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Standard library header <memory> - cppreference.com

#include <compare>

namespace std {
  // pointer Traits
  template<class Ptr>
  struct pointer_traits; // freestanding
  template<class T>
  struct pointer_traits<T*>; // freestanding

  // pointer conversion
  template<class T>
  constexpr T* to_address(T* p) noexcept; // freestanding
  template<class Ptr>
  constexpr auto to_address(const Ptr& p) noexcept; // freestanding

  // pointer alignment
  void* align(size_t alignment, size_t size, void*& ptr, size_t& space); // freestanding
  template<size_t N, class T>
  constexpr T* assume_aligned(T* ptr); // freestanding
  template<size_t Alignment, class T>
  bool is_sufficiently_aligned(T* ptr);

  // explicit lifetime management
  template<class T>
  T* start_lifetime_as(void* p) noexcept; // freestanding
  template<class T>
  const T* start_lifetime_as(const void* p) noexcept; // freestanding
  template<class T>
  volatile T* start_lifetime_as(volatile void* p) noexcept; // freestanding
  template<class T>
  const volatile T* start_lifetime_as(const volatile void* p) noexcept; // freestanding
  template<class T>
  T* start_lifetime_as_array(void* p, size_t n) noexcept; // freestanding
  template<class T>
  const T* start_lifetime_as_array(const void* p, size_t n) noexcept; // freestanding
  template<class T>
  volatile T* start_lifetime_as_array(volatile void* p,
                                      size_t n) noexcept; // freestanding
  template<class T>
  const volatile T* start_lifetime_as_array(const volatile void* p, // freestanding
                                            size_t n) noexcept;
  template<class T>
  T* trivially_relocate(T* first, T* last, T* result); // freestanding
  template<class T>
  constexpr T* relocate(T* first, T* last, T* result); // freestanding

  // allocator argument tag
  struct allocator_arg_t
  {
    explicit allocator_arg_t() = default;
  };                                                // freestanding
  inline constexpr allocator_arg_t allocator_arg{}; // freestanding

  // uses_allocator
  template<class T, class Alloc>
  struct uses_allocator; // freestanding

  // uses_allocator
  template<class T, class Alloc>
  constexpr bool uses_allocator_v = uses_allocator<T, Alloc>::value; // freestanding

  // uses-allocator construction
  template<class T, class Alloc, class... Args>
  constexpr auto uses_allocator_construction_args(const Alloc& alloc, // freestanding
                                                  Args&&... args) noexcept;
  template<class T, class Alloc, class Tuple1, class Tuple2>
  constexpr auto uses_allocator_construction_args(const Alloc& alloc, // freestanding
                                                  piecewise_construct_t,
                                                  Tuple1&& x,
                                                  Tuple2&& y) noexcept;
  template<class T, class Alloc>
  constexpr auto uses_allocator_construction_args(
    const Alloc& alloc) noexcept; // freestanding
  template<class T, class Alloc, class U, class V>
  constexpr auto uses_allocator_construction_args(const Alloc& alloc, // freestanding
                                                  U&& u,
                                                  V&& v) noexcept;
  template<class T, class Alloc, class U, class V>
  constexpr auto uses_allocator_construction_args(const Alloc& alloc, // freestanding
                                                  pair<U, V>& pr) noexcept;
  template<class T, class Alloc, class U, class V>
  constexpr auto uses_allocator_construction_args(const Alloc& alloc, // freestanding
                                                  const pair<U, V>& pr) noexcept;
  template<class T, class Alloc, class U, class V>
  constexpr auto uses_allocator_construction_args(const Alloc& alloc, // freestanding
                                                  pair<U, V>&& pr) noexcept;
  template<class T, class Alloc, class U, class V>
  constexpr auto uses_allocator_construction_args(const Alloc& alloc, // freestanding
                                                  const pair<U, V>&& pr) noexcept;
  template<class T, class Alloc, /*pair-like*/ P>
  constexpr auto uses_allocator_construction_args(const Alloc& alloc, // freestanding
                                                  P&& p) noexcept;
  template<class T, class Alloc, class U>
  constexpr auto uses_allocator_construction_args(const Alloc& alloc, // freestanding
                                                  U&& u) noexcept;
  template<class T, class Alloc, class... Args>
  constexpr T make_obj_using_allocator(const Alloc& alloc,
                                       Args&&... args); // freestanding
  template<class T, class Alloc, class... Args>
  constexpr T* uninitialized_construct_using_allocator(T* p, // freestanding
                                                       const Alloc& alloc,
                                                       Args&&... args);

  // allocator Traits
  template<class Alloc>
  struct allocator_traits; // freestanding

  template<class Pointer, class SizeType = size_t>
  struct allocation_result
  { // freestanding
    Pointer ptr;
    SizeType count;
  };

  // the default allocator
  template<class T>
  class allocator;
  template<class T, class U>
  constexpr bool operator==(const allocator<T>&, const allocator<U>&) noexcept;

  // addressof
  template<class T>
  constexpr T* addressof(T& r) noexcept; // freestanding
  template<class T>
  const T* addressof(const T&&) = delete; // freestanding

  // specialized algorithms
  // special memory concepts
  template<class I>
  concept nothrow-input-iterator = /* see description */; // exposition-only
  template<class I>
  concept nothrow-forward-iterator = /* see description */; // exposition-only
  template<class I>
  concept nothrow-bidirectional-iterator = /* see description */; // exposition-only
  template<class I>
  concept nothrow-random-access-iterator = /* see description */; // exposition-only
  template<class S, class I>
  concept nothrow-sentinel-for = /* see description */; // exposition-only
  template<class S, class I>
  concept nothrow-sized-sentinel-for = /* see description */; // exposition-only
  template<class R>
  concept nothrow-input-range = /* see description */; // exposition-only
  template<class R>
  concept nothrow-forward-range = /* see description */; // exposition-only
  template<class R>
  concept nothrow-bidirectional-range = /* see description */; // exposition-only
  template<class R>
  concept nothrow-random-access-range = /* see description */; // exposition-only
  template<class R>
  concept nothrow-sized-random-access-range = /* see description */; // exposition-only

  template<class NoThrowForwardIter>
  constexpr void uninitialized_default_construct(NoThrowForwardIter first, // freestanding
                                                 NoThrowForwardIter last);
  template<class ExecutionPolicy, class NoThrowForwardIter>
  void uninitialized_default_construct(ExecutionPolicy&& exec, // freestanding-deleted
                                       NoThrowForwardIter first,
                                       NoThrowForwardIter last);
  template<class NoThrowForwardIter, class Size>
  constexpr NoThrowForwardIter uninitialized_default_construct_n(NoThrowForwardIter first,
                                                                 Size n); // freestanding
  template<class ExecutionPolicy, class NoThrowForwardIter, class Size>
  NoThrowForwardIter uninitialized_default_construct_n(
    ExecutionPolicy&& exec, // freestanding-deleted
    NoThrowForwardIter first,
    Size n);

  namespace ranges {
    template<no-throw-forward-iterator I, no-throw-sentinel-for<I> S>
      requires default_initializable<iter_value_t<I>>
    constexpr I uninitialized_default_construct(I first, S last); // freestanding
    template<no-throw-forward-range R>
      requires default_initializable<range_value_t<R>>
    constexpr borrowed_iterator_t<R> uninitialized_default_construct(
      R&& r); // freestanding

