src/google/protobuf/arena.h

// Protocol Buffers - Google's data interchange format
// Copyright 2008 Google Inc.  All rights reserved.
//
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file or at
// https://developers.google.com/open-source/licenses/bsd

// This file defines an Arena allocator for better allocation performance.

#ifndef GOOGLE_PROTOBUF_ARENA_H__
#define GOOGLE_PROTOBUF_ARENA_H__

#include <cstddef>
#include <cstdint>
#include <limits>
#include <new>  // IWYU pragma: keep for operator new().
#include <string>
#include <type_traits>
#include <utility>
#include <vector>
#if defined(_MSC_VER) && !defined(_LIBCPP_STD_VER) && !_HAS_EXCEPTIONS
// Work around bugs in MSVC <typeinfo> header when _HAS_EXCEPTIONS=0.
#include <exception>
#include <typeinfo>
namespace std {
using type_info = ::type_info;
}
#endif

#include "absl/base/attributes.h"
#include "google/protobuf/stubs/common.h"
#include "absl/log/absl_check.h"
#include "absl/utility/internal/if_constexpr.h"
#include "google/protobuf/arena_align.h"
#include "google/protobuf/arena_allocation_policy.h"
#include "google/protobuf/port.h"
#include "google/protobuf/serial_arena.h"
#include "google/protobuf/thread_safe_arena.h"

// Must be included last.
#include "google/protobuf/port_def.inc"

#ifdef SWIG
#error "You cannot SWIG proto headers"
#endif

namespace google {
namespace protobuf {

struct ArenaOptions;  // defined below
class Arena;    // defined below
class Message;  // defined in message.h
class MessageLite;
template <typename Key, typename T>
class Map;
namespace internal {
struct RepeatedFieldBase;
}  // namespace internal

namespace arena_metrics {

void EnableArenaMetrics(ArenaOptions* options);

}  // namespace arena_metrics

namespace TestUtil {
class ReflectionTester;  // defined in test_util.h
}  // namespace TestUtil

namespace internal {

struct ArenaTestPeer;        // defined in arena_test_util.h
class InternalMetadata;      // defined in metadata_lite.h
class LazyField;             // defined in lazy_field.h
class EpsCopyInputStream;    // defined in parse_context.h
class UntypedMapBase;        // defined in map.h
class RepeatedPtrFieldBase;  // defined in repeated_ptr_field.h
class TcParser;              // defined in generated_message_tctable_impl.h

template <typename Type>
class GenericTypeHandler;  // defined in repeated_field.h

template <typename T>
void arena_delete_object(void* object) {
  delete reinterpret_cast<T*>(object);
}
}  // namespace internal

// ArenaOptions provides optional additional parameters to arena construction
// that control its block-allocation behavior.
struct ArenaOptions {
  // This defines the size of the first block requested from the system malloc.
  // Subsequent block sizes will increase in a geometric series up to a maximum.
  size_t start_block_size = internal::AllocationPolicy::kDefaultStartBlockSize;

  // This defines the maximum block size requested from system malloc (unless an
  // individual arena allocation request occurs with a size larger than this
  // maximum). Requested block sizes increase up to this value, then remain
  // here.
  size_t max_block_size = internal::AllocationPolicy::kDefaultMaxBlockSize;

  // An initial block of memory for the arena to use, or nullptr for none. If
  // provided, the block must live at least as long as the arena itself. The
  // creator of the Arena retains ownership of the block after the Arena is
  // destroyed.
  char* initial_block = nullptr;

  // The size of the initial block, if provided.
  size_t initial_block_size = 0;

