tvl-depot/absl/memory/memory.h

// Copyright 2017 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
//
//      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.
//
// -----------------------------------------------------------------------------
// File: memory.h
// -----------------------------------------------------------------------------
//
// This header file contains utility functions for managing the creation and
// conversion of smart pointers. This file is an extension to the C++
// standard <memory> library header file.

#ifndef ABSL_MEMORY_MEMORY_H_
#define ABSL_MEMORY_MEMORY_H_

#include <cstddef>
#include <limits>
#include <memory>
#include <new>
#include <type_traits>
#include <utility>

#include "absl/base/macros.h"
#include "absl/meta/type_traits.h"

namespace absl {

// -----------------------------------------------------------------------------
// Function Template: WrapUnique()
// -----------------------------------------------------------------------------
//
//  Adopts ownership from a raw pointer and transfers it to the returned
//  `std::unique_ptr`, whose type is deduced. DO NOT specify the template type T
//  when calling WrapUnique.
//
// Example:
//   X* NewX(int, int);
//   auto x = WrapUnique(NewX(1, 2));  // 'x' is std::unique_ptr<X>.
//
// `absl::WrapUnique` is useful for capturing the output of a raw pointer
// factory. However, prefer 'absl::make_unique<T>(args...) over
// 'absl::WrapUnique(new T(args...))'.
//
//   auto x = WrapUnique(new X(1, 2));  // works, but nonideal.
//   auto x = make_unique<X>(1, 2);     // safer, standard, avoids raw 'new'.
//
// Note that `absl::WrapUnique(p)` is valid only if `delete p` is a valid
// expression. In particular, `absl::WrapUnique()` cannot wrap pointers to
// arrays, functions or void, and it must not be used to capture pointers
// obtained from array-new expressions (even though that would compile!).
template <typename T>
std::unique_ptr<T> WrapUnique(T* ptr) {
  static_assert(!std::is_array<T>::value, "array types are unsupported");
  static_assert(std::is_object<T>::value, "non-object types are unsupported");
  return std::unique_ptr<T>(ptr);
}

namespace memory_internal {

// Traits to select proper overload and return type for `absl::make_unique<>`.
template <typename T>
struct MakeUniqueResult {
  using scalar = std::unique_ptr<T>;
};
template <typename T>
struct MakeUniqueResult<T[]> {
  using array = std::unique_ptr<T[]>;
};
template <typename T, size_t N>
struct MakeUniqueResult<T[N]> {
  using invalid = void;
};

}  // namespace memory_internal

// gcc 4.8 has __cplusplus at 201301 but doesn't define make_unique.  Other
// supported compilers either just define __cplusplus as 201103 but have
// make_unique (msvc), or have make_unique whenever __cplusplus > 201103 (clang)
#if (__cplusplus > 201103L || defined(_MSC_VER)) && \
    !(defined(__GNUC__) && __GNUC__ == 4 && __GNUC_MINOR__ == 8)
using std::make_unique;
#else
// -----------------------------------------------------------------------------
// Function Template: make_unique<T>()
// -----------------------------------------------------------------------------
//
// Creates a `std::unique_ptr<>`, while avoiding issues creating temporaries
// during the construction process. `absl::make_unique<>` also avoids redundant
// type declarations, by avoiding the need to explicitly use the `new` operator.
//
// This implementation of `absl::make_unique<>` is designed for C++11 code and
// will be replaced in C++14 by the equivalent `std::make_unique<>` abstraction.
// `absl::make_unique<>` is designed to be 100% compatible with
// `std::make_unique<>` so that the eventual migration will involve a simple
// rename operation.
//
// For more background on why `std::unique_ptr<T>(new T(a,b))` is problematic,
// see Herb Sutter's explanation on
// (Exception-Safe Function Calls)[http://herbsutter.com/gotw/_102/].
// (In general, reviewers should treat `new T(a,b)` with scrutiny.)
//
// Example usage:
//
//    auto p = make_unique<X>(args...);  // 'p'  is a std::unique_ptr<X>
//    auto pa = make_unique<X[]>(5);     // 'pa' is a std::unique_ptr<X[]>
//
// Three overloads of `absl::make_unique` are required:
//
//   - For non-array T:
//
//       Allocates a T with `new T(std::forward<Args> args...)`,
//       forwarding all `args` to T's constructor.
//       Returns a `std::unique_ptr<T>` owning that object.
//
//   - For an array of unknown bounds T[]:
//
//       `absl::make_unique<>` will allocate an array T of type U[] with
//       `new U[n]()` and return a `std::unique_ptr<U[]>` owning that array.
//
//       Note that 'U[n]()' is different from 'U[n]', and elements will be
//       value-initialized. Note as well that `std::unique_ptr` will perform its
//       own destruction of the array elements upon leaving scope, even though
//       the array [] does not have a default destructor.
//
//       NOTE: an array of unknown bounds T[] may still be (and often will be)
//       initialized to have a size, and will still use this overload. E.g:
//
//         auto my_array = absl::make_unique<int[]>(10);
//
//   - For an array of known bounds T[N]:
//
//       `absl::make_unique<>` is deleted (like with `std::make_unique<>`) as
//       this overload is not useful.
//
//       NOTE: an array of known bounds T[N] is not considered a useful
//       construction, and may cause undefined behavior in templates. E.g:
//
//         auto my_array = absl::make_unique<int[10]>();
//
//       In those cases, of course, you can still use the overload above and
//       simply initialize it to its desired size:
//
//         auto my_array = absl::make_unique<int[]>(10);

