),
                subnetwork(std::move(args2)...)
            {
            }

            template <typename ...T, typename ...U>
            repeat(
                decorator_repeat_group<T...>&& arg1,
                U ...args2
            ):
                details(num, arg1.data),
                subnetwork(std::move(args2)...)
            {
            }

            template <typename T, typename ...U>
            repeat(
                std::tuple<>,
                T arg1,
                U ...args2
            ):
                details(num, std::move(arg1)),
                subnetwork(std::move(args2)...)
            {
            }

            template <typename forward_iterator>
            void to_tensor (
                forward_iterator ibegin,
                forward_iterator iend,
                resizable_tensor& data
            ) const
            {
                subnetwork.to_tensor(ibegin,iend,data);
                // call to_tensor on the networks in details just to populate the
                // _sample_expansion_factor values in those networks.  Other than that this
                // call is a noop.
                for (auto& d : details)
                    d.to_tensor(ibegin, iend, data);
            }

            template <typename forward_iterator>
            const tensor& operator() (
                forward_iterator ibegin,
                forward_iterator iend
            )
            {
                to_tensor(ibegin,iend,temp_tensor);
                return forward(temp_tensor);
            }

            const tensor& operator() (const input_type& x)
            {
                return (*this)(&x, &x+1);
            }

            const tensor& forward(const tensor& x)
            {
                subnetwork.forward(x);
                details[details.size()-1].forward(subnetwork.get_output());
                for (long i = details.size()-2; i >= 0; --i)
                    details[i].forward(details[i+1].get_output());
                return private_get_output();
            }

        private:
            tensor& private_get_output() const
            {
                return details[0].private_get_output();
            }
            tensor& private_get_gradient_input()
            {
                return details[0].private_get_gradient_input();
            }
        public:
            const tensor& get_output() const
            {
                return details[0].get_output();
            }
            tensor& get_gradient_input()
            {
                return details[0].get_gradient_input();
            }

            const tensor& get_final_data_gradient(
            ) const { return subnetwork.get_final_data_gradient(); }

            const tensor& get_parameter_gradient(
            ) const { return details[0].get_parameter_gradient(); }

            tensor& get_parameter_gradient (
            ) { return details[0].get_parameter_gradient(); }

            void back_propagate_error(
                const tensor& x,
                zero_gradients zero_grads = zero_gradients::yes
            )
            {
                back_propagate_error(x, private_get_gradient_input(), zero_grads);
            }
            void back_propagate_error(
                const tensor& x,
                const tensor& gradient_input,
                zero_gradients zero_grads = zero_gradients::yes
            )
            {
                if (details.size() > 1)
                {
                    details[0].back_propagate_error(details[1].get_output(), gradient_input, zero_grads);
                    for (size_t i = 1; i < details.size(); ++i)
                    {
                        if (i+1 < details.size())
                            details[i].back_propagate_error(details[i+1].get_output(), details[i-1].get_final_data_gradient(), zero_grads);
                        else
                            details[i].back_propagate_error(subnetwork.get_output(), details[i-1].get_final_data_gradient(), zero_grads);
                    }
                }
                else
                {
                    details[0].back_propagate_error(subnetwork.get_output(), gradient_input, zero_grads);
                }
                subnetwork.back_propagate_error(x, details.back().get_final_data_gradient(), zero_grads);
            }

            template <typename solver_type>
            void update_parameters(sstack<solver_type> solvers, double learning_rate)
            {
                for (size_t i = 0; i < details.size(); ++i)
                    details[i].update_parameters(solvers.pop(comp_layers_in_each_group*i),learning_rate);
                subnetwork.update_parameters(solvers.pop(comp_layers_in_each_group*details.size()),learning_rate);
            }

            template <typename solver_type>
            void update_parameters(std::vector<solver_type>& solvers, double learning_rate)
            {
                update_parameters(make_sstack(solvers), learning_rate);
            }

            const subnet_type& subnet() const { return subnetwork; }
            subnet_type& subnet() { return subnetwork; }

            const input_layer_type& input_layer() const { return subnet().input_layer(); }
            input_layer_type& input_layer() { return subnet().input_layer(); }

            unsigned int sample_expansion_factor() const { return subnet().sample_expansion_factor(); }

            void set_gradient_inputs_to_zero()
            {
                subnetwork.set_gradient_inputs_to_zero();
            }

            void clean()
            {
                temp_tensor.clear();
                subnetwork.clean();
                for (auto&& d : details)
                    d.clean();
            }

            friend void serialize(const repeat& item, std::ostream& out)
            {
                int version = 1;
                serialize(version, out);
                serialize(item.details, out);
                serialize(item.subnetwork, out);
            }

            friend void deserialize(repeat& item, std::istream& in)
            {
                int version = 0;
                deserialize(version, in);
                if (version != 1)
                    throw serialization_error("Unexpected version found while deserializing dlib::repeat.");
                deserialize(item.details, in);
                deserialize(item.subnetwork, in);
            }

            friend std::ostream& operator<< (std::ostream& out, const repeat& item)
            {
                int min_length = 0;
                item.print(out, 0, min_length);
                return out;
            }

            void print (std::ostream& out, unsigned long idx, int& min_length) const
            {
                for (size_t i = 0; i < num_repetitions(); ++i)
                {
                    get_repeated_layer(i).print(out, idx, min_length);
                    idx += layers_in_each_group;
                }
                subnet().print(out, idx, min_length);
            }
        private:

            template <typename T, typename U, typename E>
            friend class add_layer;
            template <typename T, bool is_first, typename E>
            friend class dimpl::subnet_wrapper;
            template <unsigned long T, typename U, typename E>
            friend class add_tag_layer;
            template <template<typename> class T, typename U>
            friend class add_skip_layer;
            template <size_t N, template<typename> class L, typename S>
            friend class repeat;

            bool this_layer_requires_forward_output(
            )
            {
                return details[0].this_layer_requires_forward_output();
            }

            void disable_output_and_gradient_getters (
            )
            {
                details[0].disable_output_and_gradient_getters();
            }

            std::vector<repeated_layer_type> details;
            subnet_type subnetwork;

            // temp_tensor doesn't logically contribute to the state of this class.
            // It is here only to void needing to reallocate it over and over.
            resizable_tensor temp_tensor;
        };

        template <
            size_t num,
            template<typename> class REPEATED_LAYER,
            typename SUBNET
            >
        struct is_nonloss_layer_type<repeat<num,REPEATED_LAYER,SUBNET>> : std::true_type {};

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

    // This version of add_tag_layer handles the special case where the subnetwork being given
    // is just an input layer object.
        template <unsigned long ID, typename INPUT_LAYER, typename enabled>
        class add_tag_layer
        {
        public:
            typedef INPUT_LAYER subnet_type;
            typedef typename subnet_type::input_type input_type;
            typedef INPUT_LAYER input_layer_type;
            typedef int layer_details_type; // not really used anywhere, but required by subnet_wrapper.
            const static size_t num_computational_layers = 0;
            const static size_t num_layers = 2;
            const static unsigned long id = ID;

            add_tag_layer():cached_output_ptr(nullptr),gradient_input_is_stale(true),_sample_expansion_factor(0) {}

            add_tag_layer(const add_tag_layer&) = default;
            add_tag_layer& operator=(const add_tag_layer&) = default;
            add_tag_layer(add_tag_layer&& item) : add_tag_layer() { swap(item); }
            add_tag_layer& operator=(add_tag_layer&& item) { swap(item); return *this; }

            template <typename T, typename E>
            add_tag_layer(
                const add_tag_layer<ID,T,E>& item
            ) : input_layer_(item.subnet()),
                cached_output(item.cached_output),
                cached_output_ptr(nullptr),
                grad_final(item.grad_final),
                gradient_input_is_stale(item.gradient_input_is_stale),
                _sample_expansion_factor(0)
            {}

            template <typename ...T>
            add_tag_layer(
                T ...args
            ) :
                input_layer_(std::move(args)...),
                cached_output_ptr(nullptr),
                gradient_input_is_stale(true),
                _sample_expansion_factor(0)
            {
            }

            add_tag_layer (
                std::tuple<>
            ) :
                cached_output_ptr(nullptr),
                gradient_input_is_stale(true),
                _sample_expansion_factor(0)
            {}

            template <typename forward_iterator>
            void to_tensor (
                forward_iterator ibegin,
                forward_iterator iend,
                resizable_tensor& data
            ) const
            {
                input_layer_.to_tensor(ibegin,iend,data);