  // A function pointer to an alloc method that returns memory blocks of size
  // requested. By default, it contains a ptr to the malloc function.
  //
  // NOTE: block_alloc and dealloc functions are expected to behave like
  // malloc and free, including Asan poisoning.
  void* (*block_alloc)(size_t) = nullptr;
  // A function pointer to a dealloc method that takes ownership of the blocks
  // from the arena. By default, it contains a ptr to a wrapper function that
  // calls free.
  void (*block_dealloc)(void*, size_t) = nullptr;

 private:
  internal::AllocationPolicy AllocationPolicy() const {
    internal::AllocationPolicy res;
    res.start_block_size = start_block_size;
    res.max_block_size = max_block_size;
    res.block_alloc = block_alloc;
    res.block_dealloc = block_dealloc;
    return res;
  }

  friend class Arena;
  friend class ArenaOptionsTestFriend;
};

// Arena allocator. Arena allocation replaces ordinary (heap-based) allocation
// with new/delete, and improves performance by aggregating allocations into
// larger blocks and freeing allocations all at once. Protocol messages are
// allocated on an arena by using Arena::Create<T>(Arena*), below, and are
// automatically freed when the arena is destroyed.
//
// This is a thread-safe implementation: multiple threads may allocate from the
// arena concurrently. Destruction is not thread-safe and the destructing
// thread must synchronize with users of the arena first.
class PROTOBUF_EXPORT PROTOBUF_ALIGNAS(8) Arena final {
 public:
  // Default constructor with sensible default options, tuned for average
  // use-cases.
  inline Arena() : impl_() {}

  // Construct an arena with default options, except for the supplied
  // initial block. It is more efficient to use this constructor
  // instead of passing ArenaOptions if the only configuration needed
  // by the caller is supplying an initial block.
  inline Arena(char* initial_block, size_t initial_block_size)
      : impl_(initial_block, initial_block_size) {}

  // Arena constructor taking custom options. See ArenaOptions above for
  // descriptions of the options available.
  explicit Arena(const ArenaOptions& options)
      : impl_(options.initial_block, options.initial_block_size,
              options.AllocationPolicy()) {}

  // Block overhead.  Use this as a guide for how much to over-allocate the
  // initial block if you want an allocation of size N to fit inside it.
  //
  // WARNING: if you allocate multiple objects, it is difficult to guarantee
  // that a series of allocations will fit in the initial block, especially if
  // Arena changes its alignment guarantees in the future!
  static const size_t kBlockOverhead =
      internal::ThreadSafeArena::kBlockHeaderSize +
      internal::ThreadSafeArena::kSerialArenaSize;

  inline ~Arena() = default;

  // Deprecated. Use Create<T> instead. TODO: depreate OSS version
  // once internal migration to Arena::Create is done.
  template <typename T, typename... Args>
  ABSL_DEPRECATED("Use Create")
  static T* CreateMessage(Arena* arena, Args&&... args) {
    using Type = std::remove_const_t<T>;
    static_assert(
        is_arena_constructable<Type>::value,
        "CreateMessage can only construct types that are ArenaConstructable");
    return Create<Type>(arena, std::forward<Args>(args)...);
  }

  // Allocates an object type T if the arena passed in is not nullptr;
  // otherwise, returns a heap-allocated object.
  template <typename T, typename... Args>
  PROTOBUF_NDEBUG_INLINE static T* Create(Arena* arena, Args&&... args) {
    return absl::utility_internal::IfConstexprElse<
        is_arena_constructable<T>::value>(
        // Arena-constructable
        [arena](auto&&... args) {
          using Type = std::remove_const_t<T>;
#ifdef __cpp_if_constexpr
          // DefaultConstruct/CopyConstruct are optimized for messages, which
          // are both arena constructible and destructor skippable and they
          // assume much. Don't use these functions unless the invariants
          // hold.
          if constexpr (is_destructor_skippable<T>::value) {
            constexpr auto construct_type = GetConstructType<T, Args&&...>();
            // We delegate to DefaultConstruct/CopyConstruct where appropriate
            // because protobuf generated classes have external templates for
            // these functions for code size reasons. When `if constexpr` is not
            // available always use the fallback.
            if constexpr (construct_type == ConstructType::kDefault) {
              return static_cast<Type*>(DefaultConstruct<Type>(arena));
            } else if constexpr (construct_type == ConstructType::kCopy) {
              return static_cast<Type*>(CopyConstruct<Type>(arena, &args...));
            }
          }
#endif
          return CreateArenaCompatible<Type>(arena,
                                             std::forward<Args>(args)...);
        },
        // Non arena-constructable
        [arena](auto&&... args) {
          if (PROTOBUF_PREDICT_FALSE(arena == nullptr)) {
            return new T(std::forward<Args>(args)...);
          }
          return new (arena->AllocateInternal<T>())
              T(std::forward<Args>(args)...);
        },
        std::forward<Args>(args)...);
  }

