node/cppgc/sentinel-pointer.h

0001 // Copyright 2021 the V8 project authors. All rights reserved.
0002 // Use of this source code is governed by a BSD-style license that can be
0003 // found in the LICENSE file.
0004
0005 #ifndef INCLUDE_CPPGC_SENTINEL_POINTER_H_
0006 #define INCLUDE_CPPGC_SENTINEL_POINTER_H_
0007
0008 #include <cstdint>
0009
0010 #include "cppgc/internal/api-constants.h"
0011
0012 namespace cppgc {
0013 namespace internal {
0014
0015 // Special tag type used to denote some sentinel member. The semantics of the
0016 // sentinel is defined by the embedder.
0017 struct SentinelPointer {
0018 #if defined(CPPGC_POINTER_COMPRESSION)
0019   static constexpr intptr_t kSentinelValue =
0020       1 << api_constants::kPointerCompressionShift;
0021 #else   // !defined(CPPGC_POINTER_COMPRESSION)
0022   static constexpr intptr_t kSentinelValue = 0b10;
0023 #endif  // !defined(CPPGC_POINTER_COMPRESSION)
0024   template <typename T>
0025   operator T*() const {
0026     return reinterpret_cast<T*>(kSentinelValue);
0027   }
0028   // Hidden friends.
0029   friend bool operator==(SentinelPointer, SentinelPointer) { return true; }
0030   friend bool operator!=(SentinelPointer, SentinelPointer) { return false; }
0031 };
0032
0033 }  // namespace internal
0034
0035 constexpr internal::SentinelPointer kSentinelPointer;
0036
0037 }  // namespace cppgc
0038
0039 #endif  // INCLUDE_CPPGC_SENTINEL_POINTER_H_