chromium/base/no_destructor.h

// Copyright 2018 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#ifndef BASE_NO_DESTRUCTOR_H_
#define BASE_NO_DESTRUCTOR_H_

#include <new>
#include <type_traits>
#include <utility>

namespace base {

// Helper type to create a function-local static variable of type `T` when `T`
// has a non-trivial destructor. Storing a `T` in a `base::NoDestructor<T>` will
// prevent `~T()` from running, even when the variable goes out of scope.
//
// Useful when a variable has static storage duration but its type has a
// non-trivial destructor. Chromium bans global constructors and destructors:
// using a function-local static variable prevents the former, while using
// `base::NoDestructor<T>` prevents the latter.
//
// ## Caveats
//
// - Must not be used for locals or fields; by definition, this does not run
//   destructors, and this will likely lead to memory leaks and other
//   surprising and undesirable behaviour.
//
// - If `T` is not constexpr constructible, must be a function-local static
//   variable, since a global `NoDestructor<T>` will still generate a static
//   initializer.
//
// - If `T` is constinit constructible, may be used as a global, but mark the
//   global `constinit`.
//
// - If the data is rarely used, consider creating it on demand rather than
//   caching it for the lifetime of the program. Though `base::NoDestructor<T>`
//   does not heap allocate, the compiler still reserves space in bss for
//   storing `T`, which costs memory at runtime.
//
// - If `T` is trivially destructible, do not use `base::NoDestructor<T>`:
//
//     const uint64_t GetUnstableSessionSeed() {
//       // No need to use `base::NoDestructor<T>` as `uint64_t` is trivially
//       // destructible and does not require a global destructor.
//       static const uint64_t kSessionSeed = base::RandUint64();
//       return kSessionSeed;
//     }
//
// ## Example Usage
//
// const std::string& GetDefaultText() {
//   // Required since `static const std::string` requires a global destructor.
//   static const base::NoDestructor<std::string> s("Hello world!");
//   return *s;
// }
//
// More complex initialization using a lambda:
//
// const std::string& GetRandomNonce() {
//   // `nonce` is initialized with random data the first time this function is
//   // called, but its value is fixed thereafter.
//   static const base::NoDestructor<std::string> nonce([] {
//     std::string s(16);
//     crypto::RandString(s.data(), s.size());
//     return s;
//   }());
//   return *nonce;
// }
//
// ## Thread safety
//
// Initialisation of function-local static variables is thread-safe since C++11.
// The standard guarantees that:
//
// - function-local static variables will be initialised the first time
//   execution passes through the declaration.
//
// - if another thread's execution concurrently passes through the declaration
//   in the middle of initialisation, that thread will wait for the in-progress
//   initialisation to complete.
template <typename T>
class NoDestructor { … };

}  // namespace base

#endif  // BASE_NO_DESTRUCTOR_H_