  // API to delete any objects not on an arena.  This can be used to safely
  // clean up messages or repeated fields without knowing whether or not they're
  // owned by an arena.  The pointer passed to this function should not be used
  // again.
  template <typename T>
  PROTOBUF_ALWAYS_INLINE static void Destroy(T* obj) {
    if (InternalGetArena(obj) == nullptr) delete obj;
  }

  // Allocates memory with the specific size and alignment.
  void* AllocateAligned(size_t size, size_t align = 8) {
    if (align <= internal::ArenaAlignDefault::align) {
      return Allocate(internal::ArenaAlignDefault::Ceil(size));
    } else {
      // We are wasting space by over allocating align - 8 bytes. Compared
      // to a dedicated function that takes current alignment in consideration.
      // Such a scheme would only waste (align - 8)/2 bytes on average, but
      // requires a dedicated function in the outline arena allocation
      // functions. Possibly re-evaluate tradeoffs later.
      auto align_as = internal::ArenaAlignAs(align);
      return align_as.Ceil(Allocate(align_as.Padded(size)));
    }
  }

  // Create an array of object type T on the arena *without* invoking the
  // constructor of T. If `arena` is null, then the return value should be freed
  // with `delete[] x;` (or `::operator delete[](x);`).
  // To ensure safe uses, this function checks at compile time
  // (when compiled as C++11) that T is trivially default-constructible and
  // trivially destructible.
  template <typename T>
  PROTOBUF_NDEBUG_INLINE static T* CreateArray(Arena* arena,
                                               size_t num_elements) {
    static_assert(std::is_trivial<T>::value,
                  "CreateArray requires a trivially constructible type");
    static_assert(std::is_trivially_destructible<T>::value,
                  "CreateArray requires a trivially destructible type");
    ABSL_CHECK_LE(num_elements, std::numeric_limits<size_t>::max() / sizeof(T))
        << "Requested size is too large to fit into size_t.";
    if (PROTOBUF_PREDICT_FALSE(arena == nullptr)) {
      return new T[num_elements];
    } else {
      // We count on compiler to realize that if sizeof(T) is a multiple of
      // 8 AlignUpTo can be elided.
      return static_cast<T*>(
          arena->AllocateAlignedForArray(sizeof(T) * num_elements, alignof(T)));
    }
  }

  // The following are routines are for monitoring. They will approximate the
  // total sum allocated and used memory, but the exact value is an
  // implementation deal. For instance allocated space depends on growth
  // policies. Do not use these in unit tests.
  // Returns the total space allocated by the arena, which is the sum of the
  // sizes of the underlying blocks.
  uint64_t SpaceAllocated() const { return impl_.SpaceAllocated(); }
  // Returns the total space used by the arena. Similar to SpaceAllocated but
  // does not include free space and block overhead.  This is a best-effort
  // estimate and may inaccurately calculate space used by other threads
  // executing concurrently with the call to this method.  These inaccuracies
  // are due to race conditions, and are bounded but unpredictable.  Stale data
  // can lead to underestimates of the space used, and race conditions can lead
  // to overestimates (up to the current block size).
  uint64_t SpaceUsed() const { return impl_.SpaceUsed(); }