                // make sure the input layer's to_tensor() function is implemented properly.
                DLIB_CASSERT(data.num_samples() >= std::distance(ibegin,iend),
                "The input layer can't produce fewer output tensors than there are inputs.");
                DLIB_CASSERT(data.num_samples()%std::distance(ibegin,iend) == 0,
                "The number of tensors produced by the input layer must be an integer multiple of the number of input objects.");

                _sample_expansion_factor = data.num_samples()/std::distance(ibegin,iend);
                data.async_copy_to_device();
            }

            unsigned int sample_expansion_factor() const { return _sample_expansion_factor; }

            template <typename forward_iterator>
            const tensor& operator() (
                forward_iterator ibegin,
                forward_iterator iend
            )
            {
                input_layer_.to_tensor(ibegin,iend,cached_output);
                cached_output_ptr = nullptr;
                return get_output();
            }

            const tensor& operator() (const input_type& x)
            {
                return (*this)(&x, &x+1);
            }

            const tensor& forward(const tensor& x)
            {
                // If this tag is the first layer in one of the sub networks inside a repeat
                // layer then we don't want it to be creating copies of x.  This is because, we
                // can just hold a pointer to x since the way repeat is constructed guarantees
                // that x will have a lifetime larger than this pointer.
                if (is_same_type<INPUT_LAYER, impl::repeat_input_layer>::value)
                    cached_output_ptr = const_cast<tensor*>(&x);
                else
                    cached_output = x;
                gradient_input_is_stale = true;
                return get_output();
            }

            const tensor& get_output() const
            {
                if (cached_output_ptr)
                    return *cached_output_ptr;
                else
                    return cached_output;
            }

            const tensor& get_final_data_gradient(
            ) const { return grad_final; }

            tensor& get_gradient_input()
            {
                if (!have_same_dimensions(get_output(), grad_final) ||
                    gradient_input_is_stale)
                {
                    grad_final.copy_size(get_output());
                    grad_final = 0;
                    gradient_input_is_stale = false;
                }
                return grad_final;
            }

            void back_propagate_error(
                const tensor& /*x*/,
                zero_gradients /*zero_grads*/ = zero_gradients::yes
            )
            {
                // nothing to do
            }
            void back_propagate_error(
                const tensor& /*x*/,
                const tensor& /*gradient_input*/,
                zero_gradients /*zero_grads*/ = zero_gradients::yes
            )
            {
                // nothing to do
            }

            template <typename solver_type>
            void update_parameters(sstack<solver_type> /*solvers*/, double /*learning_rate*/)
            {
                // nothing to do
            }

            template <typename solver_type>
            void update_parameters(std::vector<solver_type>& solvers, double learning_rate)
            {
                update_parameters(make_sstack(solvers), learning_rate);
            }

            const subnet_type& subnet() const { return input_layer_; }
            subnet_type& subnet() { return input_layer_; }

            const input_layer_type& input_layer() const { return input_layer_; }
            input_layer_type& input_layer() { return input_layer_; }

            void set_gradient_inputs_to_zero()
            {
                // nothing to do
            }

            void clean()
            {
                grad_final.clear();
                cached_output.clear();
                cached_output_ptr = 0;
            }

            friend void serialize(const add_tag_layer& item, std::ostream& out)
            {
                int version = 2;
                serialize(version, out);
                serialize(item.input_layer_, out);
                serialize(item.cached_output, out);
                serialize(item.grad_final, out);
                serialize(item.gradient_input_is_stale, out);
                serialize(item._sample_expansion_factor, out);
            }

            friend void deserialize(add_tag_layer& item, std::istream& in)
            {
                int version = 0;
                deserialize(version, in);
                if (!(1 <= version && version <= 2))
                    throw serialization_error("Unexpected version found while deserializing dlib::add_tag_layer.");
                deserialize(item.input_layer_, in);
                deserialize(item.cached_output, in);
                deserialize(item.grad_final, in);
                deserialize(item.gradient_input_is_stale, in);
                item.cached_output_ptr = nullptr;
                if (version >= 2)
                    deserialize(item._sample_expansion_factor, in);
                else
                    item._sample_expansion_factor = 1; // all layer types set this to 1 in older dlib versions, so that's what we put here.

            }

            friend std::ostream& operator<< (std::ostream& out, const add_tag_layer& item)
            {
                int min_length = 0;
                item.print(out, 0, min_length);
                return out;
            }

            void print (std::ostream& out, unsigned long idx, int& min_length) const
            {
                out << "layer<"<<idx << ">\t"<<impl::tensor_to_str(private_get_output(), min_length)<< "tag" << ID << "\n";
                // Don't print the repeat_input_layer since it doesn't exist from the user's
                // point of view.  It's just an artifact of how repeat<> works.
                if (!std::is_same<subnet_type, impl::repeat_input_layer>::value)
                    out << "layer<"<< idx+1 << ">\t" << subnet() << "\n";
            }

        private:

            template <typename T, typename U, typename E>
            friend class add_layer;
            template <typename T, bool is_first, typename E>
            friend class dimpl::subnet_wrapper;
            template <unsigned long T, typename U, typename E>
            friend class add_tag_layer;
            template <template<typename> class T, typename U>
            friend class add_skip_layer;
            template <size_t N, template<typename> class L, typename S>
            friend class repeat;

            // You woudln't put a tag on a layer if you didn't want to access its forward
            // outputs.  So this is always true.
            bool this_layer_requires_forward_output(
            ) { return true; }

            void disable_output_and_gradient_getters (
            )
            {
                // This should never happen because only inplace layers call
                // disable_output_and_gradient_getters(), however, putting a tag layer right
                // before an inplace layer basically means you don't want the following layer
                // to operate in place.  So the inplace layer should turn itself into an
                // out-of-place layer and not call disable_output_and_gradient_getters().
                DLIB_CASSERT(false,"This should never happen");
            }

            tensor& private_get_output() const
            { return const_cast<tensor&>(get_output()); }
            tensor& private_get_gradient_input()
            { return get_gradient_input(); }

            void swap(add_tag_layer& item)
            {
                std::swap(input_layer_, item.input_layer_);
                std::swap(cached_output, item.cached_output);
                std::swap(cached_output_ptr, item.cached_output_ptr);
                std::swap(grad_final, item.grad_final);
                std::swap(gradient_input_is_stale, item.gradient_input_is_stale);
                std::swap(_sample_expansion_factor, item._sample_expansion_factor);
            }

            subnet_type input_layer_;
            resizable_tensor cached_output;
            tensor* cached_output_ptr;
            resizable_tensor grad_final;
            bool gradient_input_is_stale;
            mutable unsigned int _sample_expansion_factor;
        };

        template <unsigned long ID, typename U, typename E>
        struct is_nonloss_layer_type<add_tag_layer<ID,U,E>> : std::true_type {};