    template<no-throw-forward-iterator I>
      requires default_initializable<iter_value_t<I>>
    constexpr I uninitialized_default_construct_n(I first, // freestanding
                                                  iter_difference_t<I> n);

    template<execution-policy Ep, nothrow-random-access-iterator I,
             nothrow-sized-sentinel-for <I> S>
      requires default_initializable<iter_value_t<I>>
    I uninitialized_default_construct(Ep&& exec, I first, S last); // freestanding-deleted
    template<execution-policy Ep, nothrow-sized-random-access-range R>
      requires default_initializable<range_value_t<R>>
    borrowed_iterator_t<R> uninitialized_default_construct // freestanding-deleted
      (Ep&& exec, R&& r);

    template<execution-policy Ep, nothrow-random-access-iterator I>
      requires default_initializable<iter_value_t<I>>
    I uninitialized_default_construct_n(Ep&& exec, I first, // freestanding-deleted
                                        iter_difference_t<I> n);
  }

  template<class NoThrowForwardIter>
  constexpr void uninitialized_value_construct(NoThrowForwardIter first, // freestanding
                                               NoThrowForwardIter last);
  template<class ExecutionPolicy, class NoThrowForwardIter>
  void uninitialized_value_construct(ExecutionPolicy&& exec, // freestanding-deleted
                                     NoThrowForwardIter first,
                                     NoThrowForwardIter last);
  template<class NoThrowForwardIter, class Size>
  constexpr NoThrowForwardIter uninitialized_value_construct_n(NoThrowForwardIter first,
                                                               Size n); // freestanding
  template<class ExecutionPolicy, class NoThrowForwardIter, class Size>
  NoThrowForwardIter uninitialized_value_construct_n(
    ExecutionPolicy&& exec, // freestanding-deleted
    NoThrowForwardIter first,
    Size n);

  namespace ranges {
    template<no-throw-forward-iterator I, no-throw-sentinel-for<I> S>
      requires default_initializable<iter_value_t<I>>
    constexpr I uninitialized_value_construct(I first, S last); // freestanding
    template<no-throw-forward-range R>
      requires default_initializable<range_value_t<R>>
    constexpr borrowed_iterator_t<R> uninitialized_value_construct(R&& r); // freestanding

    template<no-throw-forward-iterator I>
      requires default_initializable<iter_value_t<I>>
    constexpr I uninitialized_value_construct_n(I first, // freestanding
                                                iter_difference_t<I> n);

    template<execution-policy Ep, nothrow-random-access-iterator I,
             nothrow-sized-sentinel-for <I> S>
      requires default_initializable<iter_value_t<I>>
    I uninitialized_value_construct(Ep&& exec, I first, S last); // freestanding-deleted
    template<execution-policy Ep, nothrow-sized-random-access-range R>
      requires default_initializable<range_value_t<R>>
    borrowed_iterator_t<R> uninitialized_value_construct // freestanding-deleted
      (Ep&& exec, R&& r);

    template<execution-policy Ep, nothrow-random-access-iterator I>
      requires default_initializable<iter_value_t<I>>
    I uninitialized_value_construct_n(Ep&& exec, I first, // freestanding-deleted
                                      iter_difference_t<I> n);
  }

  template<class InputIter, class NoThrowForwardIter>
  constexpr NoThrowForwardIter uninitialized_copy(InputIter first, // freestanding
                                                  InputIter last,
                                                  NoThrowForwardIter result);
  template<class ExecutionPolicy, class ForwardIter, class NoThrowForwardIter>
  NoThrowForwardIter uninitialized_copy(ExecutionPolicy&& exec, // freestanding-deleted
                                        ForwardIter first,
                                        ForwardIter last,
                                        NoThrowForwardIter result);
  template<class InputIter, class Size, class NoThrowForwardIter>
  constexpr NoThrowForwardIter uninitialized_copy_n(InputIter first, // freestanding
                                                    Size n,
                                                    NoThrowForwardIter result);
  template<class ExecutionPolicy,
           class ForwardIter,
           class Size,
           class NoThrowForwardIter>
  NoThrowForwardIter uninitialized_copy_n(ExecutionPolicy&& exec, // freestanding-deleted
                                          ForwardIter first,
                                          Size n,
                                          NoThrowForwardIter result);

  namespace ranges {
    template<class I, class O>
    using uninitialized_copy_result = in_out_result<I, O>; // freestanding
    template<input_iterator I,
             sentinel_for<I> S1,
             no-throw-forward-iterator O,
             no-throw-sentinel-for<O> S2>
      requires constructible_from<iter_value_t<O>, iter_reference_t<I>>
    constexpr uninitialized_copy_result<I, O>
    uninitialized_copy(I ifirst, S1 ilast, O ofirst, S2 olast); // freestanding
    template<input_range IR, no-throw-forward-range OR>
      requires constructible_from<range_value_t<OR>, range_reference_t<IR>>
    constexpr uninitialized_copy_result<borrowed_iterator_t<IR>, borrowed_iterator_t<OR>>
    uninitialized_copy(IR&& in_range, OR&& out_range); // freestanding

    template<class I, class O>
    using uninitialized_copy_n_result = in_out_result<I, O>; // freestanding
    template<input_iterator I,
             no-throw-forward-iterator O,
             no-throw-sentinel-for<O> S>
      requires constructible_from<iter_value_t<O>, iter_reference_t<I>>
    constexpr uninitialized_copy_n_result<I, O> uninitialized_copy_n(
      I ifirst,
      iter_difference_t<I> n, // freestanding
      O ofirst,
      S olast);

    template<execution-policy Ep, random_access_iterator I, sized_sentinel_for<I> S1,
             nothrow-random-access-iterator O, nothrow-sized-sentinel-for <O> S2>
      requires constructible_from<iter_value_t<O>, iter_reference_t<I>>
    uninitialized_copy_result<I, O>
      uninitialized_copy(Ep&& exec, I ifirst, S1 ilast, // freestanding-deleted
                         O ofirst, S2 olast);
    template<execution-policy Ep, sized-random-access-range IR,
             nothrow-sized-random-access-range OR>
      requires constructible_from<range_value_t<OR>, range_reference_t<IR>>
    uninitialized_copy_result<borrowed_iterator_t<IR>, borrowed_iterator_t<OR>>
      uninitialized_copy(Ep&& exec, IR&& in_range, OR&& out_range); // freestanding-deleted

    template<execution-policy Ep, random_access_iterator I,
             nothrow-random-access-iterator O, nothrow-sized-sentinel-for <O> S>
      requires constructible_from<iter_value_t<O>, iter_reference_t<I>>
    uninitialized_copy_n_result<I, O>
      uninitialized_copy_n(Ep&& exec, I ifirst, // freestanding-deleted
                           iter_difference_t<I> n, O ofirst, S olast);
  }

  template<class InputIter, class NoThrowForwardIter>
  constexpr NoThrowForwardIter uninitialized_move(InputIter first, // freestanding
                                                  InputIter last,
                                                  NoThrowForwardIter result);
  template<class ExecutionPolicy, class ForwardIter, class NoThrowForwardIter>
  NoThrowForwardIter uninitialized_move(ExecutionPolicy&& exec, // freestanding-deleted
                                        ForwardIter first,
                                        ForwardIter last,
                                        NoThrowForwardIter result);

