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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_THREADING_THREAD_CHECKER_H_
      6 #define BASE_THREADING_THREAD_CHECKER_H_
      7 
      8 // Apart from debug builds, we also enable the thread checker in
      9 // builds with DCHECK_ALWAYS_ON so that trybots and waterfall bots
     10 // with this define will get the same level of thread checking as
     11 // debug bots.
     12 //
     13 // Note that this does not perfectly match situations where DCHECK is
     14 // enabled.  For example a non-official release build may have
     15 // DCHECK_ALWAYS_ON undefined (and therefore ThreadChecker would be
     16 // disabled) but have DCHECKs enabled at runtime.
     17 #if (!defined(NDEBUG) || defined(DCHECK_ALWAYS_ON))
     18 #define ENABLE_THREAD_CHECKER 1
     19 #else
     20 #define ENABLE_THREAD_CHECKER 0
     21 #endif
     22 
     23 #if ENABLE_THREAD_CHECKER
     24 #include "base/threading/thread_checker_impl.h"
     25 #endif
     26 
     27 namespace base {
     28 
     29 // Do nothing implementation, for use in release mode.
     30 //
     31 // Note: You should almost always use the ThreadChecker class to get the
     32 // right version for your build configuration.
     33 class ThreadCheckerDoNothing {
     34  public:
     35   bool CalledOnValidThread() const {
     36     return true;
     37   }
     38 
     39   void DetachFromThread() {}
     40 };
     41 
     42 // ThreadChecker is a helper class used to help verify that some methods of a
     43 // class are called from the same thread. It provides identical functionality to
     44 // base::NonThreadSafe, but it is meant to be held as a member variable, rather
     45 // than inherited from base::NonThreadSafe.
     46 //
     47 // While inheriting from base::NonThreadSafe may give a clear indication about
     48 // the thread-safety of a class, it may also lead to violations of the style
     49 // guide with regard to multiple inheritance. The choice between having a
     50 // ThreadChecker member and inheriting from base::NonThreadSafe should be based
     51 // on whether:
     52 //  - Derived classes need to know the thread they belong to, as opposed to
     53 //    having that functionality fully encapsulated in the base class.
     54 //  - Derived classes should be able to reassign the base class to another
     55 //    thread, via DetachFromThread.
     56 //
     57 // If neither of these are true, then having a ThreadChecker member and calling
     58 // CalledOnValidThread is the preferable solution.
     59 //
     60 // Example:
     61 // class MyClass {
     62 //  public:
     63 //   void Foo() {
     64 //     DCHECK(thread_checker_.CalledOnValidThread());
     65 //     ... (do stuff) ...
     66 //   }
     67 //
     68 //  private:
     69 //   ThreadChecker thread_checker_;
     70 // }
     71 //
     72 // In Release mode, CalledOnValidThread will always return true.
     73 #if ENABLE_THREAD_CHECKER
     74 class ThreadChecker : public ThreadCheckerImpl {
     75 };
     76 #else
     77 class ThreadChecker : public ThreadCheckerDoNothing {
     78 };
     79 #endif  // ENABLE_THREAD_CHECKER
     80 
     81 #undef ENABLE_THREAD_CHECKER
     82 
     83 }  // namespace base
     84 
     85 #endif  // BASE_THREADING_THREAD_CHECKER_H_
     86