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

        template <typename LOSS_DETAILS, typename SUBNET>
        class add_loss_layer;

        class no_label_type
        {
        private:
            // We don't want anyone making these no_label_type objects.  They are here only to
            // allow add_loss_layer::training_label_type and dnn_trainer::training_label_type
            // to exist which avoids needing to overload add_loss_layer and dnn_trainer for
            // supervised an unsupervised losses.  It also can be a type to use in template
            // metaprogramming to indicate "no label".  So here we make the constructor private
            // with the exception that add_loss_layer objects can make it (again, just to
            // simplify add_loss_layer's implementation).
            no_label_type(){};
            template <typename LOSS_DETAILS, typename SUBNET> friend class add_loss_layer;
            template < typename net_type, typename solver_type > friend class dnn_trainer;
        };

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

        template <typename LOSS_DETAILS, typename SUBNET>
        class add_loss_layer
        {
            template <typename T, typename enabled=void>
            struct get_loss_layer_training_label_type
            {
                typedef no_label_type type;
            };
            template <typename T>
            struct get_loss_layer_training_label_type<T,typename std::enable_if<sizeof(typename T::training_label_type)!=0>::type>
            {
                typedef typename T::training_label_type type;
            };

            template <typename T, typename enabled=void>
            struct get_loss_layer_output_label_type
            {
                typedef no_label_type type;
            };
            template <typename T>
            struct get_loss_layer_output_label_type<T,typename std::enable_if<sizeof(typename T::output_label_type)!=0>::type>
            {
                typedef typename T::output_label_type type;
            };

        public:
            typedef LOSS_DETAILS loss_details_type;
            typedef SUBNET subnet_type;
            typedef typename subnet_type::input_type input_type;
            typedef typename subnet_type::input_layer_type input_layer_type;
            const static size_t num_layers = subnet_type::num_layers + 1;
            // Note that the loss layer doesn't count as an additional computational layer.
            const static size_t num_computational_layers = subnet_type::num_computational_layers;
            typedef typename get_loss_layer_training_label_type<LOSS_DETAILS>::type training_label_type;
            typedef typename get_loss_layer_output_label_type<LOSS_DETAILS>::type output_label_type;

            static_assert(is_nonloss_layer_type<SUBNET>::value,
                "SUBNET must be of type add_layer, add_skip_layer, or add_tag_layer.");

            add_loss_layer() {};
            add_loss_layer(const add_loss_layer&) = default;
            add_loss_layer& operator=(const add_loss_layer&) = default;
            add_loss_layer(add_loss_layer&& item) : add_loss_layer() { swap(item); }
            add_loss_layer& operator=(add_loss_layer&& item) { swap(item); return *this; }

            template <typename T, typename U>
            add_loss_layer(
                const add_loss_layer<T,U>& item
            ) :
                loss(item.loss_details()),
                subnetwork(item.subnet())
            {}

            template <typename ...T>
            add_loss_layer(
                const LOSS_DETAILS& layer_det,
                T&& ...args
            ) :
                loss(layer_det),
                subnetwork(std::forward<T>(args)...)
            {
            }

            template <typename ...T>
            add_loss_layer(
                LOSS_DETAILS&& layer_det,
                T&& ...args
            ) :
                loss(std::move(layer_det)),
                subnetwork(std::forward<T>(args)...)
            {
            }

            template <typename T, typename ...U>
            struct disable_forwarding_constr
            {
                const static bool value = std::is_constructible<LOSS_DETAILS,T>::value;
            };
            template <typename ...T>
            struct disable_forwarding_constr<add_loss_layer<T...>>
            {
                const static bool value = true;
            };

            template <
                typename ...T,
                typename = typename std::enable_if<!disable_forwarding_constr<typename std::remove_reference<T>::type...>::value>::type
                >
            add_loss_layer(
                T&& ...args
            ) :
                subnetwork(std::forward<T>(args)...)
            {
            }

            template <typename forward_iterator>
            void to_tensor (
                forward_iterator ibegin,
                forward_iterator iend,
                resizable_tensor& data
            ) const
            {
                subnetwork.to_tensor(ibegin,iend,data);
            }

            unsigned int sample_expansion_factor() const { return subnet().sample_expansion_factor(); }

            template <typename output_iterator>
            void operator() (
                const tensor& x,
                output_iterator obegin
            )
            {
                subnetwork.forward(x);
                const dimpl::subnet_wrapper<subnet_type> wsub(subnetwork);
                loss.to_label(x, wsub, obegin);
            }

            template <typename forward_iterator, typename output_iterator>
            void operator() (
                forward_iterator ibegin,
                forward_iterator iend,
                output_iterator obegin
            )
            {
                to_tensor(ibegin,iend,temp_tensor);
                (*this)(temp_tensor, obegin);
            }

            const output_label_type& operator() (const input_type& x)
            {
                (*this)(&x, &x+1, &temp_label);
                return temp_label;
            }

            template <typename ...T>
            const output_label_type& process (const input_type& x, T&& ...args)
            {
                to_tensor(&x,&x+1,temp_tensor);
                subnetwork.forward(temp_tensor);
                const dimpl::subnet_wrapper<subnet_type> wsub(subnetwork);
                loss.to_label(temp_tensor, wsub, &temp_label, std::forward<T>(args)...);
                return temp_label;
            }

            template <typename iterable_type, typename ...T>
            std::vector<output_label_type> process_batch (const iterable_type& data, size_t batch_size, T&& ...args)
            {
                std::vector<output_label_type> results(std::distance(data.begin(), data.end()));
                auto o = results.begin();
                auto i = data.begin();
                auto num_remaining = results.size();
                while(num_remaining != 0)
                {
                    auto inc = std::min(batch_size, num_remaining);
                    to_tensor(i,i+inc,temp_tensor);
                    subnetwork.forward(temp_tensor);
                    const dimpl::subnet_wrapper<subnet_type> wsub(subnetwork);
                    loss.to_label(temp_tensor, wsub, o, std::forward<T>(args)...);

                    i += inc;
                    o += inc;
                    num_remaining -= inc;
                }
                return results;
            }

            void back_propagate_error(
                const tensor& x,
                zero_gradients zero_grads = zero_gradients::yes
            )
            {
                subnet().back_propagate_error(x, zero_grads);
            }

            void back_propagate_error(
                const tensor& x,
                const tensor& gradient_input,
                zero_gradients zero_grads = zero_gradients::yes
            )
            {
                subnet().back_propagate_error(x, gradient_input, zero_grads);
            }

            const tensor& get_final_data_gradient(
            ) const
            {
                return subnet().get_final_data_gradient();
            }

            const tensor& forward(const tensor& x)
            {
                return subnet().forward(x);
            }

            template <typename iterable_type>
            std::vector<output_label_type> operator() (
                const iterable_type& data,
                size_t batch_size = 128
            )
            {
                std::vector<output_label_type> results(std::distance(data.begin(), data.end()));
                auto o = results.begin();
                auto i = data.begin();
                auto num_remaining = results.size();
                while(num_remaining != 0)
                {
                    auto inc = std::min(batch_size, num_remaining);
                    (*this)(i, i+inc, o);
                    i += inc;
                    o += inc;
                    num_remaining -= inc;
                }
                return results;
            }

            template <typename label_iterator>
            double compute_loss (
                const tensor& x,
                label_iterator lbegin
            )
            {
                subnetwork.forward(x);
                dimpl::subnet_wrapper<subnet_type> wsub(subnetwork);
                return loss.compute_loss_value_and_gradient(x, lbegin, wsub);
            }

            template <typename forward_iterator, typename label_iterator>
            double compute_loss (
                forward_iterator ibegin,
                forward_iterator iend,
                label_iterator lbegin
            )
            {
                to_tensor(ibegin,iend,temp_tensor);
                return compute_loss(temp_tensor, lbegin);
            }

            double compute_loss (
                const tensor& x
            )
            {
                subnetwork.forward(x);
                dimpl::subnet_wrapper<subnet_type> wsub(subnetwork);
                return loss.compute_loss_value_and_gradient(x, wsub);
            }

            template <typename forward_iterator>
            double compute_loss (
                forward_iterator ibegin,
                forward_iterator iend
            )
            {
                to_tensor(ibegin,iend,temp_tensor);
                return compute_loss(temp_tensor);
            }