  template<class InputIter, class Size, class NoThrowForwardIter>
  constexpr pair<InputIter, NoThrowForwardIter> uninitialized_move_n(
    InputIter first,
    Size n, // freestanding
    NoThrowForwardIter result);
  template<class ExecutionPolicy, class ForwardIter, class Size, class NoThrowForwardIter>
  pair<ForwardIter, NoThrowForwardIter> uninitialized_move_n(
    ExecutionPolicy&& exec, // freestanding-deleted
    ForwardIter first,
    Size n,
    NoThrowForwardIter result);

  namespace ranges {
    template<class I, class O>
    using uninitialized_move_result = in_out_result<I, O>; // freestanding
    template<input_iterator I,
             sentinel_for<I> S1,
             no-throw-forward-iterator O,
             no-throw-sentinel-for<O> S2>
      requires constructible_from<iter_value_t<O>, iter_rvalue_reference_t<I>>
    constexpr uninitialized_move_result<I, O>
    uninitialized_move(I ifirst, S1 ilast, O ofirst, S2 olast); // freestanding
    template<input_range IR, no-throw-forward-range OR>
      requires constructible_from<range_value_t<OR>, range_rvalue_reference_t<IR>>
    constexpr uninitialized_move_result<borrowed_iterator_t<IR>, borrowed_iterator_t<OR>>
    uninitialized_move(IR&& in_range, OR&& out_range); // freestanding

    template<class I, class O>
    using uninitialized_move_n_result = in_out_result<I, O>; // freestanding
    template<input_iterator I,
             no-throw-forward-iterator O,
             no-throw-sentinel-for<O> S>
      requires constructible_from<iter_value_t<O>, iter_rvalue_reference_t<I>>
    constexpr uninitialized_move_n_result<I, O> uninitialized_move_n(
      I ifirst,
      iter_difference_t<I> n, // freestanding
      O ofirst,
      S olast);

    template<execution-policy Ep, random_access_iterator I, sized_sentinel_for<I> S1,
             nothrow-random-access-iterator O, nothrow-sized-sentinel-for <O> S2>
      requires constructible_from<iter_value_t<O>, iter_rvalue_reference_t<I>>
    uninitialized_move_result<I, O>
      uninitialized_move(Ep&& exec, I ifirst, S1 ilast, // freestanding-deleted
                         O ofirst, S2 olast);
    template<execution-policy Ep, sized-random-access-range IR,
             nothrow-sized-random-access-range OR>
      requires constructible_from<range_value_t<OR>, range_rvalue_reference_t<IR>>
    uninitialized_move_result<borrowed_iterator_t<IR>, borrowed_iterator_t<OR>>
      uninitialized_move(Ep&& exec, IR&& in_range, OR&& out_range); // freestanding-deleted

    template<execution-policy Ep, random_access_iterator I,
             nothrow-random-access-iterator O, nothrow-sized-sentinel-for <O> S>
      requires constructible_from<iter_value_t<O>, iter_rvalue_reference_t<I>>
    uninitialized_move_n_result<I, O>
      uninitialized_move_n(Ep&& exec, I ifirst, // freestanding-deleted
                           iter_difference_t<I> n, O ofirst, S olast);
  }

  template<class NoThrowForwardIter, class T>
  constexpr void uninitialized_fill(NoThrowForwardIter first, // freestanding
                                    NoThrowForwardIter last,
                                    const T& x);
  template<class ExecutionPolicy, class NoThrowForwardIter, class T>
  void uninitialized_fill(ExecutionPolicy&& exec, // freestanding-deleted
                          NoThrowForwardIter first,
                          NoThrowForwardIter last,
                          const T& x);

  template<class NoThrowForwardIter, class Size, class T>
  constexpr NoThrowForwardIter uninitialized_fill_n(NoThrowForwardIter first,
                                                    Size n,
                                                    const T& x); // freestanding
  template<class ExecutionPolicy, class NoThrowForwardIter, class Size, class T>
  NoThrowForwardIter uninitialized_fill_n(ExecutionPolicy&& exec, // freestanding-deleted
                                          NoThrowForwardIter first,
                                          Size n,
                                          const T& x);

  namespace ranges {
    template<no-throw-forward-iterator I, no-throw-sentinel-for<I> S, class T>
      requires constructible_from<iter_value_t<I>, const T&>
    constexpr I uninitialized_fill(I first, S last, const T& x); // freestanding
    template<no-throw-forward-range R, class T>
      requires constructible_from<range_value_t<R>, const T&>
    constexpr borrowed_iterator_t<R> uninitialized_fill(R&& r,
                                                        const T& x); // freestanding

    template<no-throw-forward-iterator I, class T>
      requires constructible_from<iter_value_t<I>, const T&>
    constexpr I uninitialized_fill_n(I first, // freestanding
                                     iter_difference_t<I> n,
                                     const T& x);

    template<execution-policy Ep, nothrow-random-access-iterator I,
             nothrow-sized-sentinel-for <I> S, class T = iter_value_t<I>>
      requires constructible_from<iter_value_t<I>, const T&>
    I uninitialized_fill(Ep&& exec, I first, S last, const T& x); // freestanding-deleted
    template<execution-policy Ep, nothrow-sized-random-access-range R,
             class T = range_value_t<R>>
      requires constructible_from<range_value_t<R>, const T&>
    borrowed_iterator_t<R> uninitialized_fill(Ep&& exec, R&& r, // freestanding-deleted
                                              const T& x);

    template<execution-policy Ep, nothrow-random-access-iterator I,
             class T = iter_value_t<I>>
      requires constructible_from<iter_value_t<I>, const T&>
    I uninitialized_fill_n(Ep&& exec, I first, // freestanding-deleted
                           iter_difference_t<I> n, const T& x);
  }

  // construct_at
  template<class T, class... Args>
  constexpr T* construct_at(T* location, Args&&... args); // freestanding

  namespace ranges {
    template<class T, class... Args>
    constexpr T* construct_at(T* location, Args&&... args); // freestanding
  }

  // destroy
  template<class T>
  constexpr void destroy_at(T* location); // freestanding
  template<class NoThrowForwardIter>
  constexpr void destroy(NoThrowForwardIter first, // freestanding
                         NoThrowForwardIter last);
  template<class ExecutionPolicy, class NoThrowForwardIter>
  void destroy(ExecutionPolicy&& exec, // freestanding-deleted
               NoThrowForwardIter first,
               NoThrowForwardIter last);
  template<class NoThrowForwardIter, class Size>
  constexpr NoThrowForwardIter destroy_n(NoThrowForwardIter first, // freestanding
                                         Size n);
  template<class ExecutionPolicy, class NoThrowForwardIter, class Size>
  NoThrowForwardIter destroy_n(ExecutionPolicy&& exec, // freestanding-deleted
                               NoThrowForwardIter first,
                               Size n);

  namespace ranges {
    template<destructible T>
    constexpr void destroy_at(T* location) noexcept; // freestanding

    template<no-throw-input-iterator I, no-throw-sentinel-for<I> S>
      requires destructible<iter_value_t<I>>
    constexpr I destroy(I first, S last) noexcept; // freestanding
    template<no-throw-input-range R>
      requires destructible<range_value_t<R>>
    constexpr borrowed_iterator_t<R> destroy(R&& r) noexcept; // freestanding

    template<no-throw-input-iterator I>
      requires destructible<iter_value_t<I>>
    constexpr I destroy_n(I first, iter_difference_t<I> n) noexcept; // freestanding

    template<execution-policy Ep, nothrow-random-access-iterator I,
             nothrow-sized-sentinel-for <I> S>
      requires destructible<iter_value_t<I>>
    I destroy(Ep&& exec, I first, S last); // freestanding-deleted
    template<execution-policy Ep, nothrow-sized-random-access-range R>
      requires destructible<range_value_t<R>>
    borrowed_iterator_t<R> destroy(Ep&& exec, R&& r); // freestanding-deleted

    template<execution-policy Ep, nothrow-random-access-iterator I>
      requires destructible<iter_value_t<I>>
    I destroy_n(Ep&& exec, I first, iter_difference_t<I> n); // freestanding-deleted
  }

