123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566676869707172737475767778798081828384858687888990919293949596979899100101102103104105106107108109110111112113114115116117118119120121122123124125126127128129130131132133134135136137138139140141142143144145146147148149150151152153154155156157158159160161162163164165166167168169170171172173174175176177 |
- /*
- * Copyright 2016-present Facebook, Inc.
- *
- * 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 <type_traits>
- #include <utility>
- #include <folly/Traits.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())) {}
- /**
- * Constructor accepting a single argument by forwarding reference, this
- * allows using list initialzation 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,
- _t<std::enable_if<std::is_constructible<T, U&&>::value>>* = nullptr,
- _t<std::enable_if<
- !std::is_same<Indestructible<T>, remove_cvref_t<U>>::value>>* =
- nullptr,
- _t<std::enable_if<!std::is_convertible<U&&, T>::value>>* = nullptr>
- explicit constexpr Indestructible(U&& u) noexcept(
- noexcept(T(std::declval<U>())))
- : storage_(std::forward<U>(u)) {}
- template <
- typename U = T,
- _t<std::enable_if<std::is_constructible<T, U&&>::value>>* = nullptr,
- _t<std::enable_if<
- !std::is_same<Indestructible<T>, remove_cvref_t<U>>::value>>* =
- nullptr,
- _t<std::enable_if<std::is_convertible<U&&, T>::value>>* = nullptr>
- /* implicit */ constexpr Indestructible(U&& u) noexcept(
- noexcept(T(std::declval<U>())))
- : storage_(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_(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_(il, std::forward<Args>(args)...) {}
- ~Indestructible() = default;
- Indestructible(Indestructible const&) = delete;
- Indestructible& operator=(Indestructible const&) = delete;
- Indestructible(Indestructible&& other) noexcept(
- noexcept(T(std::declval<T>())))
- : storage_(std::move(other.storage_.value)) {
- other.erased_ = true;
- }
- Indestructible& operator=(Indestructible&& other) noexcept(
- noexcept(T(std::declval<T>()))) {
- storage_.value = std::move(other.storage_.value);
- other.erased_ = true;
- }
- T* get() noexcept {
- check();
- return &storage_.value;
- }
- T const* get() const noexcept {
- check();
- return &storage_.value;
- }
- T& operator*() noexcept {
- return *get();
- }
- T const& operator*() const noexcept {
- return *get();
- }
- T* operator->() noexcept {
- return get();
- }
- T const* operator->() const noexcept {
- return get();
- }
- private:
- void check() const noexcept {
- assert(!erased_);
- }
- union Storage {
- T value;
- template <typename S = T, typename = decltype(S())>
- constexpr Storage() noexcept(noexcept(T())) : value() {}
- template <typename... Args, typename = decltype(T(std::declval<Args>()...))>
- explicit constexpr Storage(Args&&... args) noexcept(
- noexcept(T(std::declval<Args>()...)))
- : value(std::forward<Args>(args)...) {}
- ~Storage() {}
- };
- Storage storage_{};
- bool erased_{false};
- };
- } // namespace folly
|