/* * Copyright (c) Meta Platforms, Inc. and affiliates. * * 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. */ #pragma once #include <cassert> #include <new> #include <type_traits> #include <utility> #include <folly/Traits.h> #include <folly/Utility.h> namespace folly { /*** * Indestructible * * When you need a Meyers singleton that will not get destructed, even at * shutdown, and you also want the object stored inline. * * Use like: * * void doSomethingWithExpensiveData(); * * void doSomethingWithExpensiveData() { * static const Indestructible<map<string, int>> data{ * map<string, int>{{"key1", 17}, {"key2", 19}, {"key3", 23}}, * }; * callSomethingTakingAMapByRef(*data); * } * * This should be used only for Meyers singletons, and, even then, only when * the instance does not need to be destructed ever. * * This should not be used more generally, e.g., as member fields, etc. * * This is designed as an alternative, but with one fewer allocation at * construction time and one fewer pointer dereference at access time, to the * Meyers singleton pattern of: * * void doSomethingWithExpensiveData() { * static const auto data = // never `delete`d * new map<string, int>{{"key1", 17}, {"key2", 19}, {"key3", 23}}; * callSomethingTakingAMapByRef(*data); * } */ template <typename T> class Indestructible final { public: template <typename S = T, typename = decltype(S())> constexpr Indestructible() noexcept(noexcept(T())) : storage_{in_place} {} /** * Constructor accepting a single argument by forwarding reference, this * allows using list initialization without the overhead of things like * in_place, etc and also works with std::initializer_list constructors * which can't be deduced, the default parameter helps there. * * auto i = folly::Indestructible<std::map<int, int>>{{{1, 2}}}; * * This provides convenience * * There are two versions of this constructor - one for when the element is * implicitly constructible from the given argument and one for when the * type is explicitly but not implicitly constructible from the given * argument. */ template < typename U = T, std::enable_if_t<std::is_constructible<T, U&&>::value>* = nullptr, std::enable_if_t< !std::is_same<Indestructible<T>, remove_cvref_t<U>>::value>* = nullptr, std::enable_if_t<!std::is_convertible<U&&, T>::value>* = nullptr> explicit constexpr Indestructible(U&& u) noexcept( noexcept(T(std::declval<U>()))) : storage_{in_place, std::forward<U>(u)} {} template < typename U = T, std::enable_if_t<std::is_constructible<T, U&&>::value>* = nullptr, std::enable_if_t< !std::is_same<Indestructible<T>, remove_cvref_t<U>>::value>* = nullptr, std::enable_if_t<std::is_convertible<U&&, T>::value>* = nullptr> /* implicit */ constexpr Indestructible(U&& u) noexcept( noexcept(T(std::declval<U>()))) : storage_{in_place, std::forward<U>(u)} {} template <typename... Args, typename = decltype(T(std::declval<Args>()...))> explicit constexpr Indestructible(Args&&... args) noexcept( noexcept(T(std::declval<Args>()...))) : storage_{in_place, std::forward<Args>(args)...} {} template < typename U, typename... Args, typename = decltype(T( std::declval<std::initializer_list<U>&>(), std::declval<Args>()...))> explicit constexpr Indestructible(std::initializer_list<U> il, Args... args) noexcept( noexcept(T( std::declval<std::initializer_list<U>&>(), std::declval<Args>()...))) : storage_{in_place, il, std::forward<Args>(args)...} {} Indestructible(Indestructible const&) = delete; Indestructible& operator=(Indestructible const&) = delete; T* get() noexcept { return reinterpret_cast<T*>(&storage_.bytes); } T const* get() const noexcept { return reinterpret_cast<T const*>(&storage_.bytes); } T& operator*() noexcept { return *get(); } T const& operator*() const noexcept { return *get(); } T* operator->() noexcept { return get(); } T const* operator->() const noexcept { return get(); } /* implicit */ operator T&() noexcept { return *get(); } /* implicit */ operator T const &() const noexcept { return *get(); } private: struct Storage { aligned_storage_for_t<T> bytes; template <typename... Args, typename = decltype(T(std::declval<Args>()...))> explicit constexpr Storage(in_place_t, Args&&... args) noexcept( noexcept(T(std::declval<Args>()...))) { ::new (&bytes) T(std::forward<Args>(args)...); } }; Storage storage_{}; }; } // namespace folly