Replace uses of hash_map in //base

base/task/cancelable_task_tracker.h

 // Copyright 2014 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.
 
 // CancelableTaskTracker posts tasks (in the form of a Closure) to a
 // TaskRunner, and is able to cancel the task later if it's not needed
 // anymore.  On destruction, CancelableTaskTracker will cancel all
 // tracked tasks.
 //
 // Each cancelable task can be associated with a reply (also a Closure). After
@@ skipping 25 matching lines @@
 #ifndef BASE_TASK_CANCELABLE_TASK_TRACKER_H_
 #define BASE_TASK_CANCELABLE_TASK_TRACKER_H_
 
 #include <stdint.h>
 
 #include <utility>
 
 #include "base/base_export.h"
 #include "base/bind.h"
 #include "base/callback.h"
-#include "base/containers/hash_tables.h"
+#include "base/containers/small_map.h"
 #include "base/macros.h"
 #include "base/memory/weak_ptr.h"
 #include "base/post_task_and_reply_with_result_internal.h"
 #include "base/sequence_checker.h"
 
 namespace tracked_objects {
 class Location;
 }  // namespace tracked_objects
 
 namespace base {
@@ skipping 78 matching lines @@
   void TryCancelAll();
 
   // Returns true iff there are in-flight tasks that are still being
   // tracked.
   bool HasTrackedTasks() const;
 
  private:
   void Track(TaskId id, CancellationFlag* flag);
   void Untrack(TaskId id);
 
-  hash_map<TaskId, CancellationFlag*> task_flags_;
+  // Typically the number of tasks are 0-2 and occationally 3-4. But since
+  // this is a general API that could be used in unexpected ways, use a
+  // small_map instead of a flat_map to avoid falling over if there are many
+  // tasks.
+  base::small_map<std::map<TaskId, CancellationFlag*>, 4> task_flags_;

[Łukasz Anforowicz, 2017/04/24 18:46:10]

optional nit: I notice that the declaration above uses an unnecessary |base::|
namespace qualifier - this is probably fine, but it is inconsistent with other
type references in this file (which omit the |base::| namespace qualifier when
referring to types like SequenceChecker).

 
   TaskId next_id_;
   SequenceChecker sequence_checker_;
 
   WeakPtrFactory<CancelableTaskTracker> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(CancelableTaskTracker);
 };
 
 }  // namespace base
 
 #endif  // BASE_TASK_CANCELABLE_TASK_TRACKER_H_