cppmp/include/cppmp/misc/generic_predicate.h

#pragma once

#include <utility>
#include <cppmp/core/types.h>
#include <cppmp/core/checker.h>

namespace cppmp
{

    namespace __impl
    {
        template<typename T>
        struct is_default_impl
        {
        private:
            template<typename X>
            static void impl(decltype(typename X::is_default(), int()));

            template<typename X>
            static bool impl(char);

        public:
            static const bool value = std::is_same<void, decltype(impl<T>(0))>::value;
        };

    }

    /**
     * @brief Helper class to create generic predicates.
     *
     * @tparam T_builder Builder class to pass arguments to.
     */
    template<template<typename...> class T_builder>
    struct generic_predicate
    {
        /**
         * @brief Helper class to check if the passed arguments would create a valid object.
         */
        template<typename... T_args>
        struct is_valid
            : public bool_t<!__impl::is_default_impl<T_builder<list<T_args...>>>::value>
            { };

        #ifdef cppmp_supports_variable_templates
        /**
         * @brief Evaluates to true if the passed arguments would create a valid object, false otherwise.
         */
        template<typename... T_args>
        static constexpr bool is_valid_v = is_valid<T_args...>::value;
        #endif

        /**
         * @brief Invoke the builder to create the requested object.
         *
         * @param args  Arguments to pass to the builder.
         *
         * @return      Object created from the builder.
         */
        template<typename... T_args>
        constexpr decltype(auto) operator()(T_args&&... args) const
            { return T_builder<list<T_args...>>::apply(std::forward<T_args>(args)...); }
    };

}