chromium/third_party/protobuf/src/google/protobuf/arena.h

// Protocol Buffers - Google's data interchange format
// Copyright 2008 Google Inc.  All rights reserved.
// https://developers.google.com/protocol-buffers/
//
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//
//     * Redistributions of source code must retain the above copyright
// notice, this list of conditions and the following disclaimer.
//     * Redistributions in binary form must reproduce the above
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// in the documentation and/or other materials provided with the
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// contributors may be used to endorse or promote products derived from
// this software without specific prior written permission.
//
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// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

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

#ifndef GOOGLE_PROTOBUF_ARENA_H__
#define GOOGLE_PROTOBUF_ARENA_H__


#include <limits>
#include <type_traits>
#include <utility>
#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;
}
#else
#include <typeinfo>
#endif

#include <type_traits>
#include <google/protobuf/arena_impl.h>
#include <google/protobuf/port.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 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 RepeatedPtrFieldBase;  // defined in repeated_ptr_field.h

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

inline PROTOBUF_ALWAYS_INLINE
void* AlignTo(void* ptr, size_t align) { … }

// Templated cleanup methods.
template <typename T>
void arena_destruct_object(void* object) { … }

template <bool destructor_skippable, typename T>
struct ObjectDestructor { … };

ObjectDestructor<true, T>;

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

// ArenaOptions provides optional additional parameters to arena construction
// that control its block-allocation behavior.
struct ArenaOptions { … };

// Support for non-RTTI environments. (The metrics hooks API uses type
// information.)
#if PROTOBUF_RTTI
#define RTTI_TYPE_ID …
#else
#define RTTI_TYPE_ID …
#endif

// 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::CreateMessage<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.
//
// An arena provides two allocation interfaces: CreateMessage<T>, which works
// for arena-enabled proto2 message types as well as other types that satisfy
// the appropriate protocol (described below), and Create<T>, which works for
// any arbitrary type T. CreateMessage<T> is better when the type T supports it,
// 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. Create<T> does not place
// any special requirements on the type T, and will invoke the object's
// destructor when the arena is destroyed.
//
// The arena message allocation protocol, required by
// CreateMessage<T>(Arena* arena, Args&&... args), is as follows:
//
// - 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|. If no
//   such type trait exists, then the instantiation CreateMessage<T> will fail
//   to compile.
//
// - 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.
//
// This protocol is implemented by all arena-enabled proto2 message classes as
// well as protobuf container types like RepeatedPtrField and Map. The protocol
// is internal to protobuf and is not guaranteed to be stable. Non-proto types
// should not rely on this protocol.
class PROTOBUF_EXPORT PROTOBUF_ALIGNAS(8) Arena final { … };

// Defined above for supporting environments without RTTI.
#undef RTTI_TYPE_ID

}  // namespace protobuf
}  // namespace google

#include <google/protobuf/port_undef.inc>

#endif  // GOOGLE_PROTOBUF_ARENA_H__