/* * 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 #include #include #include #include #include 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> data{ * map{{"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{{"key1", 17}, {"key2", 19}, {"key3", 23}}; * callSomethingTakingAMapByRef(*data); * } */ template class Indestructible final { public: template 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>{{{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::value>* = nullptr, std::enable_if_t< !std::is_same, remove_cvref_t>::value>* = nullptr, std::enable_if_t::value>* = nullptr> explicit constexpr Indestructible(U&& u) noexcept( noexcept(T(std::declval()))) : storage_{in_place, std::forward(u)} {} template < typename U = T, std::enable_if_t::value>* = nullptr, std::enable_if_t< !std::is_same, remove_cvref_t>::value>* = nullptr, std::enable_if_t::value>* = nullptr> /* implicit */ constexpr Indestructible(U&& u) noexcept( noexcept(T(std::declval()))) : storage_{in_place, std::forward(u)} {} template ()...))> explicit constexpr Indestructible(Args&&... args) noexcept( noexcept(T(std::declval()...))) : storage_{in_place, std::forward(args)...} {} template < typename U, typename... Args, typename = decltype(T( std::declval&>(), std::declval()...))> explicit constexpr Indestructible(std::initializer_list il, Args... args) noexcept( noexcept(T( std::declval&>(), std::declval()...))) : storage_{in_place, il, std::forward(args)...} {} Indestructible(Indestructible const&) = delete; Indestructible& operator=(Indestructible const&) = delete; T* get() noexcept { return reinterpret_cast(&storage_.bytes); } T const* get() const noexcept { return reinterpret_cast(&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 bytes; template ()...))> explicit constexpr Storage(in_place_t, Args&&... args) noexcept( noexcept(T(std::declval()...))) { ::new (&bytes) T(std::forward(args)...); } }; Storage storage_{}; }; } // namespace folly