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 // 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.
             typename std::enable_if<!std::is_same<void(std::decay_t<Ts>&...),
                                                   void(NoDestructor&)>::value,
                                     int>::type = 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(); }
   absl::Nonnull<T*> operator->() { return get(); }
   absl::Nonnull<T*> get() { return impl_.get(); }
   const T& operator*() const { return *get(); }
   absl::Nonnull<const T*> operator->() const { return get(); }
   absl::Nonnull<const T*> get() const { return impl_.get(); }
 
  private:
   class DirectImpl {
    public:
     template <typename... Args>
     explicit constexpr DirectImpl(Args&&... args)
         : value_(std::forward<Args>(args)...) {}
     absl::Nonnull<const T*> get() const { return &value_; }
     absl::Nonnull<T*> get() { return &value_; }
 
    private:
     T value_;
   };
 
   class PlacementImpl {
    public:
     template <typename... Args>
     explicit PlacementImpl(Args&&... args) {
       new (&space_) T(std::forward<Args>(args)...);
     }
     absl::Nonnull<const T*> get() const {
       return Launder(reinterpret_cast<const T*>(&space_));
     }
     absl::Nonnull<T*> get() { return Launder(reinterpret_cast<T*>(&space_)); }
 
    private:
     template <typename P>
     static absl::Nonnull<P*> Launder(absl::Nonnull<P*> p) {
 #if defined(__cpp_lib_launder) && __cpp_lib_launder >= 201606L
       return std::launder(p);
 #elif ABSL_HAVE_BUILTIN(__builtin_launder)
       return __builtin_launder(p);
 #else
       // When `std::launder` or equivalent are not available, we rely on
       // undefined behavior, which works as intended on Abseil's officially
       // supported platforms as of Q3 2023.
 #if defined(__GNUC__) && !defined(__clang__)
 #pragma GCC diagnostic push
 #pragma GCC diagnostic ignored "-Wstrict-aliasing"
 #endif
       return p;
 #if defined(__GNUC__) && !defined(__clang__)
 #pragma GCC diagnostic pop
 #endif
 #endif
     }
 
     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<T>::value, DirectImpl,
                      PlacementImpl>
       impl_;
 };
 
 #ifdef ABSL_HAVE_CLASS_TEMPLATE_ARGUMENT_DEDUCTION
 // 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>;
 #endif  // ABSL_HAVE_CLASS_TEMPLATE_ARGUMENT_DEDUCTION
 
 ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_BASE_NO_DESTRUCTOR_H_

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