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diff --git a/src/flatbuffer_builder.h b/src/flatbuffer_builder.h
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+++ b/src/flatbuffer_builder.h
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+// Copyright 2018 Bert Belder <bertbelder@gmail.com>
+// All rights reserved. MIT License.
+#ifndef FLATBUFFER_BUILDER_H_
+#define FLATBUFFER_BUILDER_H_
+
+#include <stddef.h>
+#include <stdint.h>
+#include <stdlib.h>
+
+#include "deno.h"
+#include "flatbuffers/flatbuffers.h"
+
+namespace deno {
+
+// Wrap the default FlatBufferBuilder class, because the default one can't give
+// us a pointer to the output buffer that we own. Nominally,
+// FlatBufferBuilder::Release() should do that, but it returns some
+// smart-pointer-like object (DetachedBuffer) that frees the buffer when it goes
+// out of scope.
+//
+// This wrapper adds the `ExportBuf` method that returns a deno_buf, which
+// is really not owned at all -- the caller is responsible for releasing the
+// allocation with free().
+//
+// The alternative allocator also uses malloc()/free(), rather than
+// new/delete[], so that the exported buffer can be later be converted to an
+// ArrayBuffer; the (default) V8 ArrayBuffer allocator also uses free().
+class FlatBufferBuilder : public flatbuffers::FlatBufferBuilder {
+  static const size_t kDefaultInitialSize = 1024;
+
+  class Allocator : public flatbuffers::Allocator {
+    uint8_t* keep_alloc_ptr_ = nullptr;
+    uint8_t* last_alloc_ptr_ = nullptr;
+    size_t last_alloc_len_ = 0;
+
+   public:
+    deno_buf GetAndKeepBuf(uint8_t* data_ptr, size_t data_len);
+
+   protected:
+    virtual uint8_t* allocate(size_t size);
+    virtual void deallocate(uint8_t* ptr, size_t size);
+  };
+
+  Allocator allocator_;
+
+ public:
+  explicit FlatBufferBuilder(size_t initial_size = kDefaultInitialSize)
+      : flatbuffers::FlatBufferBuilder(initial_size, &allocator_) {}
+
+  // Export the finalized flatbuffer as a deno_buf structure. The caller takes
+  // ownership of the underlying memory allocation, which must be released with
+  // free().
+  // Afer calling ExportBuf() the FlatBufferBuilder should no longer be used;
+  // However it can be used again once it is reset with the Reset() method.
+  deno_buf ExportBuf();
+
+  // Don't use these.
+  flatbuffers::DetachedBuffer Release() = delete;
+  flatbuffers::DetachedBuffer ReleaseBufferPointer() = delete;
+};
+
+}  // namespace deno
+
+#endif  // FLATBUFFER_BUILDER_H_