base/sequenced_task_runner_helpers.h

   1 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
   2 // Use of this source code is governed by a BSD-style license that can be
   3 // found in the LICENSE file.
   4
   5 #ifndef BASE_SEQUENCED_TASK_RUNNER_HELPERS_H_
   6 #define BASE_SEQUENCED_TASK_RUNNER_HELPERS_H_
   7
   8 #include "base/basictypes.h"
   9
  10 // TODO(akalin): Investigate whether it's possible to just have
  11 // SequencedTaskRunner use these helpers (instead of MessageLoop).
  12 // Then we can just move these to sequenced_task_runner.h.
  13
  14 namespace tracked_objects {
  15 class Location;
  16 }
  17
  18 namespace base {
  19
  20 namespace subtle {
  21 template <class T, class R> class DeleteHelperInternal;
  22 template <class T, class R> class ReleaseHelperInternal;
  23 }
  24
  25 // Template helpers which use function indirection to erase T from the
  26 // function signature while still remembering it so we can call the
  27 // correct destructor/release function.
  28 //
  29 // We use this trick so we don't need to include bind.h in a header
  30 // file like sequenced_task_runner.h. We also wrap the helpers in a
  31 // templated class to make it easier for users of DeleteSoon to
  32 // declare the helper as a friend.
  33 template <class T>
  34 class DeleteHelper {
  35  private:
  36   template <class T2, class R> friend class subtle::DeleteHelperInternal;
  37
  38   static void DoDelete(const void* object) {
  39     delete reinterpret_cast<const T*>(object);
  40   }
  41
  42   DISALLOW_COPY_AND_ASSIGN(DeleteHelper);
  43 };
  44
  45 template <class T>
  46 class ReleaseHelper {
  47  private:
  48   template <class T2, class R> friend class subtle::ReleaseHelperInternal;
  49
  50   static void DoRelease(const void* object) {
  51     reinterpret_cast<const T*>(object)->Release();
  52   }
  53
  54   DISALLOW_COPY_AND_ASSIGN(ReleaseHelper);
  55 };
  56
  57 namespace subtle {
  58
  59 // An internal SequencedTaskRunner-like class helper for DeleteHelper
  60 // and ReleaseHelper.  We don't want to expose the Do*() functions
  61 // directly directly since the void* argument makes it possible to
  62 // pass/ an object of the wrong type to delete.  Instead, we force
  63 // callers to go through these internal helpers for type
  64 // safety. SequencedTaskRunner-like classes which expose DeleteSoon or
  65 // ReleaseSoon methods should friend the appropriate helper and
  66 // implement a corresponding *Internal method with the following
  67 // signature:
  68 //
  69 // bool(const tracked_objects::Location&,
  70 //      void(*function)(const void*),
  71 //      void* object)
  72 //
  73 // An implementation of this function should simply create a
  74 // base::Closure from (function, object) and return the result of
  75 // posting the task.
  76 template <class T, class ReturnType>
  77 class DeleteHelperInternal {
  78  public:
  79   template <class SequencedTaskRunnerType>
  80   static ReturnType DeleteViaSequencedTaskRunner(
  81       SequencedTaskRunnerType* sequenced_task_runner,
  82       const tracked_objects::Location& from_here,
  83       const T* object) {
  84     return sequenced_task_runner->DeleteSoonInternal(
  85         from_here, &DeleteHelper<T>::DoDelete, object);
  86   }
  87
  88  private:
  89   DISALLOW_COPY_AND_ASSIGN(DeleteHelperInternal);
  90 };
  91
  92 template <class T, class ReturnType>
  93 class ReleaseHelperInternal {
  94  public:
  95   template <class SequencedTaskRunnerType>
  96   static ReturnType ReleaseViaSequencedTaskRunner(
  97       SequencedTaskRunnerType* sequenced_task_runner,
  98       const tracked_objects::Location& from_here,
  99       const T* object) {
 100     return sequenced_task_runner->ReleaseSoonInternal(
 101         from_here, &ReleaseHelper<T>::DoRelease, object);
 102   }
 103
 104  private:
 105   DISALLOW_COPY_AND_ASSIGN(ReleaseHelperInternal);
 106 };
 107
 108 }  // namespace subtle
 109
 110 }  // namespace base
 111
 112 #endif  // BASE_SEQUENCED_TASK_RUNNER_HELPERS_H_