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      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_
    113