            template <typename label_iterator>
            double compute_parameter_gradients (
                const tensor& x,
                label_iterator lbegin,
                zero_gradients zero_grads = zero_gradients::yes
            )
            {
                subnetwork.forward(x);
                dimpl::subnet_wrapper<subnet_type> wsub(subnetwork);
                double l = loss.compute_loss_value_and_gradient(x, lbegin, wsub);
                subnetwork.back_propagate_error(x, zero_grads);
                return l;
            }
            template <typename forward_iterator, typename label_iterator>
            double compute_parameter_gradients (
                forward_iterator ibegin,
                forward_iterator iend,
                label_iterator lbegin,
                zero_gradients zero_grads = zero_gradients::yes
            )
            {
                to_tensor(ibegin,iend,temp_tensor);
                return compute_parameter_gradients(temp_tensor, lbegin, zero_grads);
            }
            double compute_parameter_gradients (
                const tensor& x,
                zero_gradients zero_grads = zero_gradients::yes
            )
            {
                subnetwork.forward(x);
                dimpl::subnet_wrapper<subnet_type> wsub(subnetwork);
                double l = loss.compute_loss_value_and_gradient(x, wsub);
                subnetwork.back_propagate_error(x, zero_grads);
                return l;
            }
            template <typename forward_iterator>
            double compute_parameter_gradients (
                forward_iterator ibegin,
                forward_iterator iend,
                zero_gradients zero_grads = zero_gradients::yes
            )
            {
                to_tensor(ibegin,iend,temp_tensor);
                return compute_parameter_gradients(temp_tensor, zero_grads);
            }

            template <typename solver_type>
            void update_parameters (
                sstack<solver_type> solvers,
                double learning_rate
            )
            {
                subnetwork.update_parameters(solvers, learning_rate);
            }

            template <typename solver_type>
            void update_parameters(std::vector<solver_type>& solvers, double learning_rate)
            {
                update_parameters(make_sstack(solvers), learning_rate);
            }

            const subnet_type& subnet() const { return subnetwork; }
            subnet_type& subnet() { return subnetwork; }

            const input_layer_type& input_layer() const { return subnet().input_layer(); }
            input_layer_type& input_layer() { return subnet().input_layer(); }

            const loss_details_type& loss_details() const { return loss; }
            loss_details_type& loss_details() { return loss; }

            void set_gradient_inputs_to_zero (
            )
            {
                subnetwork.set_gradient_inputs_to_zero();
            }

            void clean (
            )
            {
                temp_tensor.clear();
                subnetwork.clean();
            }

            template <typename T, typename U>
            friend void serialize(const add_loss_layer<T,U>& item, std::ostream& out);
            template <typename T, typename U>
            friend void deserialize(add_loss_layer<T,U>& item, std::istream& in);

            friend std::ostream& operator<< (std::ostream& out, const add_loss_layer& item)
            {
                int min_length = 0;
                item.print(out, 0, min_length);
                return out;
            }

            void print (std::ostream& out, unsigned long idx, int& min_length) const
            {
                out << "layer<" << idx << ">\t" << loss_details() << "\n";
                subnet().print(out, idx+1, min_length);
            }

        private:

            void swap(add_loss_layer& item)
            {
                std::swap(loss, item.loss);
                std::swap(subnetwork, item.subnetwork);
            }

            loss_details_type loss;
            subnet_type subnetwork;

            // These two objects don't logically contribute to the state of this object.  They
            // are here to prevent them from being reallocated over and over.
            output_label_type temp_label;
            resizable_tensor temp_tensor;
        };

        template <typename LOSS_DETAILS, typename SUBNET>
        void serialize(const add_loss_layer<LOSS_DETAILS,SUBNET>& item, std::ostream& out)
        {
            int version = 1;
            serialize(version, out);
            serialize(item.loss, out);
            serialize(item.subnetwork, out);
        }

        template <typename LOSS_DETAILS, typename SUBNET>
        void deserialize(add_loss_layer<LOSS_DETAILS,SUBNET>& item, std::istream& in)
        {
            int version = 0;
            deserialize(version, in);
            if (version != 1)
                throw serialization_error("Unexpected version found while deserializing dlib::add_loss_layer.");
            deserialize(item.loss, in);
            deserialize(item.subnetwork, in);
        }

        template <typename T, typename U>
        struct is_loss_layer_type<add_loss_layer<T,U>> : std::true_type {};

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

        namespace impl
        {
            template <unsigned int i, typename T, typename enabled = void>
            struct layer_helper
            {
                static_assert(i < T::num_layers, "Call to layer() attempted to access non-existing layer in neural network.");
                static T& makeT();
                // If you get error here mentioning lack of member "subnet" in "dlib::input<...>",
                // then likely your "dlib::layer<...>" invocation wasn't able to find requested layer.
                // This could happen for instance when trying to use skip layer for non-existing tag.
                using next_type = typename std::remove_reference<decltype(makeT().subnet())>::type;
                using type = typename layer_helper<i-1,next_type>::type;
                static type& layer(T& n)
                {
                    return layer_helper<i-1,next_type>::layer(n.subnet());
                }
            };
            template <
                unsigned int i,
                size_t N, template<typename> class L, typename S
            >
            struct layer_helper<i,repeat<N,L,S>, typename std::enable_if<(i!=0&&i>=repeat<N,L,S>::layers_in_repeated_group)>::type>
            {
                const static size_t layers_in_repeated_group = repeat<N,L,S>::layers_in_repeated_group;

                static repeat<N,L,S>& makeT();
                using next_type = typename std::remove_reference<decltype(makeT().subnet())>::type;
                using type = typename layer_helper<i-layers_in_repeated_group,next_type>::type;
                static type& layer(repeat<N,L,S>& n)
                {
                    return layer_helper<i-layers_in_repeated_group,next_type>::layer(n.subnet());
                }
            };
            template <
                unsigned int i,
                size_t N, template<typename> class L, typename S
            >
            struct layer_helper<i,repeat<N,L,S>, typename std::enable_if<(i!=0&&i<repeat<N,L,S>::layers_in_repeated_group)>::type>
            {
                const static size_t layers_in_each_group = repeat<N,L,S>::layers_in_each_group;
                typedef typename repeat<N,L,S>::repeated_layer_type repeated_layer_type;
                using next_type = repeated_layer_type;
                using type = typename layer_helper<i%layers_in_each_group,next_type>::type;
                static type& layer(repeat<N,L,S>& n)
                {
                    return layer_helper<i%layers_in_each_group,next_type>::layer(n.get_repeated_layer(i/layers_in_each_group));
                }
            };
            template <
                size_t N, template<typename> class L, typename S
            >
            struct layer_helper<0,repeat<N,L,S>, void>
            {
                typedef typename repeat<N,L,S>::repeated_layer_type repeated_layer_type;
                using type = repeated_layer_type;
                static type& layer(repeat<N,L,S>& n)
                {
                    return n.get_repeated_layer(0);
                }
            };

            template <
                unsigned int i,
                size_t N, template<typename> class L, typename S
            >
            struct layer_helper<i,const repeat<N,L,S>, typename std::enable_if<(i!=0&&i>=repeat<N,L,S>::layers_in_repeated_group)>::type>
            {
                const static size_t layers_in_repeated_group = repeat<N,L,S>::layers_in_repeated_group;

