libcruft-util/variadic.hpp

/*
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 *
 * 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> && ...))
    { ; }


    ///////////////////////////////////////////////////////////////////////////
    /// Returns the argument at index `IndexV', as if we called:
    ///     std::get<N> (std::make_tuple (...))
    template <std::size_t IndexV, typename HeadT, typename ...TailT>
    auto
    get (HeadT &&head, TailT &&...tail) noexcept ((std::is_nothrow_move_constructible_v<TailT> && ...))
    {
        static_assert (IndexV < sizeof... (TailT) + 1, "Index is out of bounds");

        if constexpr (IndexV == 0)
            return std::forward<HeadT> (head);
        else
            return get<IndexV-1> (std::forward<TailT> (tail)...);
    }


    ///////////////////////////////////////////////////////////////////////////
    /// 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