56 lines
2.3 KiB
C++
56 lines
2.3 KiB
C++
/*
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* TypeTraits.h
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*
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* This source file is part of the FoundationDB open source project
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*
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* Copyright 2013-2022 Apple Inc. and the FoundationDB project authors
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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// This file, similar to `type_traits` in the standard library, contains utility types that can be used for template
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// metaprogramming. While they can be very useful and simplify certain things, please be aware that their use will
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// increase compilation times significantly. Therefore it is not recommended to use them in header file if not
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// absolutely necessary.
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#pragma once
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#include <variant>
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// This type class will take two std::variant types and concatenate them
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template <class L, class R>
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struct variant_concat_t;
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template <class... Args1, class... Args2>
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struct variant_concat_t<std::variant<Args1...>, std::variant<Args2...>> {
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using type = std::variant<Args1..., Args2...>;
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};
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// Helper definition for variant_concat_t. Instead of using `typename variant_concat_t<...>::type` one can simply use
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// `variant_concat<...>`
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template <class L, class R>
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using variant_concat = typename variant_concat_t<L, R>::type;
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// Takes a std::variant as first argument and applies Fun to all of them. For example: typename
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// variant_map_t<std::variant<int, bool>, std::add_pointer_t>::type will be defined as std::variant<int*, bool*>
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template <class T, template <class> class Fun>
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struct variant_map_t;
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template <class... Args, template <class> class Fun>
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struct variant_map_t<std::variant<Args...>, Fun> {
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using type = std::variant<Fun<Args>...>;
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};
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// Helper definition for variant_map_t. Instead of using `typename variant_map<...>::type` one can simple use
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// `variant_map<...>` which is equivalent but shorter.
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template <class T, template <class> class Fun>
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using variant_map = typename variant_map_t<T, Fun>::type;
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