sql::MetaTable.next_download_id, DownloadManager::GetNextId()

chrome/browser/download/download_id.h

+// Copyright (c) 2011 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 CHROME_BROWSER_DOWNLOAD_DOWNLOAD_ID_H_
+#define CHROME_BROWSER_DOWNLOAD_DOWNLOAD_ID_H_
+#pragma once
+
+#include <iostream>
+
+#include "base/hash_tables.h"
+
+class DownloadManager;
+
+// DownloadId combines per-profile Download ids with an indication of which
+// profile in order to be globally unique. DownloadIds are not persistent across
+// sessions, but their local() field is.
+class DownloadId {
+ public:
+  explicit DownloadId(const DownloadManager* manager = NULL,
+                      int32 local_id = -1)
+    : manager_(manager),
+      local_id_(local_id) {
+  }
+
+  // Return the per-profile and persistent part of this DownloadId.
+  int32 local() const { return local_id_; }
+
+  // Returns true if this DownloadId has been allocated and could possibly refer
+  // to a DownloadItem that exists.
+  bool IsValid() const { return ((manager_ != NULL) && (local_id_ >= 0)); }
+
+  // Returns true if this DownloadId was allocated by test_man.
+  bool BelongsTo(const DownloadManager* test_man) const {

[Randy Smith (Not in Mondays), 2011/08/03 21:10:29]

What's the intended use for this routine?  It doesn't seem to be used in this
CL.

[benjhayden, 2011/08/04 17:15:00]

I was imagining sprinkling DCHECK(global_id.BelongsTo(this)) throughout
DownloadManager when I make it only deal in DownloadIds, just in case somebody
ever plays fast and loose with their DM*s. WDYT?

[Randy Smith (Not in Mondays), 2011/08/04 19:52:27]

I'm fine with that.  The perfectionist in me sorta wants this routine going in
when you actually start using it, but that's not a big deal at all. 

I think I would request that you change the argument to "manager" or
"download_manager" from "test_man", both to avoid the abbreviation, and because
the current name sorta implies that the routine is here for testing only.

+    return test_man == manager_;
+  }
+
+  // The following methods (operator==, operator<, hash(), copy, and assign)
+  // provide support for STL containers such as hash_map and set.
+
+  // Returns true if this refers to the same DownloadItem as that.
+  bool operator==(const DownloadId& that) const {
+    return ((that.local_id_ == local_id_) &&
+            (that.manager_ == manager_));
+  }
+  bool operator<(const DownloadId& that) const {
+    return ((manager_ < that.manager_) ||
+            (local_id_ < that.local_id_));
+  }
+
+  size_t hash() const {
+    // The top half of manager is unlikely to be distinct, and the user is
+    // unlikely to have >64K downloads. If these assumptions are incorrect, then
+    // DownloadFileManager's hash_map might have a few collisions, but it will
+    // use operator== to safely disambiguate.
+    return reinterpret_cast<size_t>(manager_) +
+           (static_cast<size_t>(local_id_) << (4 * sizeof(size_t)));
+  }
+
+ private:
+  // DownloadId is used mostly off the UI thread, so manager's methods can't be
+  // called, but the pointer can be compared.
+  const DownloadManager* manager_;
+
+  int32 local_id_;
+
+  friend std::ostream& operator<<(std::ostream& out,
+                                  const DownloadId& global_id);
+
+  // Allow copy and assign.
+};
+
+// Allow logging DownloadIds. Looks like "0x01234567:42".
+std::ostream& operator<<(std::ostream& out, const DownloadId& global_id);
+
+// Allow using DownloadIds as keys in hash_maps.
+namespace BASE_HASH_NAMESPACE {
+template<> struct hash<DownloadId> {
+  std::size_t operator()(const DownloadId& did) const {
+    return did.hash();
+  }
+};
+}
+#endif  // CHROME_BROWSER_DOWNLOAD_DOWNLOAD_ID_H_