libcruft-util/variadic.hpp

/*
 * This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/.
 *
 * Copyright 2017-2018 Danny Robson <danny@nerdcruft.net>
 */

#ifndef CRUFT_UTIL_VARIADIC_HPP
#define CRUFT_UTIL_VARIADIC_HPP

#include <cstddef>
#include <type_traits>
#include <utility>
#include <tuple>


namespace util::variadic {
    ///////////////////////////////////////////////////////////////////////////
    /// do nothing with a set of parameters.
    ///
    /// useful for temporarily silencing unused argument warnings in parameter
    /// packs, or for avoiding assignment of [[gnu::warn_unused_result]] to a
    /// temporary value we'd just cast to void anyway (GCC#66425).
    ///
    /// it is guaranteed that this function will never be defined out in
    /// debug/release/whatever builds. so it is safe to use to guarantee
    /// parameter evaluation.
    template <typename ...Args>
    void
    ignore (const Args&...) noexcept ((std::is_nothrow_destructible_v<Args> && ...))
    { ; }


    ///////////////////////////////////////////////////////////////////////////
    namespace detail {
        // using the strategy of discarding assignment from:
        //     https://attugit.github.io/2015/02/Accessing-nth-element-of-parameter-pack/
        struct discard {
            template <typename ArgsT>
            constexpr discard (ArgsT&&...) { ; }
        };

        template <size_t>
        using discard_n = discard;

        template <std::size_t N, typename = std::make_index_sequence<N>>
        struct getter;

        template <std::size_t N, std::size_t... ignore>
        struct getter<N, std::index_sequence<ignore...>> {
            template <typename ValueT, typename ...TailT>
            decltype(auto)
            operator() (discard_n<ignore>..., ValueT &&val, TailT&&...) const
            {
                return std::forward<ValueT> (val);
            }
        };
    }

    /// Returns the argument at index `IndexV', as if we called:
    ///     std::get<N> (std::make_tuple (...))
    template <size_t IndexV, typename ...ArgsT>
    decltype(auto)
    get (ArgsT&& ...args)
    {
        return detail::getter<IndexV> {}(std::forward<ArgsT> (args)...);
    }


    ///////////////////////////////////////////////////////////////////////////
    /// returns a tuple of all arguments that satisfy the trait QueryT
    template <template <typename> class QueryT>
    auto filter () { return std::tuple {}; }


    /// returns a tuple of all arguments that satisfy the trait QueryT
    template <
        template <typename> class QueryT,
        typename HeadT,
        typename ...ArgsT
    >
    auto
    filter (HeadT &&head, ArgsT &&...args)
    {
        if constexpr (QueryT<HeadT>::value)
            return std::tuple_cat (
                std::tuple (std::forward<HeadT> (head)),
                filter<QueryT> (std::forward<ArgsT> (args)...)
            );
        else
            return filter<QueryT> (std::forward<ArgsT> (args)...);
    }
};


#endif