  // Frees all storage allocated by this arena after calling destructors
  // registered with OwnDestructor() and freeing objects registered with Own().
  // Any objects allocated on this arena are unusable after this call. It also
  // returns the total space used by the arena which is the sums of the sizes
  // of the allocated blocks. This method is not thread-safe.
  uint64_t Reset() { return impl_.Reset(); }

  // Adds |object| to a list of heap-allocated objects to be freed with |delete|
  // when the arena is destroyed or reset.
  template <typename T>
  PROTOBUF_ALWAYS_INLINE void Own(T* object) {
    // Collapsing all template instantiations to one for generic Message reduces
    // code size, using the virtual destructor instead.
    using TypeToUse =
        std::conditional_t<std::is_convertible<T*, MessageLite*>::value,
                           MessageLite, T>;
    if (object != nullptr) {
      impl_.AddCleanup(static_cast<TypeToUse*>(object),
                       &internal::arena_delete_object<TypeToUse>);
    }
  }

  // Adds |object| to a list of objects whose destructors will be manually
  // called when the arena is destroyed or reset. This differs from Own() in
  // that it does not free the underlying memory with |delete|; hence, it is
  // normally only used for objects that are placement-newed into
  // arena-allocated memory.
  template <typename T>
  PROTOBUF_ALWAYS_INLINE void OwnDestructor(T* object) {
    if (object != nullptr) {
      impl_.AddCleanup(object, &internal::cleanup::arena_destruct_object<T>);
    }
  }

  // Adds a custom member function on an object to the list of destructors that
  // will be manually called when the arena is destroyed or reset. This differs
  // from OwnDestructor() in that any member function may be specified, not only
  // the class destructor.
  PROTOBUF_ALWAYS_INLINE void OwnCustomDestructor(void* object,
                                                  void (*destruct)(void*)) {
    impl_.AddCleanup(object, destruct);
  }

  // Retrieves the arena associated with |value| if |value| is an arena-capable
  // message, or nullptr otherwise. If possible, the call resolves at compile
  // time. Note that we can often devirtualize calls to `value->GetArena()` so
  // usually calling this method is unnecessary.
  // TODO: remove this function.
  template <typename T>
  ABSL_DEPRECATED(
      "This will be removed in a future release. Call value->GetArena() "
      "instead.")
  PROTOBUF_ALWAYS_INLINE static Arena* GetArena(T* value) {
    return GetArenaInternal(value);
  }

  template <typename T>
  class InternalHelper {
   private:
    // A SFINAE friendly trait that probes for `U` but always evalues to
    // `Arena*`.
    template <typename U>
    using EnableIfArena =
        typename std::enable_if<std::is_same<Arena*, U>::value, Arena*>::type;

    // Use go/ranked-overloads for dispatching.
    struct Rank0 {};
    struct Rank1 : Rank0 {};

    static void InternalSwap(T* a, T* b) { a->InternalSwap(b); }

    static Arena* GetArena(T* p) { return GetArena(Rank1{}, p); }

    template <typename U>
    static auto GetArena(Rank1,
                         U* p) -> EnableIfArena<decltype(p->GetArena())> {
      return p->GetArena();
    }

    template <typename U>
    static Arena* GetArena(Rank0, U*) {
      return nullptr;
    }

