Add support for turning pointers back to references.

base/metrics/persistent_memory_allocator.h

 // Copyright (c) 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef BASE_METRICS_PERSISTENT_MEMORY_ALLOCATOR_H_
 #define BASE_METRICS_PERSISTENT_MEMORY_ALLOCATOR_H_
 
 #include <stdint.h>
 
 #include <atomic>
@@ skipping 298 matching lines @@
     return const_cast<T*>(reinterpret_cast<volatile T*>(
         GetBlockData(ref, type_id, count * sizeof(T))));
   }
   template <typename T>
   const T* GetAsArray(Reference ref, uint32_t type_id, size_t count) const {
     static_assert(std::is_fundamental<T>::value, "use GetAsObject<>()");
     return const_cast<const char*>(reinterpret_cast<const volatile T*>(
         GetBlockData(ref, type_id, count * sizeof(T))));
   }
 
+  // Get the corresponding reference for an object held in persistent memory.
+  // If the |memory| is not valid or the type does not match, a kReferenceNull
+  // result will be returned.
+  Reference GetAsReference(const void* memory, uint32_t type_id) const;
+
   // Get the number of bytes allocated to a block. This is useful when storing
   // arrays in order to validate the ending boundary. The returned value will
   // include any padding added to achieve the required alignment and so could
   // be larger than given in the original Allocate() request.
   size_t GetAllocSize(Reference ref) const;
 
   // Access the internal "type" of an object. This generally isn't necessary
   // but can be used to "clear" the type and so effectively mark it as deleted
   // even though the memory stays valid and allocated. Changing the type is
   // an atomic compare/exchange and so requires knowing the existing value.
@@ skipping 197 matching lines @@
  private:
   std::unique_ptr<MemoryMappedFile> mapped_file_;
 
   DISALLOW_COPY_AND_ASSIGN(FilePersistentMemoryAllocator);
 };
 #endif  // !defined(OS_NACL)
 
 }  // namespace base
 
 #endif  // BASE_METRICS_PERSISTENT_MEMORY_ALLOCATOR_H_