  // class template unique_ptr
  template<class T>
  struct default_delete; // freestanding
  template<class T>
  struct default_delete<T[]>; // freestanding
  template<class T, class D = default_delete<T>>
  class unique_ptr; // freestanding
  template<class T, class D>
  class unique_ptr<T[], D>; // freestanding

  template<class T, class... Args>
  constexpr unique_ptr<T> make_unique(Args&&... args); // T is not array
  template<class T>
  constexpr unique_ptr<T> make_unique(size_t n); // T is U[]
  template<class T, class... Args>
  /* unspecified */ make_unique(Args&&...) = delete; // T is U[N]

  template<class T>
  constexpr unique_ptr<T> make_unique_for_overwrite(); // T is not array
  template<class T>
  constexpr unique_ptr<T> make_unique_for_overwrite(size_t n); // T is U[]
  template<class T, class... Args>
  /* unspecified */ make_unique_for_overwrite(Args&&...) = delete; // T is U[N]

  template<class T, class D>
  constexpr void swap(unique_ptr<T, D>& x, unique_ptr<T, D>& y) noexcept; // freestanding

  template<class T1, class D1, class T2, class D2>
  constexpr bool operator==(const unique_ptr<T1, D1>& x, // freestanding
                            const unique_ptr<T2, D2>& y);
  template<class T1, class D1, class T2, class D2>
  constexpr bool operator<(const unique_ptr<T1, D1>& x,
                           const unique_ptr<T2, D2>& y); // freestanding
  template<class T1, class D1, class T2, class D2>
  constexpr bool operator>(const unique_ptr<T1, D1>& x,
                           const unique_ptr<T2, D2>& y); // freestanding
  template<class T1, class D1, class T2, class D2>
  constexpr bool operator<=(const unique_ptr<T1, D1>& x,
                            const unique_ptr<T2, D2>& y); // freestanding
  template<class T1, class D1, class T2, class D2>
  constexpr bool operator>=(const unique_ptr<T1, D1>& x,
                            const unique_ptr<T2, D2>& y); // freestanding
  template<class T1, class D1, class T2, class D2>
    requires three_way_comparable_with<typename unique_ptr<T1, D1>::pointer,
                                       typename unique_ptr<T2, D2>::pointer>
  constexpr compare_three_way_result_t<typename unique_ptr<T1, D1>::pointer,
                                       typename unique_ptr<T2, D2>::pointer>
  operator<=>(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y); // freestanding

  template<class T, class D>
  constexpr bool operator==(const unique_ptr<T, D>& x,
                            nullptr_t) noexcept; // freestanding
  template<class T, class D>
  constexpr bool operator<(const unique_ptr<T, D>& x, nullptr_t); // freestanding
  template<class T, class D>
  constexpr bool operator<(nullptr_t, const unique_ptr<T, D>& y); // freestanding
  template<class T, class D>
  constexpr bool operator>(const unique_ptr<T, D>& x, nullptr_t); // freestanding
  template<class T, class D>
  constexpr bool operator>(nullptr_t, const unique_ptr<T, D>& y); // freestanding
  template<class T, class D>
  constexpr bool operator<=(const unique_ptr<T, D>& x, nullptr_t); // freestanding
  template<class T, class D>
  constexpr bool operator<=(nullptr_t, const unique_ptr<T, D>& y); // freestanding
  template<class T, class D>
  constexpr bool operator>=(const unique_ptr<T, D>& x, nullptr_t); // freestanding
  template<class T, class D>
  constexpr bool operator>=(nullptr_t, const unique_ptr<T, D>& y); // freestanding
  template<class T, class D>
    requires three_way_comparable<typename unique_ptr<T, D>::pointer>
  constexpr compare_three_way_result_t<typename unique_ptr<T, D>::pointer> operator<=>(
    const unique_ptr<T, D>& x,
    nullptr_t); // freestanding

  template<class E, class T, class Y, class D>
  basic_ostream<E, T>& operator<<(basic_ostream<E, T>& os, const unique_ptr<Y, D>& p);

  // class bad_weak_ptr
  class bad_weak_ptr;

  // class template shared_ptr
  template<class T>
  class shared_ptr;

  // shared_ptr creation
  template<class T, class... Args>
  constexpr shared_ptr<T> make_shared(Args&&... args); // T is not array
  template<class T, class A, class... Args>
  sconstexpr hared_ptr<T> allocate_shared(const A& a, Args&&... args); // T is not array

  template<class T>
  constexpr shared_ptr<T> make_shared(size_t N); // T is U[]
  template<class T, class A>
  constexpr shared_ptr<T> allocate_shared(const A& a, size_t N); // T is U[]

  template<class T>
  constexpr shared_ptr<T> make_shared(); // T is U[N]
  template<class T, class A>
  constexpr shared_ptr<T> allocate_shared(const A& a); // T is U[N]

  template<class T>
  constexpr shared_ptr<T> make_shared(size_t N, const remove_extent_t<T>& u); // T is U[]
  template<class T, class A>
  constexpr shared_ptr<T> allocate_shared(const A& a,
                                          size_t N,
                                          const remove_extent_t<T>& u); // T is U[]

  template<class T>
  constexpr shared_ptr<T> make_shared(const remove_extent_t<T>& u); // T is U[N]
  template<class T, class A>
  constexpr shared_ptr<T> allocate_shared(const A& a,
                                          const remove_extent_t<T>& u); // T is U[N]

  template<class T>
  constexpr shared_ptr<T> make_shared_for_overwrite(); // T is not U[]
  template<class T, class A>
  constexpr shared_ptr<T> allocate_shared_for_overwrite(const A& a); // T is not U[]

  template<class T>
  constexpr shared_ptr<T> make_shared_for_overwrite(size_t N); // T is U[]
  template<class T, class A>
  constexpr shared_ptr<T> allocate_shared_for_overwrite(const A& a, size_t N); // T is U[]

  // shared_ptr comparisons
  template<class T, class U>
  constexpr bool operator==(const shared_ptr<T>& a, const shared_ptr<U>& b) noexcept;
  template<class T, class U>
  constexpr strong_ordering operator<=>(const shared_ptr<T>& a,
                                        const shared_ptr<U>& b) noexcept;

  template<class T>
  constexpr bool operator==(const shared_ptr<T>& x, nullptr_t) noexcept;
  template<class T>
  constexpr strong_ordering operator<=>(const shared_ptr<T>& x, nullptr_t) noexcept;

  // shared_ptr specialized algorithms
  template<class T>
  constexpr void swap(shared_ptr<T>& a, shared_ptr<T>& b) noexcept;

