101 lines
2.9 KiB
C++
101 lines
2.9 KiB
C++
/*
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* Copyright (c) Facebook, Inc. and its affiliates.
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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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#pragma once
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#include <type_traits>
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#include <utility>
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#include <folly/Traits.h>
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#include <folly/functional/Invoke.h>
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/**
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* folly implementation of `std::overload` like functionality
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*
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* Example:
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* struct One {};
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* struct Two {};
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* boost::variant<One, Two> value;
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*
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* variant_match(value,
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* [] (const One& one) { ... },
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* [] (const Two& two) { ... });
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*/
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namespace folly {
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namespace detail {
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template <typename...>
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struct Overload;
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template <typename Case, typename... Cases>
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struct Overload<Case, Cases...> : Overload<Cases...>, Case {
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Overload(Case c, Cases... cs)
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: Overload<Cases...>(std::move(cs)...), Case(std::move(c)) {}
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using Case::operator();
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using Overload<Cases...>::operator();
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};
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template <typename Case>
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struct Overload<Case> : Case {
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explicit Overload(Case c) : Case(std::move(c)) {}
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using Case::operator();
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};
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} // namespace detail
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/*
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* Combine multiple `Cases` in one function object
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*/
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template <typename... Cases>
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decltype(auto) overload(Cases&&... cases) {
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return detail::Overload<typename std::decay<Cases>::type...>{
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std::forward<Cases>(cases)...};
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}
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namespace overload_detail {
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FOLLY_CREATE_MEMBER_INVOKER(valueless_by_exception, valueless_by_exception);
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FOLLY_CREATE_FREE_INVOKER(visit, visit);
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FOLLY_CREATE_FREE_INVOKER(apply_visitor, apply_visitor);
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} // namespace overload_detail
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/*
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* Match `Variant` with one of the `Cases`
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*
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* Note: you can also use `[] (const auto&) {...}` as default case
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*
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* Selects `visit` if `v.valueless_by_exception()` available and the call to
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* `visit` is valid (e.g. `std::variant`). Otherwise, selects `apply_visitor`
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* (e.g. `boost::variant`, `folly::DiscriminatedPtr`).
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*/
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template <typename Variant, typename... Cases>
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decltype(auto) variant_match(Variant&& variant, Cases&&... cases) {
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using invoker = std::conditional_t<
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folly::Conjunction<
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is_invocable<overload_detail::valueless_by_exception, Variant>,
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is_invocable<
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overload_detail::visit,
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decltype(overload(std::forward<Cases>(cases)...)),
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Variant>>::value,
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overload_detail::visit,
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overload_detail::apply_visitor>;
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return invoker{}(
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overload(std::forward<Cases>(cases)...), std::forward<Variant>(variant));
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}
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} // namespace folly
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