// `absl::make_unique` overload for non-array types.
template <typename T, typename... Args>
typename memory_internal::MakeUniqueResult<T>::scalar make_unique(
    Args&&... args) {
  return std::unique_ptr<T>(new T(std::forward<Args>(args)...));
}

// `absl::make_unique` overload for an array T[] of unknown bounds.
// The array allocation needs to use the `new T[size]` form and cannot take
// element constructor arguments. The `std::unique_ptr` will manage destructing
// these array elements.
template <typename T>
typename memory_internal::MakeUniqueResult<T>::array make_unique(size_t n) {
  return std::unique_ptr<T>(new typename absl::remove_extent_t<T>[n]());
}

// `absl::make_unique` overload for an array T[N] of known bounds.
// This construction will be rejected.
template <typename T, typename... Args>
typename memory_internal::MakeUniqueResult<T>::invalid make_unique(
    Args&&... /* args */) = delete;
#endif

// -----------------------------------------------------------------------------
// Function Template: RawPtr()
// -----------------------------------------------------------------------------
//
// Extracts the raw pointer from a pointer-like value `ptr`. `absl::RawPtr` is
// useful within templates that need to handle a complement of raw pointers,
// `std::nullptr_t`, and smart pointers.
template <typename T>
auto RawPtr(T&& ptr) -> decltype(std::addressof(*ptr)) {
  // ptr is a forwarding reference to support Ts with non-const operators.
  return (ptr != nullptr) ? std::addressof(*ptr) : nullptr;
}
inline std::nullptr_t RawPtr(std::nullptr_t) { return nullptr; }

// -----------------------------------------------------------------------------
// Function Template: ShareUniquePtr()
// -----------------------------------------------------------------------------
//
// Adopts a `std::unique_ptr` rvalue and returns a `std::shared_ptr` of deduced
// type. Ownership (if any) of the held value is transferred to the returned
// shared pointer.
//
// Example:
//
//     auto up = absl::make_unique<int>(10);
//     auto sp = absl::ShareUniquePtr(std::move(up));  // shared_ptr<int>
//     CHECK_EQ(*sp, 10);
//     CHECK(up == nullptr);
//
// Note that this conversion is correct even when T is an array type, and more
// generally it works for *any* deleter of the `unique_ptr` (single-object
// deleter, array deleter, or any custom deleter), since the deleter is adopted
// by the shared pointer as well. The deleter is copied (unless it is a
// reference).
//
// Implements the resolution of [LWG 2415](http://wg21.link/lwg2415), by which a
// null shared pointer does not attempt to call the deleter.
template <typename T, typename D>
std::shared_ptr<T> ShareUniquePtr(std::unique_ptr<T, D>&& ptr) {
  return ptr ? std::shared_ptr<T>(std::move(ptr)) : std::shared_ptr<T>();
}

