folly/folly/Indestructible.h

/*
 * 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);
 *    }
 */

struct factory_constructor_t { … };

constexpr factory_constructor_t factory_constructor{ … };

template <typename T>
class Indestructible final { … };
} // namespace folly