  // shared_ptr casts
  template<class T, class U>
  constexpr shared_ptr<T> static_pointer_cast(const shared_ptr<U>& r) noexcept;
  template<class T, class U>
  constexpr shared_ptr<T> static_pointer_cast(shared_ptr<U>&& r) noexcept;
  template<class T, class U>
  constexpr shared_ptr<T> dynamic_pointer_cast(const shared_ptr<U>& r) noexcept;
  template<class T, class U>
  constexpr shared_ptr<T> dynamic_pointer_cast(shared_ptr<U>&& r) noexcept;
  template<class T, class U>
  constexpr shared_ptr<T> const_pointer_cast(const shared_ptr<U>& r) noexcept;
  template<class T, class U>
  constexpr shared_ptr<T> const_pointer_cast(shared_ptr<U>&& r) noexcept;
  template<class T, class U>
  shared_ptr<T> reinterpret_pointer_cast(const shared_ptr<U>& r) noexcept;
  template<class T, class U>
  shared_ptr<T> reinterpret_pointer_cast(shared_ptr<U>&& r) noexcept;

  // shared_ptr get_deleter
  template<class D, class T>
  constexpr D* get_deleter(const shared_ptr<T>& p) noexcept;

  // shared_ptr I/O
  template<class E, class T, class Y>
  basic_ostream<E, T>& operator<<(basic_ostream<E, T>& os, const shared_ptr<Y>& p);

  // class template weak_ptr
  template<class T>
  class weak_ptr;

  // weak_ptr specialized algorithms
  template<class T>
  constexpr void swap(weak_ptr<T>& a, weak_ptr<T>& b) noexcept;

  // class template owner_less
  template<class T = void>
  struct owner_less;

  // struct owner_hash
  struct owner_hash;

  // struct owner_equal
  struct owner_equal;

  // class template enable_shared_from_this
  template<class T>
  class enable_shared_from_this;

  // hash support
  template<class T>
  struct hash; // freestanding
  template<class T, class D>
  struct hash<unique_ptr<T, D>>; // freestanding
  template<class T>
  struct hash<shared_ptr<T>>;

  // atomic smart pointers
  template<class T>
  struct atomic; // freestanding
  template<class T>
  struct atomic<shared_ptr<T>>;
  template<class T>
  struct atomic<weak_ptr<T>>;

  // class template out_ptr_t
  template<class Smart, class Pointer, class... Args>
  class out_ptr_t; // freestanding

  // function template out_ptr
  template<class Pointer = void, class Smart, class... Args>
  constexpr auto out_ptr(Smart& s, Args&&... args); // freestanding

  // class template inout_ptr_t
  template<class Smart, class Pointer, class... Args>
  class inout_ptr_t; // freestanding

  // function template inout_ptr
  template<class Pointer = void, class Smart, class... Args>
  constexpr auto inout_ptr(Smart& s, Args&&... args); // freestanding

  // class template indirect
  template<class T, class Allocator = allocator<T>>
  class indirect;

  // hash support
  template<class T, class Alloc>
  struct hash<indirect<T, Alloc>>;

  // class template polymorphic
  template<class T, class Allocator = allocator<T>>
  class polymorphic;

  namespace pmr {
    template<class T>
    using indirect = indirect<T, polymorphic_allocator<T>>;
    template<class T>
    using polymorphic = polymorphic<T, polymorphic_allocator<T>>;
  }
}

namespace std {
  template<class Ptr>
  struct pointer_traits
  {
    /* see description */;
  };

  template<class T>
  struct pointer_traits<T*>
  {
    using pointer         = T*;
    using element_type    = T;
    using difference_type = ptrdiff_t;

    template<class U>
    using rebind = U*;

    static constexpr pointer pointer_to(/* see description */ r) noexcept;
  };
}

namespace std {
  struct allocator_arg_t { explicit allocator_arg_t() = default; };
  inline constexpr allocator_arg_t allocator_arg{};
}

namespace std {
  template<class Alloc>
  struct allocator_traits
  {
    using allocator_type                         = Alloc;

    using value_type                             = typename Alloc::value_type;

    using pointer                                = /* see description */;
    using const_pointer                          = /* see description */;
    using void_pointer                           = /* see description */;
    using const_void_pointer                     = /* see description */;

    using difference_type                        = /* see description */;
    using size_type                              = /* see description */;

    using propagate_on_container_copy_assignment = /* see description */;
    using propagate_on_container_move_assignment = /* see description */;
    using propagate_on_container_swap            = /* see description */;
    using is_always_equal                        = /* see description */;

    template<class T>
    using rebind_alloc = /* see description */;
    template<class T>
    using rebind_traits = allocator_traits<rebind_alloc<T>>;

    static constexpr pointer allocate(Alloc& a, size_type n);
    static constexpr pointer allocate(Alloc& a, size_type n, const_void_pointer hint);
    static constexpr allocation_result<pointer, size_type> allocate_at_least(Alloc& a,
                                                                             size_type n);

    static constexpr void deallocate(Alloc& a, pointer p, size_type n);

    template<class T, class... Args>
    static constexpr void construct(Alloc& a, T* p, Args&&... args);

    template<class T>
    static constexpr void destroy(Alloc& a, T* p);

    static constexpr size_type max_size(const Alloc& a) noexcept;

    static constexpr Alloc select_on_container_copy_construction(const Alloc& rhs);
  };
}

namespace std {
  template<class T>
  class allocator
  {
  public:
    using value_type                             = T;
    using size_type                              = size_t;
    using difference_type                        = ptrdiff_t;
    using propagate_on_container_move_assignment = true_type;

    constexpr allocator() noexcept;
    constexpr allocator(const allocator&) noexcept;
    template<class U>
    constexpr allocator(const allocator<U>&) noexcept;
    constexpr ~allocator();
    constexpr allocator& operator=(const allocator&) = default;

    constexpr T* allocate(size_t n);
    constexpr allocation_result<T*> allocate_at_least(size_t n);
    constexpr void deallocate(T* p, size_t n);
  };
}

namespace std {
  template<class T>
  struct default_delete
  {
    constexpr default_delete() noexcept = default;
    template<class U>
    constexpr default_delete(const default_delete<U>&) noexcept;
    constexpr void operator()(T*) const;
  };

  template<class T>
  struct default_delete<T[]>
  {
    constexpr default_delete() noexcept = default;
    template<class U>
    constexpr default_delete(const default_delete<U[]>&) noexcept;
    template<class U>
    constexpr void operator()(U* ptr) const;
  };
}

namespace std {
  template<class T, class D = default_delete<T>>
  class unique_ptr
  {
  public:
    using pointer      = /* see description */;
    using element_type = T;
    using deleter_type = D;

    // constructors
    constexpr unique_ptr() noexcept;
    constexpr explicit unique_ptr(type_identity_t<pointer> p) noexcept;
    constexpr unique_ptr(type_identity_t<pointer> p, /* see description */ d1) noexcept;
    constexpr unique_ptr(type_identity_t<pointer> p, /* see description */ d2) noexcept;
    constexpr unique_ptr(unique_ptr&& u) noexcept;
    constexpr unique_ptr(nullptr_t) noexcept;
    template<class U, class E>
    constexpr unique_ptr(unique_ptr<U, E>&& u) noexcept;

    // destructor
    constexpr ~unique_ptr();