// -----------------------------------------------------------------------------
// Function Template: WeakenPtr()
// -----------------------------------------------------------------------------
//
// Creates a weak pointer associated with a given shared pointer. The returned
// value is a `std::weak_ptr` of deduced type.
//
// Example:
//
//    auto sp = std::make_shared<int>(10);
//    auto wp = absl::WeakenPtr(sp);
//    CHECK_EQ(sp.get(), wp.lock().get());
//    sp.reset();
//    CHECK(wp.lock() == nullptr);
//
template <typename T>
std::weak_ptr<T> WeakenPtr(const std::shared_ptr<T>& ptr) {
  return std::weak_ptr<T>(ptr);
}

namespace memory_internal {

// ExtractOr<E, O, D>::type evaluates to E<O> if possible. Otherwise, D.
template <template <typename> class Extract, typename Obj, typename Default,
          typename>
struct ExtractOr {
  using type = Default;
};

template <template <typename> class Extract, typename Obj, typename Default>
struct ExtractOr<Extract, Obj, Default, void_t<Extract<Obj>>> {
  using type = Extract<Obj>;
};

template <template <typename> class Extract, typename Obj, typename Default>
using ExtractOrT = typename ExtractOr<Extract, Obj, Default, void>::type;

// Extractors for the features of allocators.
template <typename T>
using GetPointer = typename T::pointer;

template <typename T>
using GetConstPointer = typename T::const_pointer;

template <typename T>
using GetVoidPointer = typename T::void_pointer;

template <typename T>
using GetConstVoidPointer = typename T::const_void_pointer;

template <typename T>
using GetDifferenceType = typename T::difference_type;

template <typename T>
using GetSizeType = typename T::size_type;

template <typename T>
using GetPropagateOnContainerCopyAssignment =
    typename T::propagate_on_container_copy_assignment;

template <typename T>
using GetPropagateOnContainerMoveAssignment =
    typename T::propagate_on_container_move_assignment;

template <typename T>
using GetPropagateOnContainerSwap = typename T::propagate_on_container_swap;

template <typename T>
using GetIsAlwaysEqual = typename T::is_always_equal;

template <typename T>
struct GetFirstArg;

template <template <typename...> class Class, typename T, typename... Args>
struct GetFirstArg<Class<T, Args...>> {
  using type = T;
};

template <typename Ptr, typename = void>
struct ElementType {
  using type = typename GetFirstArg<Ptr>::type;
};

template <typename T>
struct ElementType<T, void_t<typename T::element_type>> {
  using type = typename T::element_type;
};

template <typename T, typename U>
struct RebindFirstArg;

template <template <typename...> class Class, typename T, typename... Args,
          typename U>
struct RebindFirstArg<Class<T, Args...>, U> {
  using type = Class<U, Args...>;
};

template <typename T, typename U, typename = void>
struct RebindPtr {
  using type = typename RebindFirstArg<T, U>::type;
};

template <typename T, typename U>
struct RebindPtr<T, U, void_t<typename T::template rebind<U>>> {
  using type = typename T::template rebind<U>;
};

template <typename T, typename U>
constexpr bool HasRebindAlloc(...) {
  return false;
}

template <typename T, typename U>
constexpr bool HasRebindAlloc(typename T::template rebind<U>::other*) {
  return true;
}

template <typename T, typename U, bool = HasRebindAlloc<T, U>(nullptr)>
struct RebindAlloc {
  using type = typename RebindFirstArg<T, U>::type;
};

template <typename T, typename U>
struct RebindAlloc<T, U, true> {
  using type = typename T::template rebind<U>::other;
};

}  // namespace memory_internal

// -----------------------------------------------------------------------------
// Class Template: pointer_traits
// -----------------------------------------------------------------------------
//
// An implementation of C++11's std::pointer_traits.
//
// Provided for portability on toolchains that have a working C++11 compiler,
// but the standard library is lacking in C++11 support. For example, some
// version of the Android NDK.
//

template <typename Ptr>
struct pointer_traits {
  using pointer = Ptr;

  // element_type:
  // Ptr::element_type if present. Otherwise T if Ptr is a template
  // instantiation Template<T, Args...>
  using element_type = typename memory_internal::ElementType<Ptr>::type;