                static const repeat<N,L,S>& makeT();
                using next_type = const typename std::remove_reference<decltype(makeT().subnet())>::type;
                using type = const typename layer_helper<i-layers_in_repeated_group,next_type>::type;
                static type& layer(const repeat<N,L,S>& n)
                {
                    return layer_helper<i-layers_in_repeated_group,next_type>::layer(n.subnet());
                }
            };
            template <
                unsigned int i,
                size_t N, template<typename> class L, typename S
            >
            struct layer_helper<i,const repeat<N,L,S>, typename std::enable_if<(i!=0&&i<repeat<N,L,S>::layers_in_repeated_group)>::type>
            {
                const static size_t layers_in_each_group = repeat<N,L,S>::layers_in_each_group;
                typedef typename repeat<N,L,S>::repeated_layer_type repeated_layer_type;
                using next_type = const repeated_layer_type;
                using type = const typename layer_helper<i%layers_in_each_group,next_type>::type;
                static type& layer(const repeat<N,L,S>& n)
                {
                    return layer_helper<i%layers_in_each_group,next_type>::layer(n.get_repeated_layer(i/layers_in_each_group));
                }
            };
            template <
                size_t N, template<typename> class L, typename S
            >
            struct layer_helper<0,const repeat<N,L,S>, void>
            {
                typedef typename repeat<N,L,S>::repeated_layer_type repeated_layer_type;
                using type = const repeated_layer_type;
                static type& layer(const repeat<N,L,S>& n)
                {
                    return n.get_repeated_layer(0);
                }
            };

            template <typename T>
            struct layer_helper<0,T,void>
            {
                using type = T;
                static type& layer(T& n)
                {
                    return n;
                }
            };

            template <template<typename> class Match, typename T, unsigned int i, typename enabled = void>
            struct layer_helper_match
            {
                static T& makeT();
                using next_type = typename std::remove_reference<decltype(makeT().subnet())>::type;
                using type = typename layer_helper_match<Match,next_type,i>::type;
                static type& layer(T& n)
                {
                    return layer_helper_match<Match,next_type,i>::layer(n.subnet());
                }
            };
            // This overload catches add_layer and add_loss_layer templates.
            template <template<typename> class Match, typename T, unsigned int i>
            struct layer_helper_match<Match,T,i,
                typename std::enable_if<std::is_same<const T,const  Match<typename T::subnet_type>>::value>::type>
            {
                using type = typename layer_helper<i,T>::type;
                static type& layer(T& n)
                {
                    return layer_helper<i,T>::layer(n);
                }
            };
            // This overload catches input templates.
            template <template<typename> class Match, typename T, unsigned int i>
            struct layer_helper_match<Match,T,i,
                typename std::enable_if<std::is_same<const T,const  Match<typename T::input_type>>::value>::type>
            {
                using type = typename layer_helper<i,T>::type;
                static type& layer(T& n)
                {
                    return layer_helper<i,T>::layer(n);
                }
            };
            // This overload catches subnet_wrapper templates.
            template <template<typename> class Match, typename T, unsigned int i>
            struct layer_helper_match<Match,T,i,
                typename std::enable_if<std::is_same<const typename T::wrapped_type,
                                                     const Match<typename T::wrapped_type::subnet_type>>::value>::type>
            {
                using type = typename layer_helper<i,T>::type;
                static type& layer(T& n)
                {
                    return layer_helper<i,T>::layer(n);
                }
            };
        }

        template <unsigned int i, typename T>
        typename impl::layer_helper<i,T>::type& layer (T& n)
        {
            return impl::layer_helper<i,T>::layer(n);
        }

        template <template<typename> class Match, typename T>
        typename impl::layer_helper_match<Match,T,0>::type& layer (T& n)
        {
            return impl::layer_helper_match<Match,T,0>::layer(n);
        }

        template <template<typename> class Match, unsigned int i, typename T>
        typename impl::layer_helper_match<Match,T,i>::type& layer (T& n)
        {
            return impl::layer_helper_match<Match,T,i>::layer(n);
        }

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

        template <typename net_type>
        typename net_type::input_layer_type& input_layer (
            net_type& net
        )
        {
            return net.input_layer();
        }

        template <typename net_type>
        const typename net_type::input_layer_type& input_layer (
            const net_type& net
        )
        {
            return net.input_layer();
        }

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

        template <template<typename> class TAG_TYPE, typename SUBNET>
        class add_skip_layer
        {
        public:
            typedef SUBNET subnet_type;
            typedef typename subnet_type::input_type input_type;
            typedef typename subnet_type::input_layer_type input_layer_type;
            typedef int layer_details_type; // not really used anywhere, but required by subnet_wrapper.
            const static size_t num_layers = subnet_type::num_layers + 1;
            const static size_t num_computational_layers = subnet_type::num_computational_layers;
            const static unsigned long id = tag_id<TAG_TYPE>::id;

            add_skip_layer() {};
            add_skip_layer(const add_skip_layer&) = default;
            add_skip_layer(add_skip_layer&&) = default;
            add_skip_layer& operator=(add_skip_layer&&) = default;
            add_skip_layer& operator=(const add_skip_layer&) = default;

            template <typename T>
            add_skip_layer(
                const add_skip_layer<TAG_TYPE,T>& item
            ) : subnetwork(item.subnet())
            {}

            template <typename ...T>
            add_skip_layer(
                T ...args
            ) :
                subnetwork(std::move(args)...)
            {
            }

            template <typename forward_iterator>
            void to_tensor (
                forward_iterator ibegin,
                forward_iterator iend,
                resizable_tensor& data
            ) const
            {
                subnetwork.to_tensor(ibegin,iend,data);
            }

            template <typename forward_iterator>
            const tensor& operator() (
                forward_iterator ibegin,
                forward_iterator iend
            )
            {
                subnetwork(ibegin,iend);
                return layer<TAG_TYPE>(subnetwork).get_output();
            }

            const tensor& operator() (const input_type& x)
            {
                subnetwork(x);
                return layer<TAG_TYPE>(subnetwork).get_output();
            }

            const tensor& forward(const tensor& x)
            {
                subnetwork.forward(x);
                return layer<TAG_TYPE>(subnetwork).get_output();
            }

            const tensor& get_output() const
            {
                return layer<TAG_TYPE>(subnetwork).get_output();
            }

            tensor& get_gradient_input()
            {
                return layer<TAG_TYPE>(subnetwork).get_gradient_input();
            }

            const tensor& get_final_data_gradient(
            ) const
            {
                return subnetwork.get_final_data_gradient();
            }

            void back_propagate_error(
                const tensor& x,
                zero_gradients zero_grads = zero_gradients::yes
            )
            {
                subnetwork.back_propagate_error(x, zero_grads);
            }

            template <typename solver_type>
            void update_parameters(sstack<solver_type> solvers, double learning_rate)
            {
                subnetwork.update_parameters(solvers, learning_rate);
            }

            template <typename solver_type>
            void update_parameters(std::vector<solver_type>& solvers, double learning_rate)
            {
                update_parameters(make_sstack(solvers), learning_rate);
            }

            const tensor& get_parameter_gradient(
            ) const { return params_grad; }

            tensor& get_parameter_gradient (
            ) { return params_grad; }

            const subnet_type& subnet() const
            {
                return subnetwork;
            }

            subnet_type& subnet()
            {
                return subnetwork;
            }

            const input_layer_type& input_layer() const { return subnet().input_layer(); }
            input_layer_type& input_layer() { return subnet().input_layer(); }

            unsigned int sample_expansion_factor() const { return subnet().sample_expansion_factor(); }

            void set_gradient_inputs_to_zero()
            {
                subnetwork.set_gradient_inputs_to_zero();
            }

            void clean()
            {
                subnetwork.clean();
            }

            friend void serialize(const add_skip_layer& item, std::ostream& out)
            {
                int version = 1;
                serialize(version, out);
                serialize(item.subnetwork, out);
            }

            friend void deserialize(add_skip_layer& item, std::istream& in)
            {
                int version = 0;
                deserialize(version, in);
                if (version != 1)
                    throw serialization_error("Unexpected version found while deserializing dlib::add_skip_layer.");
                deserialize(item.subnetwork, in);
            }

