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122 lines
3.3 KiB
C++
122 lines
3.3 KiB
C++
/*
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* Copyright 2016 Facebook, Inc.
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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 <utility>
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#include <glog/logging.h>
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#include <folly/Likely.h>
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#include <folly/Portability.h>
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namespace folly {
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/***
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* Indestructible
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*
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* When you need a Meyers singleton that will not get destructed, even at
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* shutdown, and you also want the object stored inline.
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*
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* Use like:
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*
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* void doSomethingWithExpensiveData();
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*
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* void doSomethingWithExpensiveData() {
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* static const Indestructible<map<string, int>> data{
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* map<string, int>{{"key1", 17}, {"key2", 19}, {"key3", 23}},
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* };
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* callSomethingTakingAMapByRef(*data);
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* }
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*
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* This should be used only for Meyers singletons, and, even then, only when
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* the instance does not need to be destructed ever.
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*
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* This should not be used more generally, e.g., as member fields, etc.
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*
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* This is designed as an alternative, but with one fewer allocation at
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* construction time and one fewer pointer dereference at access time, to the
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* Meyers singleton pattern of:
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*
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* void doSomethingWithExpensiveData() {
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* static const auto data = // never `delete`d
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* new map<string, int>{{"key1", 17}, {"key2", 19}, {"key3", 23}};
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* callSomethingTakingAMapByRef(*data);
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* }
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*/
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template <typename T>
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class Indestructible final {
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public:
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template <typename... Args>
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explicit constexpr Indestructible(Args&&... args) noexcept(
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std::is_nothrow_constructible<T, Args&&...>::value)
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: storage_(std::forward<Args>(args)...), inited_(true) {}
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~Indestructible() = default;
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Indestructible(Indestructible const&) = delete;
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Indestructible& operator=(Indestructible const&) = delete;
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Indestructible(Indestructible&& other) noexcept(
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std::is_nothrow_move_constructible<T>::value)
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: storage_(std::move(other.storage_.value)) {
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other.inited_ = false;
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}
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Indestructible& operator=(Indestructible&& other) noexcept(
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std::is_nothrow_move_assignable<T>::value) {
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storage_.value = std::move(other.storage_.value);
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other.inited_ = false;
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}
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T* get() {
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check();
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return &storage_.value;
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}
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T const* get() const {
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check();
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return &storage_.value;
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}
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T& operator*() { return *get(); }
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T const& operator*() const { return *get(); }
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T* operator->() { return get(); }
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T const* operator->() const { return get(); }
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private:
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void check() const {
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if (UNLIKELY(!inited_)) {
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fail();
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}
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}
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[[noreturn]] FOLLY_NOINLINE static void fail() {
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LOG(FATAL) << "Indestructible is not initialized";
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}
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union Storage {
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T value;
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template <typename... Args>
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explicit constexpr Storage(Args&&... args)
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: value(std::forward<Args>(args)...) {}
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~Storage() {}
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};
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Storage storage_;
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bool inited_{false};
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};
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}
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