    // If an object type T satisfies the appropriate protocol, it is deemed
    // "arena compatible" and handled more efficiently because this interface
    // (i) passes the arena pointer to the created object so that its
    // sub-objects and internal allocations can use the arena too, and (ii)
    // elides the object's destructor call when possible; e.g. protobuf
    // messages, RepeatedField, etc. Otherwise, the arena will invoke the
    // object's destructor when the arena is destroyed.
    //
    // To be "arena-compatible", a type T must satisfy the following:
    //
    // - The type T must have (at least) two constructors: a constructor
    //   callable with `args` (without `arena`), called when a T is allocated on
    //   the heap; and a constructor callable with `Arena* arena, Args&&...
    //   args`, called when a T is allocated on an arena. If the second
    //   constructor is called with a null arena pointer, it must be equivalent
    //   to invoking the first
    //   (`args`-only) constructor.
    //
    // - The type T must have a particular type trait: a nested type
    //   |InternalArenaConstructable_|. This is usually a typedef to |void|.
    //
    // - The type T *may* have the type trait |DestructorSkippable_|. If this
    //   type trait is present in the type, then its destructor will not be
    //   called if and only if it was passed a non-null arena pointer. If this
    //   type trait is not present on the type, then its destructor is always
    //   called when the containing arena is destroyed.
    //
    // The protocol is implemented by all protobuf message classes as well as
    // protobuf container types like RepeatedPtrField and Map. It is internal to
    // protobuf and is not guaranteed to be stable. Non-proto types should not
    // rely on this protocol.
    template <typename U>
    static char DestructorSkippable(const typename U::DestructorSkippable_*);
    template <typename U>
    static double DestructorSkippable(...);

    typedef std::integral_constant<
        bool, sizeof(DestructorSkippable<T>(static_cast<const T*>(0))) ==
                      sizeof(char) ||
                  std::is_trivially_destructible<T>::value>
        is_destructor_skippable;

    template <typename U>
    static char ArenaConstructable(
        const typename U::InternalArenaConstructable_*);
    template <typename U>
    static double ArenaConstructable(...);

    typedef std::integral_constant<bool, sizeof(ArenaConstructable<T>(
                                             static_cast<const T*>(0))) ==
                                             sizeof(char)>
        is_arena_constructable;


    template <typename... Args>
    static T* Construct(void* ptr, Args&&... args) {
      return new (ptr) T(static_cast<Args&&>(args)...);
    }

    static inline PROTOBUF_ALWAYS_INLINE T* New() {
      return new T(nullptr);
    }

    friend class Arena;
    friend class TestUtil::ReflectionTester;
  };

  // Provides access to protected GetArena to generated messages.
  // For internal use only.
  template <typename T>
  static Arena* InternalGetArena(T* p) {
    return InternalHelper<T>::GetArena(p);
  }

  // Helper typetraits that indicates support for arenas in a type T at compile
  // time. This is public only to allow construction of higher-level templated
  // utilities.
  //
  // is_arena_constructable<T>::value is true if the message type T has arena
  // support enabled, and false otherwise.
  //
  // is_destructor_skippable<T>::value is true if the message type T has told
  // the arena that it is safe to skip the destructor, and false otherwise.
  //
  // This is inside Arena because only Arena has the friend relationships
  // necessary to see the underlying generated code traits.
  template <typename T>
  struct is_arena_constructable : InternalHelper<T>::is_arena_constructable {};
  template <typename T>
  struct is_destructor_skippable : InternalHelper<T>::is_destructor_skippable {
  };

 private:
  internal::ThreadSafeArena impl_;

  enum class ConstructType { kUnknown, kDefault, kCopy, kMove };
  // Overload set to detect which kind of construction is going to happen for a
  // specific set of input arguments. This is used to dispatch to different
  // helper functions.
  template <typename T>
  static auto ProbeConstructType()
      -> std::integral_constant<ConstructType, ConstructType::kDefault>;
  template <typename T>
  static auto ProbeConstructType(const T&)
      -> std::integral_constant<ConstructType, ConstructType::kCopy>;
  template <typename T>
  static auto ProbeConstructType(T&)
      -> std::integral_constant<ConstructType, ConstructType::kCopy>;
  template <typename T>
  static auto ProbeConstructType(const T&&)
      -> std::integral_constant<ConstructType, ConstructType::kCopy>;
  template <typename T>
  static auto ProbeConstructType(T&&)
      -> std::integral_constant<ConstructType, ConstructType::kMove>;
  template <typename T, typename... U>
  static auto ProbeConstructType(U&&...)
      -> std::integral_constant<ConstructType, ConstructType::kUnknown>;