            friend std::ostream& operator<< (std::ostream& out, const add_skip_layer& item)
            {
                int min_length = 0;
                item.print(out, 0, min_length);
                return out;
            }

            void print (std::ostream& out, unsigned long idx, int& min_length) const
            {
                out << "layer<" << idx << ">\t"<<impl::tensor_to_str(private_get_output(), min_length) <<"skip"<<id<<"\n";
                subnet().print(out, idx+1, min_length);
            }

        private:

            template <typename T, typename U, typename E>
            friend class add_layer;
            template <typename T, bool is_first, typename E>
            friend class dimpl::subnet_wrapper;
            template <unsigned long T, typename U, typename E>
            friend class add_tag_layer;
            template <template<typename> class T, typename U>
            friend class add_skip_layer;
            template <size_t N, template<typename> class L, typename S>
            friend class repeat;

            bool this_layer_requires_forward_output(
            ) { return layer<TAG_TYPE>(subnetwork).this_layer_requires_forward_output(); }

            void disable_output_and_gradient_getters (
            ) { layer<TAG_TYPE>(subnetwork).disable_output_and_gradient_getters(); }

            tensor& private_get_output() const
            { return layer<TAG_TYPE>(subnetwork).private_get_output(); }
            tensor& private_get_gradient_input()
            { return layer<TAG_TYPE>(subnetwork).private_get_gradient_input(); }

            subnet_type subnetwork;

            // This member doesn't logically contribute to the state of the object since it is
            // always empty. It's just here so we can have the get_parameter_gradient() methods
            // which have to return something.  So they return this empty tensor.
            resizable_tensor params_grad;
        };
        template <template<typename> class T, typename U>
        struct is_nonloss_layer_type<add_skip_layer<T,U>> : std::true_type {};

        template <typename SUBNET> using tag1  = add_tag_layer< 1, SUBNET>;
        template <typename SUBNET> using tag2  = add_tag_layer< 2, SUBNET>;
        template <typename SUBNET> using tag3  = add_tag_layer< 3, SUBNET>;
        template <typename SUBNET> using tag4  = add_tag_layer< 4, SUBNET>;
        template <typename SUBNET> using tag5  = add_tag_layer< 5, SUBNET>;
        template <typename SUBNET> using tag6  = add_tag_layer< 6, SUBNET>;
        template <typename SUBNET> using tag7  = add_tag_layer< 7, SUBNET>;
        template <typename SUBNET> using tag8  = add_tag_layer< 8, SUBNET>;
        template <typename SUBNET> using tag9  = add_tag_layer< 9, SUBNET>;
        template <typename SUBNET> using tag10 = add_tag_layer<10, SUBNET>;

        template <typename SUBNET> using skip1  = add_skip_layer< tag1, SUBNET>;
        template <typename SUBNET> using skip2  = add_skip_layer< tag2, SUBNET>;
        template <typename SUBNET> using skip3  = add_skip_layer< tag3, SUBNET>;
        template <typename SUBNET> using skip4  = add_skip_layer< tag4, SUBNET>;
        template <typename SUBNET> using skip5  = add_skip_layer< tag5, SUBNET>;
        template <typename SUBNET> using skip6  = add_skip_layer< tag6, SUBNET>;
        template <typename SUBNET> using skip7  = add_skip_layer< tag7, SUBNET>;
        template <typename SUBNET> using skip8  = add_skip_layer< tag8, SUBNET>;
        template <typename SUBNET> using skip9  = add_skip_layer< tag9, SUBNET>;
        template <typename SUBNET> using skip10 = add_skip_layer<tag10, SUBNET>;

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

        namespace timpl
        {
            inline void fill_with_gassuan_random_numbers (
                tensor& t,
                dlib::rand& rnd,
                double sigma = 1
            )
            {
                float* data = t.host();
                for (size_t i = 0; i < t.size(); ++i)
                    data[i] = rnd.get_random_gaussian()*sigma;
            }

            class test_layer_subnet
            {
            public:
                test_layer_subnet (
                    dlib::rand& rnd_
                ) : rnd(rnd_)
                {
                    // Output and gradient_input have to have the same dimensions in each
                    // layer.
                    const long num_samples = rnd.get_random_32bit_number()%4+3;
                    const long k  = rnd.get_random_32bit_number()%4+2;
                    const long nr = ((rnd.get_random_32bit_number()%4)/2)*2+2;
                    const long nc = ((rnd.get_random_32bit_number()%4)/2)*2+2;

                    output.set_size(num_samples, k, nr, nc);
                    gradient_input.set_size(num_samples, k, nr, nc);

                    // Use a non-zero initial gradient to make sure the layers add to it
                    // rather than assign and blow away the initial value.
                    fill_with_gassuan_random_numbers(gradient_input, rnd, 0.01);

                    fill_with_gassuan_random_numbers(output, rnd);
                }

                tensor& get_mutable_output() { return output; }
                const tensor& get_output() const { return output; }
                const tensor& private_get_output() const { return get_output(); }
                const test_layer_subnet& subnet() const { init_sub(); return *subnetwork; }

                tensor& get_gradient_input() { return gradient_input; }
                tensor& private_get_gradient_input() { return get_gradient_input(); }
                test_layer_subnet& subnet() { init_sub(); return *subnetwork; }

                unsigned long count_outputs() const
                {
                    if (subnetwork)
                        return subnetwork->count_outputs() + output.size();
                    else
                        return output.size();
                }

                float& get_output_element(unsigned long i)
                {
                    if (i < output.size())
                        return output.host()[i];
                    else
                        return subnet().get_output_element(i-output.size());
                }

                float get_gradient_input_element(unsigned long i) const
                {
                    if (i < gradient_input.size())
                        return gradient_input.host()[i];
                    else
                        return subnet().get_gradient_input_element(i-gradient_input.size());
                }

            private:
                // We lazily initialize sub-layers as needed when someone tries to call
                // subnet()
                void init_sub() const
                {
                    if (!subnetwork)
                        subnetwork.reset(new test_layer_subnet(rnd));
                }

                dlib::rand& rnd;
                mutable std::unique_ptr<test_layer_subnet> subnetwork;
                resizable_tensor output;
                resizable_tensor gradient_input;
            };

        }

        struct layer_test_results
        {
            layer_test_results() : was_good(true) {}
            explicit layer_test_results(const std::string& l) : log(l),was_good(false) {}

            std::string log;
            bool was_good;

            operator bool() const { return was_good; }
        };

        inline std::ostream& operator<< (std::ostream& out, const layer_test_results& item)
        {
            out << item.log;
            return out;
        }

        template <
            typename layer_details_type
            >
        layer_test_results impl_test_layer (
            layer_details_type l,
            const float base_eps
        )
        {
            using namespace timpl;
            // Do some setup
            running_stats<double> rs_data, rs_params;
            dlib::rand rnd;
            std::ostringstream sout;
            for (int iter = 0; iter < 10; ++iter)
            {
                test_layer_subnet subnetwork(rnd);
                resizable_tensor output, out2, out3;
                // Run setup() and forward() as well to make sure any calls to subnet() have
                // happened before we start assuming we know how many data elements there are
                // (since we do a lazy layer creation thing based on calls to subnet() inside
                // test_layer_subnet).
                l.setup(subnetwork);
                impl::call_layer_forward(l, subnetwork, output);

                resizable_tensor input_grad;
                input_grad.copy_size(output);
                fill_with_gassuan_random_numbers(input_grad, rnd);

                // The f() we are computing gradients of is this thing.  It's value at the current
                // parameter and data values is:
                //sout << "f(data,params): " << dot(output, input_grad) << std::endl;

                // We are going to save a copy of the subnetwork.get_gradient_input() data before we do
                // backpropagation since the backward() function is supposed to *add* to the
                // gradients rather than overwrite them.  We will use this saved data to check if
                // that is the case.
                const unsigned long num_data_inputs = subnetwork.count_outputs();
                std::vector<float> initial_gradient_input(num_data_inputs);
                for (unsigned long i = 0; i < num_data_inputs; ++i)
                    initial_gradient_input[i] = subnetwork.get_gradient_input_element(i);