  // difference_type:
  // Ptr::difference_type if present, otherwise std::ptrdiff_t
  using difference_type =
      memory_internal::ExtractOrT<memory_internal::GetDifferenceType, Ptr,
                                  std::ptrdiff_t>;

  // rebind:
  // Ptr::rebind<U> if exists, otherwise Template<U, Args...> if Ptr is a
  // template instantiation Template<T, Args...>
  template <typename U>
  using rebind = typename memory_internal::RebindPtr<Ptr, U>::type;

  // pointer_to:
  // Calls Ptr::pointer_to(r)
  static pointer pointer_to(element_type& r) {  // NOLINT(runtime/references)
    return Ptr::pointer_to(r);
  }
};

// Specialization for T*.
template <typename T>
struct pointer_traits<T*> {
  using pointer = T*;
  using element_type = T;
  using difference_type = std::ptrdiff_t;

  template <typename U>
  using rebind = U*;

  // pointer_to:
  // Calls std::addressof(r)
  static pointer pointer_to(
      element_type& r) noexcept {  // NOLINT(runtime/references)
    return std::addressof(r);
  }
};

// -----------------------------------------------------------------------------
// Class Template: allocator_traits
// -----------------------------------------------------------------------------
//
// A C++11 compatible implementation of C++17's std::allocator_traits.
//
template <typename Alloc>
struct allocator_traits {
  using allocator_type = Alloc;

  // value_type:
  // Alloc::value_type
  using value_type = typename Alloc::value_type;

  // pointer:
  // Alloc::pointer if present, otherwise value_type*
  using pointer = memory_internal::ExtractOrT<memory_internal::GetPointer,
                                              Alloc, value_type*>;

  // const_pointer:
  // Alloc::const_pointer if present, otherwise
  // absl::pointer_traits<pointer>::rebind<const value_type>
  using const_pointer =
      memory_internal::ExtractOrT<memory_internal::GetConstPointer, Alloc,
                                  typename absl::pointer_traits<pointer>::
                                      template rebind<const value_type>>;

  // void_pointer:
  // Alloc::void_pointer if present, otherwise
  // absl::pointer_traits<pointer>::rebind<void>
  using void_pointer = memory_internal::ExtractOrT<
      memory_internal::GetVoidPointer, Alloc,
      typename absl::pointer_traits<pointer>::template rebind<void>>;

  // const_void_pointer:
  // Alloc::const_void_pointer if present, otherwise
  // absl::pointer_traits<pointer>::rebind<const void>
  using const_void_pointer = memory_internal::ExtractOrT<
      memory_internal::GetConstVoidPointer, Alloc,
      typename absl::pointer_traits<pointer>::template rebind<const void>>;

  // difference_type:
  // Alloc::difference_type if present, otherwise
  // absl::pointer_traits<pointer>::difference_type
  using difference_type = memory_internal::ExtractOrT<
      memory_internal::GetDifferenceType, Alloc,
      typename absl::pointer_traits<pointer>::difference_type>;

  // size_type:
  // Alloc::size_type if present, otherwise
  // std::make_unsigned<difference_type>::type
  using size_type = memory_internal::ExtractOrT<
      memory_internal::GetSizeType, Alloc,
      typename std::make_unsigned<difference_type>::type>;

  // propagate_on_container_copy_assignment:
  // Alloc::propagate_on_container_copy_assignment if present, otherwise
  // std::false_type
  using propagate_on_container_copy_assignment = memory_internal::ExtractOrT<
      memory_internal::GetPropagateOnContainerCopyAssignment, Alloc,
      std::false_type>;

  // propagate_on_container_move_assignment:
  // Alloc::propagate_on_container_move_assignment if present, otherwise
  // std::false_type
  using propagate_on_container_move_assignment = memory_internal::ExtractOrT<
      memory_internal::GetPropagateOnContainerMoveAssignment, Alloc,
      std::false_type>;

  // propagate_on_container_swap:
  // Alloc::propagate_on_container_swap if present, otherwise std::false_type
  using propagate_on_container_swap =
      memory_internal::ExtractOrT<memory_internal::GetPropagateOnContainerSwap,
                                  Alloc, std::false_type>;