  template <typename T, typename... Args>
  static constexpr auto GetConstructType() {
    return std::is_base_of<MessageLite, T>::value
               ? decltype(ProbeConstructType<T>(std::declval<Args>()...))::value
               : ConstructType::kUnknown;
  }

  void ReturnArrayMemory(void* p, size_t size) {
    impl_.ReturnArrayMemory(p, size);
  }

  template <typename T, typename... Args>
  PROTOBUF_NDEBUG_INLINE static T* CreateArenaCompatible(Arena* arena,
                                                         Args&&... args) {
    static_assert(is_arena_constructable<T>::value,
                  "Can only construct types that are ArenaConstructable");
    if (PROTOBUF_PREDICT_FALSE(arena == nullptr)) {
      return new T(nullptr, static_cast<Args&&>(args)...);
    } else {
      return arena->DoCreateMessage<T>(static_cast<Args&&>(args)...);
    }
  }

  // This specialization for no arguments is necessary, because its behavior is
  // slightly different.  When the arena pointer is nullptr, it calls T()
  // instead of T(nullptr).
  template <typename T>
  PROTOBUF_NDEBUG_INLINE static T* CreateArenaCompatible(Arena* arena) {
    static_assert(is_arena_constructable<T>::value,
                  "Can only construct types that are ArenaConstructable");
    if (PROTOBUF_PREDICT_FALSE(arena == nullptr)) {
      // Generated arena constructor T(Arena*) is protected. Call via
      // InternalHelper.
      return InternalHelper<T>::New();
    } else {
      return arena->DoCreateMessage<T>();
    }
  }

  template <typename T, bool trivial = std::is_trivially_destructible<T>::value>
  PROTOBUF_NDEBUG_INLINE void* AllocateInternal() {
    if (trivial) {
      return AllocateAligned(sizeof(T), alignof(T));
    } else {
      // We avoid instantiating arena_destruct_object<T> in the trivial case.
      constexpr auto dtor = &internal::cleanup::arena_destruct_object<
          std::conditional_t<trivial, std::string, T>>;
      return AllocateAlignedWithCleanup(sizeof(T), alignof(T), dtor);
    }
  }

  // DefaultConstruct/CopyConstruct:
  //
  // Functions with a generic signature to support taking the address in generic
  // contexts, like RepeatedPtrField, etc.
  // These are also used as a hook for `extern template` instantiations where
  // codegen can offload the instantiations to the respective .pb.cc files. This
  // has two benefits:
  //  - It reduces the library bloat as callers don't have to instantiate the
  //  function.
  //  - It allows the optimizer to see the constructors called to
  //  further optimize the instantiation.
  template <typename T>
  static void* DefaultConstruct(Arena* arena);
  template <typename T>
  static void* CopyConstruct(Arena* arena, const void* from);

  template <typename T, typename... Args>
  PROTOBUF_NDEBUG_INLINE T* DoCreateMessage(Args&&... args) {
    return InternalHelper<T>::Construct(
        AllocateInternal<T, is_destructor_skippable<T>::value>(), this,
        std::forward<Args>(args)...);
  }

  // CreateInArenaStorage is used to implement map field. Without it,
  // Map need to call generated message's protected arena constructor,
  // which needs to declare Map as friend of generated message.
  template <typename T, typename... Args>
  static void CreateInArenaStorage(T* ptr, Arena* arena, Args&&... args) {
    CreateInArenaStorageInternal(ptr, arena, is_arena_constructable<T>(),
                                 std::forward<Args>(args)...);
    if (PROTOBUF_PREDICT_TRUE(arena != nullptr)) {
      RegisterDestructorInternal(ptr, arena, is_destructor_skippable<T>());
    }
  }

  template <typename T, typename... Args>
  static void CreateInArenaStorageInternal(T* ptr, Arena* arena,
                                           std::true_type, Args&&... args) {
    InternalHelper<T>::Construct(ptr, arena, std::forward<Args>(args)...);
  }
  template <typename T, typename... Args>
  static void CreateInArenaStorageInternal(T* ptr, Arena* /* arena */,
                                           std::false_type, Args&&... args) {
    new (ptr) T(std::forward<Args>(args)...);
  }

  template <typename T>
  static void RegisterDestructorInternal(T* /* ptr */, Arena* /* arena */,
                                         std::true_type) {}
  template <typename T>
  static void RegisterDestructorInternal(T* ptr, Arena* arena,
                                         std::false_type) {
    arena->OwnDestructor(ptr);
  }