                // Now tell the layer to compute all the gradients.  In the rest of this function
                // we will just be checking that these gradients were computed correctly by
                // comparing them to a central differences approximation.
                resizable_tensor params_grad;
                params_grad.copy_size(l.get_layer_params());
                // But first, set the params grad to something crazy so that it's very obvious if
                // it doesn't get fully assigned.
                params_grad = std::numeric_limits<float>::infinity();
                impl::call_layer_backward(l, output, input_grad, subnetwork, params_grad);

                static_assert(impl::is_inplace_layer(l, subnetwork) == impl::has_inplace_backward(l, subnetwork),
                    "Layer not defined correctly.  forward and backward methods must either both be in-place or both out-of-place. ");

                // Make sure the outputs of forward() and backward() are the same when they are run
                // in in-place mode.
                if (impl::is_inplace_layer(l, subnetwork))
                {
                    test_layer_subnet subnetwork2(rnd);
                    layer_details_type ll(l);
                    ll.setup(subnetwork2);
                    resizable_tensor ip_out;
                    impl::call_layer_forward(ll, subnetwork2, ip_out);
                    impl::call_layer_forward(ll, subnetwork2, subnetwork2.get_mutable_output());
                    const auto forward_error = max(abs(mat(ip_out) - mat(subnetwork2.get_output())));
                    if (forward_error > 0.00001)
                    {
                        sout << "This layer is supposed to support in-place computations but the output of forward_inplace()\n";
                        sout << "changes when invoked in-place vs. out-of-place. The error was: " << forward_error << std::endl;
                        return layer_test_results(sout.str());
                    }

                    resizable_tensor params_grad;
                    params_grad.copy_size(ll.get_layer_params());
                    params_grad = std::numeric_limits<float>::infinity();

                    resizable_tensor input_grad;
                    input_grad.copy_size(ip_out);
                    fill_with_gassuan_random_numbers(input_grad, rnd);
                    resizable_tensor params_grad1, params_grad2, data_grad1, data_grad2;
                    params_grad1 = params_grad;
                    params_grad2 = params_grad;
                    // Now call backward() and make sure it works as well.  Recall that when an
                    // in-place layer works in-place it assigns to it's outputs but when it's
                    // not running in-place it adds.  So we initialize to a non-zero value to
                    // check that this is the behavior that really executes.
                    subnetwork2.get_gradient_input() = 9;
                    impl::call_layer_backward(ll, ip_out, input_grad, subnetwork2, params_grad1);
                    data_grad1 = subnetwork2.get_gradient_input();

                    subnetwork2.get_gradient_input() = mat(input_grad);
                    impl::call_layer_backward(ll, ip_out, subnetwork2.get_gradient_input(), subnetwork2, params_grad2);
                    data_grad2 = subnetwork2.get_gradient_input();
                    if (params_grad.size() != 0)
                    {
                        const auto backward_param_error = max(abs(mat(params_grad1) - mat(params_grad2)));
                        if (backward_param_error > 0.00001)
                        {
                            sout << "This layer is supposed to support in-place computations but the output of backward_inplace()\n";
                            sout << "changes when invoked in-place vs. out-of-place. The error was: " << backward_param_error << std::endl;
                            return layer_test_results(sout.str());
                        }
                    }
                    const auto backward_data_error = max(abs(mat(data_grad1)-9 - mat(data_grad2)));
                    if (backward_data_error > 0.00001)
                    {
                        sout << "This layer is supposed to support in-place computations but the output of backward_inplace()\n";
                        sout << "changes when invoked in-place vs. out-of-place. The error was: " << backward_data_error << std::endl;
                        return layer_test_results(sout.str());
                    }
                }

                // ==================================================================
                // first validate the way the parameter gradients are computed
                for (unsigned long i = 0; i < params_grad.size(); ++i)
                {
                    layer_details_type l1(l);

                    float eps = l1.get_layer_params().host()[i]*base_eps;
                    if (eps == 0)
                        eps = base_eps;
                    const float oldval = l1.get_layer_params().host()[i];
                    l1.get_layer_params().host()[i] = oldval+eps;
                    impl::call_layer_forward(l1, subnetwork, out2);
                    l1.get_layer_params().host()[i] = oldval-eps;
                    impl::call_layer_forward(l1, subnetwork, out3);
                    l1.get_layer_params().host()[i] = oldval;

                    // Compute a reference derivative via a central differences approximation and
                    // compare it to the one output by the layer and make sure they match.
                    double reference_derivative = (dot(out2,input_grad)-dot(out3, input_grad))/(2*eps);
                    double output_derivative = params_grad.host()[i];
                    double relative_error;
                    if (reference_derivative*output_derivative != 0)
                        relative_error = (reference_derivative - output_derivative)/(reference_derivative);
                    else
                        relative_error = (reference_derivative - output_derivative);
                    double absolute_error = (reference_derivative - output_derivative);
                    rs_params.add(std::abs(relative_error));
                    if (std::abs(relative_error) > 0.05 && std::abs(absolute_error) > 0.006)
                    {
                        sout << "Gradient error in parameter #" << i <<".  Relative error: "<< relative_error << std::endl;
                        sout << "expected derivative: " << reference_derivative << std::endl;
                        sout << "output derivative:   " << output_derivative << std::endl;
                        sout << "iteration:           " << iter << std::endl;
                        return layer_test_results(sout.str());
                    }
                }

                // ==================================================================
                // now validate the data gradients
                for (unsigned long i = 0; i < num_data_inputs; ++i)
                {
                    const float oldval = subnetwork.get_output_element(i);
                    float eps = oldval*base_eps;
                    if (eps == 0)
                        eps = base_eps;
                    subnetwork.get_output_element(i) = oldval+eps;
                    impl::call_layer_forward(l, subnetwork, out2);
                    subnetwork.get_output_element(i) = oldval-eps;
                    impl::call_layer_forward(l, subnetwork, out3);
                    subnetwork.get_output_element(i) = oldval;

                    // Compute a reference derivative via a central differences approximation and
                    // compare it to the one output by the layer and make sure they match.
                    double reference_derivative = (dot(out2,input_grad)-dot(out3, input_grad))/(2*eps);
                    double output_derivative = subnetwork.get_gradient_input_element(i);
                    output_derivative -= initial_gradient_input[i];
                    double relative_error;
                    if (reference_derivative*output_derivative != 0)
                        relative_error = (reference_derivative - output_derivative)/(reference_derivative);
                    else
                        relative_error = (reference_derivative - output_derivative);
                    double absolute_error = (reference_derivative - output_derivative);
                    rs_data.add(std::abs(relative_error));
                    if (std::abs(relative_error) > 0.05 && std::abs(absolute_error) > 0.006)
                    {
                        sout << "Gradient error in data variable #" << i <<".  Relative error: "<< relative_error << std::endl;
                        sout << "expected derivative: " << reference_derivative << std::endl;
                        sout << "output derivative:   " << output_derivative << std::endl;
                        sout << "iteration:           " << iter << std::endl;
                        return layer_test_results(sout.str());
                    }
                }

            } // end for (int iter = 0; iter < 10; ++iter)

            if (rs_params.mean() > 0.003)
            {
                sout << "Average parameter gradient error is somewhat large at: "<< rs_params.mean() << std::endl;
                return layer_test_results(sout.str());
            }
            if (rs_data.mean() > 0.003)
            {
                sout << "Average data gradient error is somewhat large at: "<< rs_data.mean() << std::endl;
                return layer_test_results(sout.str());
            }

            return layer_test_results();
        }

        template <
            typename layer_details_type
            >
        layer_test_results test_layer (
            layer_details_type l
        )
        {
            // Try a few different derivative step sizes to see if any work.
            for (float base_eps = 0.0001; base_eps < 0.1; base_eps *= 2)
            {
                auto result = impl_test_layer(l, base_eps);
                if (result)
                    return result;
            }
            // However, if none of the step sizes worked then try this one and probably result
            // in returning an error.
            return impl_test_layer(l, 0.01);
        }

