absl/base/no_destructor.h

// Copyright 2023 The Abseil Authors.
//
// 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
//
//      https://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.
//
// -----------------------------------------------------------------------------
// File: no_destructor.h
// -----------------------------------------------------------------------------
//
// This header file defines the absl::NoDestructor<T> wrapper for defining a
// static type that does not need to be destructed upon program exit. Instead,
// such an object survives during program exit (and can be safely accessed at
// any time).
//
// absl::NoDestructor<T> is useful when when a variable has static storage
// duration but its type has a non-trivial destructor. Global constructors are
// not recommended because of the C++'s static initialization order fiasco (See
// https://en.cppreference.com/w/cpp/language/siof). Global destructors are not
// allowed due to similar concerns about destruction ordering. Using
// absl::NoDestructor<T> as a function-local static prevents both of these
// issues.
//
// See below for complete details.


#ifndef ABSL_BASE_NO_DESTRUCTOR_H_
#define ABSL_BASE_NO_DESTRUCTOR_H_

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

#include "absl/base/config.h"
#include "absl/base/nullability.h"

namespace absl {
ABSL_NAMESPACE_BEGIN

// absl::NoDestructor<T>
//
// NoDestructor<T> is a wrapper around an object of type T that behaves as an
// object of type T but never calls T's destructor. NoDestructor<T> makes it
// safer and/or more efficient to use such objects in static storage contexts,
// ideally as function scope static variables.
//
// An instance of absl::NoDestructor<T> has similar type semantics to an
// instance of T:
//
// * Constructs in the same manner as an object of type T through perfect
//   forwarding.
// * Provides pointer/reference semantic access to the object of type T via
//   `->`, `*`, and `get()`.
//   (Note that `const NoDestructor<T>` works like a pointer to const `T`.)
//
// Additionally, NoDestructor<T> provides the following benefits:
//
// * Never calls T's destructor for the object
// * If the object is a function-local static variable, the type can be
//   lazily constructed.
//
// An object of type NoDestructor<T> is "trivially destructible" in the notion
// that its destructor is never run.
//
// Usage as Function Scope Static Variables
//
// Function static objects will be lazily initialized within static storage:
//
//    // Function scope.
//    const std::string& MyString() {
//      static const absl::NoDestructor<std::string> x("foo");
//      return *x;
//    }
//
// For function static variables, NoDestructor avoids heap allocation and can be
// inlined in static storage, resulting in exactly-once, thread-safe
// construction of an object, and very fast access thereafter (the cost is a few
// extra cycles).
//
// Using NoDestructor<T> in this manner is generally better than other patterns
// which require pointer chasing:
//
//   // Prefer using absl::NoDestructor<T> instead for the static variable.
//   const std::string& MyString() {
//     static const std::string* x = new std::string("foo");
//     return *x;
//   }
//
// Usage as Global Static Variables
//
// NoDestructor<T> allows declaration of a global object of type T that has a
// non-trivial destructor since its destructor is never run. However, such
// objects still need to worry about initialization order, so such use is not
// recommended, strongly discouraged by the Google C++ Style Guide, and outright
// banned in Chromium.
// See https://google.github.io/styleguide/cppguide.html#Static_and_Global_Variables
//
//    // Global or namespace scope.
//    absl::NoDestructor<MyRegistry> reg{"foo", "bar", 8008};
//
// Note that if your object already has a trivial destructor, you don't need to
// use NoDestructor<T>.
//
template <typename T>
class NoDestructor {
 public:
  // Forwards arguments to the T's constructor: calls T(args...).
  template <typename... Ts,
            // Disable this overload when it might collide with copy/move.
            std::enable_if_t<!std::is_same_v<void(std::decay_t<Ts>&...),
                                             void(NoDestructor&)>,
                             int> = 0>
  explicit constexpr NoDestructor(Ts&&... args)
      : impl_(std::forward<Ts>(args)...) {}

  // Forwards copy and move construction for T. Enables usage like this:
  //   static NoDestructor<std::array<string, 3>> x{{{"1", "2", "3"}}};
  //   static NoDestructor<std::vector<int>> x{{1, 2, 3}};
  explicit constexpr NoDestructor(const T& x) : impl_(x) {}
  explicit constexpr NoDestructor(T&& x)
      : impl_(std::move(x)) {}

  // No copying.
  NoDestructor(const NoDestructor&) = delete;
  NoDestructor& operator=(const NoDestructor&) = delete;

  // Pretend to be a smart pointer to T with deep constness.
  // Never returns a null pointer.
  T& operator*() { return *get(); }
  T* absl_nonnull operator->() { return get(); }
  T* absl_nonnull get() { return impl_.get(); }
  const T& operator*() const { return *get(); }
  const T* absl_nonnull operator->() const { return get(); }
  const T* absl_nonnull get() const { return impl_.get(); }

 private:
  class DirectImpl {
   public:
    template <typename... Args>
    explicit constexpr DirectImpl(Args&&... args)
        : value_(std::forward<Args>(args)...) {}
    const T* absl_nonnull get() const { return &value_; }
    T* absl_nonnull get() { return &value_; }

   private:
    T value_;
  };

  class PlacementImpl {
   public:
    template <typename... Args>
    explicit PlacementImpl(Args&&... args) {
      new (&space_) T(std::forward<Args>(args)...);
    }
    const T* absl_nonnull get() const {
      return std::launder(reinterpret_cast<const T*>(&space_));
    }
    T* absl_nonnull get() {
      return std::launder(reinterpret_cast<T*>(&space_));
    }

   private:
    alignas(T) unsigned char space_[sizeof(T)];
  };

  // If the object is trivially destructible we use a member directly to avoid
  // potential once-init runtime initialization. It somewhat defeats the
  // purpose of NoDestructor in this case, but this makes the class more
  // friendly to generic code.
  std::conditional_t<std::is_trivially_destructible_v<T>, DirectImpl,
                     PlacementImpl>
      impl_;
};

// Provide 'Class Template Argument Deduction': the type of NoDestructor's T
// will be the same type as the argument passed to NoDestructor's constructor.
template <typename T>
NoDestructor(T) -> NoDestructor<T>;

ABSL_NAMESPACE_END
}  // namespace absl

#endif  // ABSL_BASE_NO_DESTRUCTOR_H_