  // is_always_equal:
  // Alloc::is_always_equal if present, otherwise std::is_empty<Alloc>::type
  using is_always_equal =
      memory_internal::ExtractOrT<memory_internal::GetIsAlwaysEqual, Alloc,
                                  typename std::is_empty<Alloc>::type>;

  // rebind_alloc:
  // Alloc::rebind<T>::other if present, otherwise Alloc<T, Args> if this Alloc
  // is Alloc<U, Args>
  template <typename T>
  using rebind_alloc = typename memory_internal::RebindAlloc<Alloc, T>::type;

  // rebind_traits:
  // absl::allocator_traits<rebind_alloc<T>>
  template <typename T>
  using rebind_traits = absl::allocator_traits<rebind_alloc<T>>;

  // allocate(Alloc& a, size_type n):
  // Calls a.allocate(n)
  static pointer allocate(Alloc& a,  // NOLINT(runtime/references)
                          size_type n) {
    return a.allocate(n);
  }

  // allocate(Alloc& a, size_type n, const_void_pointer hint):
  // Calls a.allocate(n, hint) if possible.
  // If not possible, calls a.allocate(n)
  static pointer allocate(Alloc& a, size_type n,  // NOLINT(runtime/references)
                          const_void_pointer hint) {
    return allocate_impl(0, a, n, hint);
  }

  // deallocate(Alloc& a, pointer p, size_type n):
  // Calls a.deallocate(p, n)
  static void deallocate(Alloc& a, pointer p,  // NOLINT(runtime/references)
                         size_type n) {
    a.deallocate(p, n);
  }

  // construct(Alloc& a, T* p, Args&&... args):
  // Calls a.construct(p, std::forward<Args>(args)...) if possible.
  // If not possible, calls
  //   ::new (static_cast<void*>(p)) T(std::forward<Args>(args)...)
  template <typename T, typename... Args>
  static void construct(Alloc& a, T* p,  // NOLINT(runtime/references)
                        Args&&... args) {
    construct_impl(0, a, p, std::forward<Args>(args)...);
  }

  // destroy(Alloc& a, T* p):
  // Calls a.destroy(p) if possible. If not possible, calls p->~T().
  template <typename T>
  static void destroy(Alloc& a, T* p) {  // NOLINT(runtime/references)
    destroy_impl(0, a, p);
  }

  // max_size(const Alloc& a):
  // Returns a.max_size() if possible. If not possible, returns
  //   std::numeric_limits<size_type>::max() / sizeof(value_type)
  static size_type max_size(const Alloc& a) { return max_size_impl(0, a); }

  // select_on_container_copy_construction(const Alloc& a):
  // Returns a.select_on_container_copy_construction() if possible.
  // If not possible, returns a.
  static Alloc select_on_container_copy_construction(const Alloc& a) {
    return select_on_container_copy_construction_impl(0, a);
  }

 private:
  template <typename A>
  static auto allocate_impl(int, A& a,  // NOLINT(runtime/references)
                            size_type n, const_void_pointer hint)
      -> decltype(a.allocate(n, hint)) {
    return a.allocate(n, hint);
  }
  static pointer allocate_impl(char, Alloc& a,  // NOLINT(runtime/references)
                               size_type n, const_void_pointer) {
    return a.allocate(n);
  }

  template <typename A, typename... Args>
  static auto construct_impl(int, A& a,  // NOLINT(runtime/references)
                             Args&&... args)
      -> decltype(a.construct(std::forward<Args>(args)...)) {
    a.construct(std::forward<Args>(args)...);
  }

  template <typename T, typename... Args>
  static void construct_impl(char, Alloc&, T* p, Args&&... args) {
    ::new (static_cast<void*>(p)) T(std::forward<Args>(args)...);
  }

  template <typename A, typename T>
  static auto destroy_impl(int, A& a,  // NOLINT(runtime/references)
                           T* p) -> decltype(a.destroy(p)) {
    a.destroy(p);
  }
  template <typename T>
  static void destroy_impl(char, Alloc&, T* p) {
    p->~T();
  }

  template <typename A>
  static auto max_size_impl(int, const A& a) -> decltype(a.max_size()) {
    return a.max_size();
  }
  static size_type max_size_impl(char, const Alloc&) {
    return std::numeric_limits<size_type>::max() / sizeof(value_type);
  }

  template <typename A>
  static auto select_on_container_copy_construction_impl(int, const A& a)
      -> decltype(a.select_on_container_copy_construction()) {
    return a.select_on_container_copy_construction();
  }
  static Alloc select_on_container_copy_construction_impl(char,
                                                          const Alloc& a) {
    return a;
  }
};

namespace memory_internal {

// This template alias transforms Alloc::is_nothrow into a metafunction with
// Alloc as a parameter so it can be used with ExtractOrT<>.
template <typename Alloc>
using GetIsNothrow = typename Alloc::is_nothrow;