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

        namespace impl
        {
            template <size_t i, size_t num>
            struct vl_loop
            {
                template <
                    typename net_type,
                    typename visitor
                    >
                static void visit(
                    net_type& net,
                    visitor&& v
                )
                {
                    // Call whatever version of the visitor the user provided.
                    call_if_valid(v, i, layer<i>(net));
                    call_if_valid(v, layer<i>(net));
                    vl_loop<i+1, num>::visit(net,v);
                }
            };

            template <size_t num>
            struct vl_loop<num,num>
            {
                template <
                    typename net_type,
                    typename visitor
                    >
                static void visit(
                    net_type&,
                    visitor&&
                )
                {
                    // Base case of recursion.  Don't do anything.
                }
            };

            template <size_t i, size_t num>
            struct vl_loop_backwards
            {
                template <
                    typename net_type,
                    typename visitor
                    >
                static void visit(
                    net_type& net,
                    visitor&& v
                )
                {
                    vl_loop_backwards<i+1, num>::visit(net,v);
                    // Call whatever version of the visitor the user provided.
                    call_if_valid(v, i, layer<i>(net));
                    call_if_valid(v, layer<i>(net));
                }
            };

            template <size_t num>
            struct vl_loop_backwards<num,num>
            {
                template <
                    typename net_type,
                    typename visitor
                    >
                static void visit(
                    net_type&,
                    visitor&&
                )
                {
                    // Base case of recursion.  Don't do anything.
                }
            };

        }

        template <
            typename net_type,
            typename visitor
            >
        void visit_layers(
            net_type& net,
            visitor v
        )
        {
            impl::vl_loop<0, net_type::num_layers>::visit(net, v);
        }

        template <
            typename net_type,
            typename visitor
            >
        void visit_layers_backwards(
            net_type& net,
            visitor v
        )
        {
            impl::vl_loop_backwards<0, net_type::num_layers>::visit(net, v);
        }

        template <
            size_t begin,
            size_t end,
            typename net_type,
            typename visitor
            >
        void visit_layers_range(
            net_type& net,
            visitor v
        )
        {
            static_assert(begin <= end, "Invalid range");
            static_assert(end <= net_type::num_layers, "Invalid range");
            impl::vl_loop<begin,end>::visit(net, v);
        }

        template <
            size_t begin,
            size_t end,
            typename net_type,
            typename visitor
            >
        void visit_layers_backwards_range(
            net_type& net,
            visitor v
        )
        {
            static_assert(begin <= end, "Invalid range");
            static_assert(end <= net_type::num_layers, "Invalid range");
            impl::vl_loop_backwards<begin,end>::visit(net, v);
        }

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

        namespace impl
        {
            template <size_t i, unsigned long tag_id>
            struct vl_until_tag
            {
                template <
                    typename net_type,
                    typename next_net_type,
                    typename visitor
                    >
                static void visit(
                    net_type& net,
                    next_net_type& next_net,
                    visitor&& v
                )
                {
                    call_if_valid(v, next_net);
                    vl_until_tag<i+1,tag_id>::visit(net,layer<i+1>(net),v);
                }

                template <
                    typename net_type,
                    typename SUBNET,
                    typename visitor
                    >
                static void visit(
                    net_type&,
                    const add_tag_layer<tag_id,SUBNET>& next_net,
                    visitor&& v
                )
                {
                    call_if_valid(v, next_net);
                }

                template <
                    typename net_type,
                    typename SUBNET,
                    typename visitor
                    >
                static void visit(
                    net_type&,
                    add_tag_layer<tag_id,SUBNET>& next_net,
                    visitor&& v
                )
                {
                    call_if_valid(v, next_net);
                }
            };
        }

        template <
            unsigned long tag_id,
            typename net_type,
            typename visitor
            >
        void visit_layers_until_tag(
            net_type& net,
            visitor v
        )
        {
            impl::vl_until_tag<0,tag_id>::visit(net, net, v);
        }

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

        namespace impl
        {
            template <
                typename visitor
                >
            class visitor_computational_layer
            {
            public:
                explicit visitor_computational_layer(visitor& v) : v_(v) {}

                template <typename layer>
                void do_visit(size_t idx, layer& l) const
                {
                    // Call whatever version of the visitor the user provided.
                    call_if_valid(v_, idx, l.layer_details());
                    call_if_valid(v_, l.layer_details());
                }

                // const case
                template <typename T, typename U, typename E>
                void operator()(size_t idx, const add_layer<T,U,E>& l) const { do_visit(idx, l); }
                // non-const cast
                template <typename T, typename U, typename E>
                void operator()(size_t idx, add_layer<T,U,E>& l) const { do_visit(idx, l); }

            private:

                visitor& v_;
            };
        }

        template <
            typename net_type,
            typename visitor
            >
        void visit_computational_layers(
            net_type& net,
            visitor v
        )
        {
            visit_layers(net, impl::visitor_computational_layer<visitor>(v));
        }

        template <
            size_t begin,
            size_t end,
            typename net_type,
            typename visitor
            >
        void visit_computational_layers_range(
            net_type& net,
            visitor v
        )
        {
            visit_layers_range<begin,end>(net, impl::visitor_computational_layer<visitor>(v));
        }

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

        namespace impl
        {
            template <
                typename visitor
                >
            class visit_layer_parameters
            {
            public:
                explicit visit_layer_parameters(visitor& v) : v_(v) {}

                template <typename layer>
                void operator()(layer& l)
                {
                    // Call whatever version of the visitor the user provided.
                    const bool visitor_called = call_if_valid(v_, computational_layer_idx, l.get_layer_params()) ||
                        call_if_valid(v_, l.get_layer_params());
                    DLIB_CASSERT(visitor_called, "A visitor function with an incorrect signature was given to visit_layer_parameters()");
                    ++computational_layer_idx;
                }
            private:

                size_t computational_layer_idx = 0;
                visitor& v_;
            };
        }

        template <
            typename net_type,
            typename visitor
            >
        void visit_layer_parameters(
            net_type& net,
            visitor v
        )
        {
            visit_computational_layers(net, impl::visit_layer_parameters<visitor>(v));
        }

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

        namespace impl
        {
            template <
                typename visitor
                >
            class visit_layer_parameter_gradients
            {
            public:
                explicit visit_layer_parameter_gradients(visitor& v) : v_(v) {}

                template <typename layer>
                void do_visit(layer& l)
                {
                    // Call whatever version of the visitor the user provided.
                    const bool visitor_called = call_if_valid(v_, computational_layer_idx, l.get_parameter_gradient()) ||
                        call_if_valid(v_, l.get_parameter_gradient());
                    DLIB_CASSERT(visitor_called, "A visitor function with an incorrect signature was given to visit_layer_parameter_gradients()");
                    ++computational_layer_idx;
                }

                // const version
                template <typename T, typename U, typename E>
                void operator()(const add_layer<T,U,E>& l) { do_visit(l); }
                // non-const version
                template <typename T, typename U, typename E>
                void operator()(add_layer<T,U,E>& l) { do_visit(l); }

            private:

                size_t computational_layer_idx = 0;
                visitor& v_;
            };
        }

        template <
            typename net_type,
            typename visitor
            >
        void visit_layer_parameter_gradients(
            net_type& net,
            visitor v
        )
        {
            visit_layers(net, impl::visit_layer_parameter_gradients<visitor>(v));
        }

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

    }

    #endif // DLIB_DNn_CORE_H_