    // Copyright (C) 2012  Davis E. King (davis@dlib.net)
    // License: Boost Software License   See LICENSE.txt for the full license.
    #ifndef DLIB_NUM_NONNEGATIVE_WEIGHtS_H__
    #define DLIB_NUM_NONNEGATIVE_WEIGHtS_H__

    #include "../enable_if.h"

    namespace dlib
    {

        namespace impl2
        {
            template <
                typename T,
                unsigned long (T::*funct)()const
                >
            struct hnnf_helper
            {
                typedef char type;
            };

            template <typename T>
            char has_num_nonnegative_weights_helper( typename hnnf_helper<T,&T::num_nonnegative_weights>::type = 0 ) { return 0;}

            struct two_bytes
            {
                char a[2];
            };

            template <typename T>
            two_bytes has_num_nonnegative_weights_helper(int) { return two_bytes();}

            template <typename T>
            struct work_around_visual_studio_bug
            {
                const static unsigned long U = sizeof(has_num_nonnegative_weights_helper<T>('a'));
            };

            // This is a template to tell you if a feature_extractor has a num_nonnegative_weights function or not.
            template <typename T, unsigned long U = work_around_visual_studio_bug<T>::U >
            struct has_num_nonnegative_weights
            {
                static const bool value = false;
            };

            template <typename T>
            struct has_num_nonnegative_weights <T,1>
            {
                static const bool value = true;
            };

        }

        // call fe.num_nonnegative_weights() if it exists, otherwise return 0.
        template <typename feature_extractor>
        typename enable_if<impl2::has_num_nonnegative_weights<feature_extractor>,unsigned long>::type num_nonnegative_weights (
        const feature_extractor& fe
        )
        {
            return fe.num_nonnegative_weights();
        }

        template <typename feature_extractor>
        typename disable_if<impl2::has_num_nonnegative_weights<feature_extractor>,unsigned long>::type num_nonnegative_weights (
        const feature_extractor& /*fe*/
        )
        {
            return 0;
        }

    }

    #endif // DLIB_NUM_NONNEGATIVE_WEIGHtS_H__