}  // namespace memory_internal

// ABSL_ALLOCATOR_NOTHROW is a build time configuration macro for user to
// specify whether the default allocation function can throw or never throws.
// If the allocation function never throws, user should define it to a non-zero
// value (e.g. via `-DABSL_ALLOCATOR_NOTHROW`).
// If the allocation function can throw, user should leave it undefined or
// define it to zero.
//
// allocator_is_nothrow<Alloc> is a traits class that derives from
// Alloc::is_nothrow if present, otherwise std::false_type. It's specialized
// for Alloc = std::allocator<T> for any type T according to the state of
// ABSL_ALLOCATOR_NOTHROW.
//
// default_allocator_is_nothrow is a class that derives from std::true_type
// when the default allocator (global operator new) never throws, and
// std::false_type when it can throw. It is a convenience shorthand for writing
// allocator_is_nothrow<std::allocator<T>> (T can be any type).
// NOTE: allocator_is_nothrow<std::allocator<T>> is guaranteed to derive from
// the same type for all T, because users should specialize neither
// allocator_is_nothrow nor std::allocator.
template <typename Alloc>
struct allocator_is_nothrow
    : memory_internal::ExtractOrT<memory_internal::GetIsNothrow, Alloc,
                                  std::false_type> {};

#if ABSL_ALLOCATOR_NOTHROW
template <typename T>
struct allocator_is_nothrow<std::allocator<T>> : std::true_type {};
struct default_allocator_is_nothrow : std::true_type {};
#else
struct default_allocator_is_nothrow : std::false_type {};
#endif

namespace memory_internal {
#ifdef ABSL_HAVE_EXCEPTIONS
template <typename Allocator, typename StorageElement, typename... Args>
void ConstructStorage(Allocator* alloc, StorageElement* first,
                      StorageElement* last, const Args&... args) {
  for (StorageElement* cur = first; cur != last; ++cur) {
    try {
      std::allocator_traits<Allocator>::construct(*alloc, cur, args...);
    } catch (...) {
      while (cur != first) {
        --cur;
        std::allocator_traits<Allocator>::destroy(*alloc, cur);
      }
      throw;
    }
  }
}
template <typename Allocator, typename StorageElement, typename Iterator>
void CopyToStorageFromRange(Allocator* alloc, StorageElement* destination,
                            Iterator first, Iterator last) {
  for (StorageElement* cur = destination; first != last;
       static_cast<void>(++cur), static_cast<void>(++first)) {
    try {
      std::allocator_traits<Allocator>::construct(*alloc, cur, *first);
    } catch (...) {
      while (cur != destination) {
        --cur;
        std::allocator_traits<Allocator>::destroy(*alloc, cur);
      }
      throw;
    }
  }
}
#else   // ABSL_HAVE_EXCEPTIONS
template <typename Allocator, typename StorageElement, typename... Args>
void ConstructStorage(Allocator* alloc, StorageElement* first,
                      StorageElement* last, const Args&... args) {
  for (; first != last; ++first) {
    std::allocator_traits<Allocator>::construct(*alloc, first, args...);
  }
}
template <typename Allocator, typename StorageElement, typename Iterator>
void CopyToStorageFromRange(Allocator* alloc, StorageElement* destination,
                            Iterator first, Iterator last) {
  for (; first != last;
       static_cast<void>(++destination), static_cast<void>(++first)) {
    std::allocator_traits<Allocator>::construct(*alloc, destination, *first);
  }
}
#endif  // ABSL_HAVE_EXCEPTIONS
}  // namespace memory_internal
}  // namespace absl

#endif  // ABSL_MEMORY_MEMORY_H_