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/**
* Copyright (C) 2020-present MongoDB, Inc.
*
* This program is free software: you can redistribute it and/or modify
* it under the terms of the Server Side Public License, version 1,
* as published by MongoDB, Inc.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* Server Side Public License for more details.
*
* You should have received a copy of the Server Side Public License
* along with this program. If not, see
* <http://www.mongodb.com/licensing/server-side-public-license>.
*
* As a special exception, the copyright holders give permission to link the
* code of portions of this program with the OpenSSL library under certain
* conditions as described in each individual source file and distribute
* linked combinations including the program with the OpenSSL library. You
* must comply with the Server Side Public License in all respects for
* all of the code used other than as permitted herein. If you modify file(s)
* with this exception, you may extend this exception to your version of the
* file(s), but you are not obligated to do so. If you do not wish to do so,
* delete this exception statement from your version. If you delete this
* exception statement from all source files in the program, then also delete
* it in the license file.
*/
#pragma once
#include <type_traits>
#include <utility>
namespace mongo {
/**
* A helper class for defining reliable static duration variables that are safe at
* shutdown. It's also useful as a greppable identifier to emphasize, track, and
* allowlist such objects across the codebase. This is a way to make an object that
* never dies and isn't allocated. T need not be destructible, as it is never destroyed.
*
* Notice that in every example, there's a `static` storage class specifier on the
* defined variable. This is very important.
*
* The `StaticImmortal<T>` contains raw memory suitable for storing a `T`, and has a trivial
* destructor. No action is taken at shutdown time, so the on-board `T` remains undestroyed.
*
* Initialization syntax examples:
* static StaticImmortal<Map> m{};
* static auto m = StaticImmortal<Map>();
*
* // Explicitly specified `T`. Curly braces cannot be collapsed here.
* static StaticImmortal<Map> m{{{"hello", 123}, {"bye", 456}}};
* static StaticImmortal<Map> m = {{{"hello", 123}, {"bye", 456}}};
*
* // `T` can also be deduced from the initializing value:
* static StaticImmortal m = Map{{"hello", 123}, {"bye", 456}};
* static StaticImmortal m = [] { return Map{{"hello", 123}, {"bye", 456}}; }();
*
* // As with `optional<T>`, dereference operators `->` and `*` yield a reference to `T`.
* auto iter = m->find("hello");
* functionTakingAMapReference(*m);
*/
template <typename T>
class StaticImmortal {
public:
using value_type = T;
constexpr StaticImmortal() {
_construct();
}
constexpr StaticImmortal(const value_type& obj) {
_construct(obj);
}
constexpr StaticImmortal(value_type&& obj) {
_construct(std::move(obj));
}
template <typename... A>
explicit constexpr StaticImmortal(A&&... args) {
_construct(std::forward<A>(args)...);
}
StaticImmortal(const StaticImmortal&) = delete;
StaticImmortal(StaticImmortal&&) = delete;
constexpr value_type& value() noexcept {
return *reinterpret_cast<value_type*>(&_storage);
}
constexpr const value_type& value() const noexcept {
return *reinterpret_cast<const value_type*>(&_storage);
}
/**
* @{
* Deref operators like optional.
*/
constexpr value_type& operator*() noexcept {
return value();
}
constexpr const value_type& operator*() const noexcept {
return value();
}
constexpr value_type* operator->() noexcept {
return &value();
}
constexpr const value_type* operator->() const noexcept {
return &value();
}
/** @} */
private:
using Storage = std::aligned_storage_t<sizeof(value_type), alignof(value_type)>;
template <typename... A>
constexpr void _construct(A&&... args) {
new (&_storage) value_type(std::forward<A>(args)...);
}
Storage _storage;
};
} // namespace mongo
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