    // assignment
    constexpr unique_ptr& operator=(unique_ptr&& u) noexcept;
    template<class U, class E>
    constexpr unique_ptr& operator=(unique_ptr<U, E>&& u) noexcept;
    constexpr unique_ptr& operator=(nullptr_t) noexcept;

    // observers
    constexpr add_lvalue_reference_t<T> operator*() const noexcept(/* see description */);
    constexpr pointer operator->() const noexcept;
    constexpr pointer get() const noexcept;
    constexpr deleter_type& get_deleter() noexcept;
    constexpr const deleter_type& get_deleter() const noexcept;
    constexpr explicit operator bool() const noexcept;

    // modifiers
    constexpr pointer release() noexcept;
    constexpr void reset(pointer p = pointer()) noexcept;
    constexpr void swap(unique_ptr& u) noexcept;

    // disable copy from lvalue
    unique_ptr(const unique_ptr&)            = delete;
    unique_ptr& operator=(const unique_ptr&) = delete;
  };

  template<class T, class D>
  class unique_ptr<T[], D>
  {
  public:
    using pointer      = /* see description */;
    using element_type = T;
    using deleter_type = D;

    // constructors
    constexpr unique_ptr() noexcept;
    template<class U>
    constexpr explicit unique_ptr(U p) noexcept;
    template<class U>
    constexpr unique_ptr(U p, /* see description */ d) noexcept;
    template<class U>
    constexpr unique_ptr(U p, /* see description */ d) noexcept;
    constexpr unique_ptr(unique_ptr&& u) noexcept;
    template<class U, class E>
    constexpr unique_ptr(unique_ptr<U, E>&& u) noexcept;
    constexpr unique_ptr(nullptr_t) noexcept;

    // destructor
    constexpr ~unique_ptr();

    // assignment
    constexpr unique_ptr& operator=(unique_ptr&& u) noexcept;
    template<class U, class E>
    constexpr unique_ptr& operator=(unique_ptr<U, E>&& u) noexcept;
    constexpr unique_ptr& operator=(nullptr_t) noexcept;

    // observers
    constexpr T& operator[](size_t i) const;
    constexpr pointer get() const noexcept;
    constexpr deleter_type& get_deleter() noexcept;
    constexpr const deleter_type& get_deleter() const noexcept;
    constexpr explicit operator bool() const noexcept;

    // modifiers
    constexpr pointer release() noexcept;
    template<class U>
    constexpr void reset(U p) noexcept;
    constexpr void reset(nullptr_t = nullptr) noexcept;
    constexpr void swap(unique_ptr& u) noexcept;

    // disable copy from lvalue
    unique_ptr(const unique_ptr&)            = delete;
    unique_ptr& operator=(const unique_ptr&) = delete;
  };
}

namespace std {
  class bad_weak_ptr : public exception
  {
  public:
    // for the specification of the special member functions
    constexpr const char* what() const noexcept override;
  };
}

namespace std {
  template<class T>
  class shared_ptr
  {
  public:
    using element_type = remove_extent_t<T>;
    using weak_type    = weak_ptr<T>;

    // constructors
    constexpr shared_ptr() noexcept;
    constexpr shared_ptr(nullptr_t) noexcept
      : shared_ptr()
    {
    }
    template<class Y>
    constexpr explicit shared_ptr(Y* p);
    template<class Y, class D>
    constexpr shared_ptr(Y* p, D d);
    template<class Y, class D, class A>
    constexpr shared_ptr(Y* p, D d, A a);
    template<class D>
    constexpr shared_ptr(nullptr_t p, D d);
    template<class D, class A>
    constexpr shared_ptr(nullptr_t p, D d, A a);
    template<class Y>
    constexpr shared_ptr(const shared_ptr<Y>& r, element_type* p) noexcept;
    template<class Y>
    constexpr shared_ptr(shared_ptr<Y>&& r, element_type* p) noexcept;
    constexpr shared_ptr(const shared_ptr& r) noexcept;
    template<class Y>
    constexpr shared_ptr(const shared_ptr<Y>& r) noexcept;
    constexpr shared_ptr(shared_ptr&& r) noexcept;
    template<class Y>
    constexpr shared_ptr(shared_ptr<Y>&& r) noexcept;
    template<class Y>
    constexpr explicit shared_ptr(const weak_ptr<Y>& r);
    template<class Y, class D>
    constexpr shared_ptr(unique_ptr<Y, D>&& r);

    // destructor
    constexpr ~shared_ptr();

    // assignment
    constexpr shared_ptr& operator=(const shared_ptr& r) noexcept;
    template<class Y>
    constexpr shared_ptr& operator=(const shared_ptr<Y>& r) noexcept;
    constexpr shared_ptr& operator=(shared_ptr&& r) noexcept;
    template<class Y>
    constexpr shared_ptr& operator=(shared_ptr<Y>&& r) noexcept;
    template<class Y, class D>
    constexpr shared_ptr& operator=(unique_ptr<Y, D>&& r);

    // modifiers
    constexpr void swap(shared_ptr& r) noexcept;
    constexpr void reset() noexcept;
    template<class Y>
    constexpr void reset(Y* p);
    template<class Y, class D>
    constexpr void reset(Y* p, D d);
    template<class Y, class D, class A>
    constexpr void reset(Y* p, D d, A a);

    // observers
    constexpr element_type* get() const noexcept;
    constexpr T& operator*() const noexcept;
    constexpr T* operator->() const noexcept;
    constexpr element_type& operator[](ptrdiff_t i) const;
    constexpr long use_count() const noexcept;
    constexpr explicit operator bool() const noexcept;
    template<class U>
    constexpr bool owner_before(const shared_ptr<U>& b) const noexcept;
    template<class U>
    constexpr bool owner_before(const weak_ptr<U>& b) const noexcept;
    size_t owner_hash() const noexcept;
    template<class U>
    constexpr bool owner_equal(const shared_ptr<U>& b) const noexcept;
    template<class U>
    constexpr bool owner_equal(const weak_ptr<U>& b) const noexcept;
  };

  template<class T>
  shared_ptr(weak_ptr<T>) -> shared_ptr<T>;
  template<class T, class D>
  shared_ptr(unique_ptr<T, D>) -> shared_ptr<T>;
}

namespace std {
  template<class T>
  class weak_ptr
  {
  public:
    using element_type = remove_extent_t<T>;

    // constructors
    constexpr weak_ptr() noexcept;
    template<class Y>
    constexpr weak_ptr(const shared_ptr<Y>& r) noexcept;
    constexpr weak_ptr(const weak_ptr& r) noexcept;
    template<class Y>
    constexpr weak_ptr(const weak_ptr<Y>& r) noexcept;
    constexpr weak_ptr(weak_ptr&& r) noexcept;
    template<class Y>
    constexpr weak_ptr(weak_ptr<Y>&& r) noexcept;

    // destructor
    constexpr ~weak_ptr();

    // assignment
    constexpr weak_ptr& operator=(const weak_ptr& r) noexcept;
    template<class Y>
    constexpr weak_ptr& operator=(const weak_ptr<Y>& r) noexcept;
    template<class Y>
    constexpr weak_ptr& operator=(const shared_ptr<Y>& r) noexcept;
    constexpr weak_ptr& operator=(weak_ptr&& r) noexcept;
    template<class Y>
    constexpr weak_ptr& operator=(weak_ptr<Y>&& r) noexcept;

