Add AtomicFlag class to base/......

base/atomic_flag.h

+// Copyright (c) 2009 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 BASE_ATOMIC_FLAG_H_
+#define BASE_ATOMIC_FLAG_H_
+
+#include "base/atomicops.h"
+
+namespace base {
+
+// AtomicFlag allows threads to notify each other for a single occurrence
+// of a single event. It maintains an abstract boolean "flag" that transitions
+// to true at most once. It provides calls to query the boolean.
+//
+// Memory ordering: For any threads X and Y, if X calls Set(), then any
+// action taken by X before it calls Set() is visible to thread Y after
+// Y receives a true return value from IsSet().
+class AtomicFlag {
+ public:
+  // Sets "flag_" to "initial_value".
+  explicit AtomicFlag(bool initial_value = false)
+      : flag_(initial_value ? 1 : 0) { }
+  ~AtomicFlag() {}
+
+  void Set();  // Set "flag_" to true. May be called only once.
+  bool IsSet() const;  // Return "flag_".
+
+ private:
+  base::subtle::Atomic32 flag_;
+
+  DISALLOW_COPY_AND_ASSIGN(AtomicFlag);
+};
+
+}  // namespace base
+
+#endif  // BASE_ATOMIC_FLAG_H_