  // Implementation for GetArena(). Only message objects with
  // InternalArenaConstructable_ tags can be associated with an arena, and such
  // objects must implement a GetArena() method.
  template <typename T>
  PROTOBUF_ALWAYS_INLINE static Arena* GetArenaInternal(T* value) {
    return InternalHelper<T>::GetArena(value);
  }

  void* AllocateAlignedForArray(size_t n, size_t align) {
    if (align <= internal::ArenaAlignDefault::align) {
      return AllocateForArray(internal::ArenaAlignDefault::Ceil(n));
    } else {
      // We are wasting space by over allocating align - 8 bytes. Compared
      // to a dedicated function that takes current alignment in consideration.
      // Such a scheme would only waste (align - 8)/2 bytes on average, but
      // requires a dedicated function in the outline arena allocation
      // functions. Possibly re-evaluate tradeoffs later.
      auto align_as = internal::ArenaAlignAs(align);
      return align_as.Ceil(AllocateForArray(align_as.Padded(n)));
    }
  }

  void* Allocate(size_t n);
  void* AllocateForArray(size_t n);
  void* AllocateAlignedWithCleanup(size_t n, size_t align,
                                   void (*destructor)(void*));

  // Test only API.
  // It returns the objects that are in the cleanup list for the current
  // SerialArena. This API is meant for tests that want to see if something was
  // added or not to the cleanup list. Sometimes adding something to the cleanup
  // list has no visible side effect so peeking into the list is the only way to
  // test.
  std::vector<void*> PeekCleanupListForTesting();

  template <typename Type>
  friend class internal::GenericTypeHandler;
  friend class internal::InternalMetadata;  // For user_arena().
  friend class internal::LazyField;         // For DefaultConstruct.
  friend class internal::EpsCopyInputStream;  // For parser performance
  friend class internal::TcParser;            // For parser performance
  friend class MessageLite;
  template <typename Key, typename T>
  friend class Map;
  template <typename>
  friend class RepeatedField;                   // For ReturnArrayMemory
  friend class internal::RepeatedPtrFieldBase;  // For ReturnArrayMemory
  friend class internal::UntypedMapBase;        // For ReturnArenaMemory

  friend struct internal::ArenaTestPeer;
};

// DefaultConstruct/CopyConstruct
//
// IMPORTANT: These have to be defined out of line and without an `inline`
// keyword to make sure the `extern template` suppresses instantiations.
template <typename T>
PROTOBUF_NOINLINE void* Arena::DefaultConstruct(Arena* arena) {
  static_assert(is_destructor_skippable<T>::value, "");
  void* mem = arena != nullptr ? arena->AllocateAligned(sizeof(T))
                               : ::operator new(sizeof(T));
  return new (mem) T(arena);
}

template <typename T>
PROTOBUF_NOINLINE void* Arena::CopyConstruct(Arena* arena, const void* from) {
  static_assert(is_destructor_skippable<T>::value, "");
  void* mem = arena != nullptr ? arena->AllocateAligned(sizeof(T))
                               : ::operator new(sizeof(T));
  return new (mem) T(arena, *static_cast<const T*>(from));
}

template <>
inline void* Arena::AllocateInternal<std::string, false>() {
  return impl_.AllocateFromStringBlock();
}

}  // namespace protobuf
}  // namespace google

#include "google/protobuf/port_undef.inc"

#endif  // GOOGLE_PROTOBUF_ARENA_H__