    // modifiers
    constexpr void swap(weak_ptr& r) noexcept;
    constexpr void reset() noexcept;

    // observers
    constexpr long use_count() const noexcept;
    constexpr bool expired() const noexcept;
    constexpr shared_ptr<T> lock() const noexcept;
    template<class U>
    constexpr bool owner_before(const shared_ptr<U>& b) const noexcept;
    template<class U>
    constexpr bool owner_before(const weak_ptr<U>& b) const noexcept;
    size_t owner_hash() const noexcept;
    template<class U>
    constexpr bool owner_equal(const shared_ptr<U>& b) const noexcept;
    template<class U>
    constexpr bool owner_equal(const weak_ptr<U>& b) const noexcept;
  };

  template<class T>
  weak_ptr(shared_ptr<T>) -> weak_ptr<T>;
}

namespace std {
  template<class T = void>
  struct owner_less;

  template<class T>
  struct owner_less<shared_ptr<T>>
  {
    constexpr bool operator()(const shared_ptr<T>&,
                              const shared_ptr<T>&) const noexcept;
    constexpr bool operator()(const shared_ptr<T>&,
                              const weak_ptr<T>&) const noexcept;
    constexpr bool operator()(const weak_ptr<T>&,
                              const shared_ptr<T>&) const noexcept;
  };

  template<class T>
  struct owner_less<weak_ptr<T>>
  {
    constexpr bool operator()(const weak_ptr<T>&,
                              const weak_ptr<T>&) const noexcept;
    constexpr bool operator()(const shared_ptr<T>&,
                              const weak_ptr<T>&) const noexcept;
    constexpr bool operator()(const weak_ptr<T>&,
                              const shared_ptr<T>&) const noexcept;
  };

  template<>
  struct owner_less<void>
  {
    template<class T, class U>
    constexpr bool operator()(const shared_ptr<T>&,
                              const shared_ptr<U>&) const noexcept;
    template<class T, class U>
    constexpr bool operator()(const shared_ptr<T>&,
                              const weak_ptr<U>&) const noexcept;
    template<class T, class U>
    constexpr bool operator()(const weak_ptr<T>&,
                              const shared_ptr<U>&) const noexcept;
    template<class T, class U>
    constexpr bool operator()(const weak_ptr<T>&,
                              const weak_ptr<U>&) const noexcept;

    using is_transparent = /* unspecified */;
  };
}

namespace std {
  struct owner_hash
  {
    template<class T>
    size_t operator()(const shared_ptr<T>&) const noexcept;

    template<class T>
    size_t operator()(const weak_ptr<T>&) const noexcept;

    using is_transparent = /* unspecified */;
  };
}

namespace std {
  struct owner_equal
  {
    template<class T, class U>
    constexpr bool operator()(const shared_ptr<T>&,
                              const shared_ptr<U>&) const noexcept;
    template<class T, class U>
    constexpr bool operator()(const shared_ptr<T>&,
                              const weak_ptr<U>&) const noexcept;
    template<class T, class U>
    constexpr bool operator()(const weak_ptr<T>&,
                              const shared_ptr<U>&) const noexcept;
    template<class T, class U>
    constexpr bool operator()(const weak_ptr<T>&,
                              const weak_ptr<U>&) const noexcept;

    using is_transparent = /* unspecified */;
  };
}

namespace std {
  template<class T>
  class enable_shared_from_this
  {
  protected:
    constexpr enable_shared_from_this() noexcept;
    constexpr enable_shared_from_this(const enable_shared_from_this&) noexcept;
    constexpr enable_shared_from_this&
      operator=(const enable_shared_from_this&) noexcept;
    constexpr ~enable_shared_from_this();

  public:
    constexpr shared_ptr<T> shared_from_this();
    constexpr shared_ptr<T const> shared_from_this() const;
    constexpr weak_ptr<T> weak_from_this() noexcept;
    constexpr weak_ptr<T const> weak_from_this() const noexcept;

  private:
    mutable weak_ptr<T> /*weak-this*/; // exposition-only
  };
}

namespace std {
  template<class T> struct atomic<shared_ptr<T>> {
    using value_type = shared_ptr<T>;

    static constexpr bool is_always_lock_free = /* implementation-defined */;
    bool is_lock_free() const noexcept;

    constexpr atomic() noexcept = default;
    constexpr atomic(nullptr_t) noexcept : atomic() {}
    constexpr atomic(shared_ptr<T> desired) noexcept;
    atomic(const atomic&) = delete;
    void operator=(const atomic&) = delete;

    constexpr shared_ptr<T>
      load(memory_order order = memory_order::seq_cst) const noexcept;
    constexpr operator shared_ptr<T>() const noexcept;
    constexpr void store(shared_ptr<T> desired,
                         memory_order order = memory_order::seq_cst) noexcept;
    constexpr void operator=(shared_ptr<T> desired) noexcept;
    constexpr void operator=(nullptr_t) noexcept;

    constexpr shared_ptr<T>
      exchange(shared_ptr<T> desired,
               memory_order order = memory_order::seq_cst) noexcept;

    constexpr bool
      compare_exchange_weak(shared_ptr<T>& expected, shared_ptr<T> desired,
                            memory_order success, memory_order failure) noexcept;
    constexpr bool
      compare_exchange_strong(shared_ptr<T>& expected, shared_ptr<T> desired,
                              memory_order success, memory_order failure) noexcept;

    constexpr bool
      compare_exchange_weak(shared_ptr<T>& expected, shared_ptr<T> desired,
                            memory_order order = memory_order::seq_cst) noexcept;
    constexpr bool
      compare_exchange_strong(shared_ptr<T>& expected, shared_ptr<T> desired,
                              memory_order order = memory_order::seq_cst) noexcept;

    constexpr void wait(shared_ptr<T> old,
                        memory_order order = memory_order::seq_cst) const noexcept;
    constexpr void notify_one() noexcept;
    constexpr void notify_all() noexcept;

  private:
    shared_ptr<T> p;            // exposition only
  };
}

namespace std {
  template<class T> struct atomic<weak_ptr<T>> {
    using value_type = weak_ptr<T>;
    static constexpr bool is_always_lock_free = /* implementation-defined */;
    bool is_lock_free() const noexcept;

    constexpr atomic() noexcept = default;
    constexpr atomic(weak_ptr<T> desired) noexcept;
    atomic(const atomic&) = delete;
    void operator=(const atomic&) = delete;

    constexpr weak_ptr<T>
      load(memory_order order = memory_order::seq_cst) const noexcept;
    constexpr operator weak_ptr<T>() const noexcept;
    constexpr void store(weak_ptr<T> desired,
                         memory_order order = memory_order::seq_cst) noexcept;
    constexpr void operator=(weak_ptr<T> desired) noexcept;

    constexpr weak_ptr<T>
      exchange(weak_ptr<T> desired,
               memory_order order = memory_order::seq_cst) noexcept;

    constexpr bool
      compare_exchange_weak(weak_ptr<T>& expected, shared_ptr<T> desired,
                            memory_order success, memory_order failure) noexcept;
    constexpr bool
      compare_exchange_strong(weak_ptr<T>& expected, shared_ptr<T> desired,
                              memory_order success, memory_order failure) noexcept;

