38b03c840e
Summary: This ports `folly::DistributedMutex` into RocksDB. The PR includes everything else needed to compile and use DistributedMutex as a component within folly. Most files are unchanged except for some portability stuff and includes. For now, I've put this under `rocksdb/third-party`, but if there is a better folder to put this under, let me know. I also am not sure how or where to put unit tests for third-party stuff like this. It seems like gtest is included already, but I need to link with it from another third-party folder. This also includes some other common components from folly - folly/Optional - folly/ScopeGuard (In particular `SCOPE_EXIT`) - folly/synchronization/ParkingLot (A portable futex-like interface) - folly/synchronization/AtomicNotification (The standard C++ interface for futexes) - folly/Indestructible (For singletons that don't get destroyed without allocations) Pull Request resolved: https://github.com/facebook/rocksdb/pull/5642 Differential Revision: D16544439 fbshipit-source-id: 179b98b5dcddc3075926d31a30f92fd064245731
60 lines
1.7 KiB
C++
60 lines
1.7 KiB
C++
// Copyright (c) 2011-present, Facebook, Inc. All rights reserved.
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// This source code is licensed under both the GPLv2 (found in the
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// COPYING file in the root directory) and Apache 2.0 License
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// (found in the LICENSE.Apache file in the root directory).
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#pragma once
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#include <type_traits>
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namespace folly {
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/// In functional programming, the degenerate case is often called "unit". In
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/// C++, "void" is often the best analogue. However, because of the syntactic
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/// special-casing required for void, it is frequently a liability for template
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/// metaprogramming. So, instead of writing specializations to handle cases like
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/// SomeContainer<void>, a library author may instead rule that out and simply
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/// have library users use SomeContainer<Unit>. Contained values may be ignored.
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/// Much easier.
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///
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/// "void" is the type that admits of no values at all. It is not possible to
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/// construct a value of this type.
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/// "unit" is the type that admits of precisely one unique value. It is
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/// possible to construct a value of this type, but it is always the same value
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/// every time, so it is uninteresting.
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struct Unit {
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constexpr bool operator==(const Unit& /*other*/) const {
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return true;
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}
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constexpr bool operator!=(const Unit& /*other*/) const {
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return false;
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}
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};
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constexpr Unit unit{};
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template <typename T>
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struct lift_unit {
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using type = T;
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};
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template <>
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struct lift_unit<void> {
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using type = Unit;
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};
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template <typename T>
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using lift_unit_t = typename lift_unit<T>::type;
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template <typename T>
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struct drop_unit {
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using type = T;
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};
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template <>
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struct drop_unit<Unit> {
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using type = void;
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};
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template <typename T>
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using drop_unit_t = typename drop_unit<T>::type;
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} // namespace folly
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