98 lines
3.1 KiB
C++
98 lines
3.1 KiB
C++
/*
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* This Source Code Form is subject to the terms of the Mozilla Public
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* License, v. 2.0. If a copy of the MPL was not distributed with this
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* file, You can obtain one at http://mozilla.org/MPL/2.0/.
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*
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* Copyright 2017-2018 Danny Robson <danny@nerdcruft.net>
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*/
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#ifndef CRUFT_UTIL_VARIADIC_HPP
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#define CRUFT_UTIL_VARIADIC_HPP
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#include <cstddef>
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#include <type_traits>
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#include <utility>
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#include <tuple>
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namespace cruft::variadic {
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///////////////////////////////////////////////////////////////////////////
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/// do nothing with a set of parameters.
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///
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/// useful for temporarily silencing unused argument warnings in parameter
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/// packs, or for avoiding assignment of [[gnu::warn_unused_result]] to a
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/// temporary value we'd just cast to void anyway (GCC#66425).
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///
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/// it is guaranteed that this function will never be defined out in
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/// debug/release/whatever builds. so it is safe to use to guarantee
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/// parameter evaluation.
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template <typename ...Args>
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void
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ignore (const Args&...) noexcept ((std::is_nothrow_destructible_v<Args> && ...))
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{ ; }
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///////////////////////////////////////////////////////////////////////////
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namespace detail {
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// using the strategy of discarding assignment from:
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// https://attugit.github.io/2015/02/Accessing-nth-element-of-parameter-pack/
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struct discard {
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template <typename ArgsT>
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constexpr discard (ArgsT&&...) { ; }
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};
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template <size_t>
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using discard_n = discard;
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template <std::size_t N, typename = std::make_index_sequence<N>>
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struct getter;
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template <std::size_t N, std::size_t... ignore>
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struct getter<N, std::index_sequence<ignore...>> {
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template <typename ValueT, typename ...TailT>
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decltype(auto)
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operator() (discard_n<ignore>..., ValueT &&val, TailT&&...) const
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{
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return std::forward<ValueT> (val);
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}
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};
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}
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/// Returns the argument at index `IndexV', as if we called:
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/// std::get<N> (std::make_tuple (...))
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template <size_t IndexV, typename ...ArgsT>
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decltype(auto)
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get (ArgsT&& ...args)
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{
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return detail::getter<IndexV> {}(std::forward<ArgsT> (args)...);
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}
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///////////////////////////////////////////////////////////////////////////
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/// returns a tuple of all arguments that satisfy the trait QueryT
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template <template <typename> class QueryT>
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auto filter () { return std::tuple {}; }
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/// returns a tuple of all arguments that satisfy the trait QueryT
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template <
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template <typename> class QueryT,
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typename HeadT,
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typename ...ArgsT
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>
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auto
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filter (HeadT &&head, ArgsT &&...args)
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{
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if constexpr (QueryT<HeadT>::value)
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return std::tuple_cat (
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std::tuple (std::forward<HeadT> (head)),
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filter<QueryT> (std::forward<ArgsT> (args)...)
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);
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else
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return filter<QueryT> (std::forward<ArgsT> (args)...);
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}
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};
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#endif
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