    constexpr bool
      compare_exchange_weak(weak_ptr<T>& expected, shared_ptr<T> desired,
                            memory_order order = memory_order::seq_cst) noexcept;
    constexpr bool
      compare_exchange_strong(weak_ptr<T>& expected, shared_ptr<T> desired,
                              memory_order order = memory_order::seq_cst) noexcept;

    constexpr void wait(weak_ptr<T> old,
                        memory_order order = memory_order::seq_cst) const noexcept;
    constexpr void notify_one() noexcept;
    constexpr void notify_all() noexcept;

  private:
    weak_ptr<T> p;              // exposition only
  };
}

namespace std {
  template<class Smart, class Pointer, class... Args>
  class out_ptr_t
  {
  public:
    constexpr explicit out_ptr_t(Smart&, Args...);
    out_ptr_t(const out_ptr_t&) = delete;

    constexpr ~out_ptr_t();

    constexpr operator Pointer*() const noexcept;
    operator void**() const noexcept;

  private:
    Smart& s;         // exposition-only
    tuple<Args...> a; // exposition-only
    Pointer p;        // exposition-only
  };
}

namespace std {
  template<class Smart, class Pointer, class... Args>
  class inout_ptr_t
  {
  public:
    constexpr explicit inout_ptr_t(Smart&, Args...);
    inout_ptr_t(const inout_ptr_t&) = delete;

    constexpr ~inout_ptr_t();

    constexpr operator Pointer*() const noexcept;
    operator void**() const noexcept;

  private:
    Smart& s;         // exposition-only
    tuple<Args...> a; // exposition-only
    Pointer p;        // exposition-only
  };
}

namespace std {
  template<class T, class Allocator = allocator<T>>
  class indirect
  {
  public:
    using value_type     = T;
    using allocator_type = Allocator;
    using pointer        = typename allocator_traits<Allocator>::pointer;
    using const_pointer  = typename allocator_traits<Allocator>::const_pointer;

    // constructors
    constexpr explicit indirect();
    constexpr explicit indirect(allocator_arg_t, const Allocator& a);
    constexpr indirect(const indirect& other);
    constexpr indirect(allocator_arg_t, const Allocator& a, const indirect& other);
    constexpr indirect(indirect&& other) noexcept;
    constexpr indirect(allocator_arg_t,
                       const Allocator& a,
                       indirect&& other) noexcept(/* see description */);
    template<class U = T>
    constexpr explicit indirect(U&& u);
    template<class U = T>
    constexpr explicit indirect(allocator_arg_t, const Allocator& a, U&& u);
    template<class... Us>
    constexpr explicit indirect(in_place_t, Us&&... us);
    template<class... Us>
    constexpr explicit indirect(allocator_arg_t,
                                const Allocator& a,
                                in_place_t,
                                Us&&... us);
    template<class I, class... Us>
    constexpr explicit indirect(in_place_t, initializer_list<I> ilist, Us&&... us);
    template<class I, class... Us>
    constexpr explicit indirect(allocator_arg_t,
                                const Allocator& a,
                                in_place_t,
                                initializer_list<I> ilist,
                                Us&&... us);

    // destructor
    constexpr ~indirect();

    // assignment
    constexpr indirect& operator=(const indirect& other);
    constexpr indirect& operator=(indirect&& other) noexcept(/* see description */);
    template<class U = T>
    constexpr indirect& operator=(U&& u);

    // observers
    constexpr const T& operator*() const& noexcept;
    constexpr T& operator*() & noexcept;
    constexpr const T&& operator*() const&& noexcept;
    constexpr T&& operator*() && noexcept;
    constexpr const_pointer operator->() const noexcept;
    constexpr pointer operator->() noexcept;
    constexpr bool valueless_after_move() const noexcept;
    constexpr allocator_type get_allocator() const noexcept;

    // swap
    constexpr void swap(indirect& other) noexcept(/* see description */);
    friend constexpr void swap(indirect& lhs,
                               indirect& rhs) noexcept(/* see description */);

    // relational operators
    template<class U, class AA>
    friend constexpr bool operator==(
      const indirect& lhs,
      const indirect<U, AA>& rhs) noexcept(/* see description */);
    template<class U, class AA>
    friend constexpr auto operator<=>(const indirect& lhs, const indirect<U, AA>& rhs)
      -> /*synth-three-way-result*/<T, U>;

    // comparison with T
    template<class U>
    friend constexpr bool operator==(const indirect& lhs,
                                     const U& rhs) noexcept(/* see description */);
    template<class U>
    friend constexpr auto operator<=>(const indirect& lhs, const U& rhs)
      -> /*synth-three-way-result*/<T, U>;

  private:
    pointer /*p*/;                     // exposition-only
    Allocator /*alloc*/ = Allocator(); // exposition-only
  };
  template<class Value>
  indirect(Value) -> indirect<Value>;
  template<class Allocator, class Value>
  indirect(allocator_arg_t, Allocator, Value)
    -> indirect<Value,
                typename allocator_traits<Allocator>::template rebind_alloc<Value>>;
}

namespace std {
  template<class T, class Allocator = allocator<T>>
  class polymorphic
  {
  public:
    using value_type     = T;
    using allocator_type = Allocator;
    using pointer        = typename allocator_traits<Allocator>::pointer;
    using const_pointer  = typename allocator_traits<Allocator>::const_pointer;

    // constructors
    constexpr explicit polymorphic();
    constexpr explicit polymorphic(allocator_arg_t, const Allocator& a);
    constexpr polymorphic(const polymorphic& other);
    constexpr polymorphic(allocator_arg_t, const Allocator& a, const polymorphic& other);
    constexpr polymorphic(polymorphic&& other) noexcept;
    constexpr polymorphic(allocator_arg_t,
                          const Allocator& a,
                          polymorphic&& other) noexcept(/* see description */);
    template<class U = T>
    constexpr explicit polymorphic(U&& u);
    template<class U = T>
    constexpr explicit polymorphic(allocator_arg_t, const Allocator& a, U&& u);
    template<class U, class... Ts>
    constexpr explicit polymorphic(in_place_type_t<U>, Ts&&... ts);
    template<class U, class... Ts>
    constexpr explicit polymorphic(allocator_arg_t,
                                   const Allocator& a,
                                   in_place_type_t<U>,
                                   Ts&&... ts);
    template<class U, class I, class... Us>
    constexpr explicit polymorphic(in_place_type_t<U>,
                                   initializer_list<I> ilist,
                                   Us&&... us);
    template<class U, class I, class... Us>
    constexpr explicit polymorphic(allocator_arg_t,
                                   const Allocator& a,
                                   in_place_type_t<U>,
                                   initializer_list<I> ilist,
                                   Us&&... us);

    // destructor
    constexpr ~polymorphic();

    // assignment
    constexpr polymorphic& operator=(const polymorphic& other);
    constexpr polymorphic& operator=(polymorphic&& other) noexcept(/* see description */);

    // observers
    constexpr const T& operator*() const noexcept;
    constexpr T& operator*() noexcept;
    constexpr const_pointer operator->() const noexcept;
    constexpr pointer operator->() noexcept;
    constexpr bool valueless_after_move() const noexcept;
    constexpr allocator_type get_allocator() const noexcept;

    // swap
    constexpr void swap(polymorphic& other) noexcept(/* see description */);
    friend constexpr void swap(polymorphic& lhs,
                               polymorphic& rhs) noexcept(/* see description */);

  private:
    Allocator /*alloc*/ = Allocator(); // exposition-only
  };
}