Create rtc::AtomicInt POD struct.

webrtc/base/atomicops.h

 /*
  *  Copyright 2011 The WebRTC Project Authors. All rights reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
 #ifndef WEBRTC_BASE_ATOMICOPS_H_
 #define WEBRTC_BASE_ATOMICOPS_H_
 
 #if defined(WEBRTC_WIN)
 // Include winsock2.h before including <windows.h> to maintain consistency with
 // win32.h.  We can't include win32.h directly here since it pulls in
 // headers such as basictypes.h which causes problems in Chromium where webrtc
 // exists as two separate projects, webrtc and libjingle.
 #include <winsock2.h>
 #include <windows.h>
 #endif  // defined(WEBRTC_WIN)
 
 namespace rtc {
+
 class AtomicOps {
  public:
 #if defined(WEBRTC_WIN)
   // Assumes sizeof(int) == sizeof(LONG), which it is on Win32 and Win64.
   static int Increment(volatile int* i) {
     return ::InterlockedIncrement(reinterpret_cast<volatile LONG*>(i));
   }
   static int Decrement(volatile int* i) {
     return ::InterlockedDecrement(reinterpret_cast<volatile LONG*>(i));
   }
@@ skipping 20 matching lines @@
   }
   static void ReleaseStore(volatile int* i, int value) {
     __atomic_store_n(i, value, __ATOMIC_RELEASE);
   }
   static int CompareAndSwap(volatile int* i, int old_value, int new_value) {
     return __sync_val_compare_and_swap(i, old_value, new_value);
   }
 #endif
 };
 
+// POD struct version of AtomicOps, prevents accidental non-atomic operator
+// usage (such as ++, -- or =). Functions are static, so that the AtomicInt::
+// prefix must be present in the code, clearly labeling the operations as
+// atomic.
+// Do not copy-initialize, since that performs non-atomic reads of value_. The
+// copy constructor needs to be present for brace initialization.

[tommi, 2015/11/06 22:53:36]

why do we need brace initialization? (are you sure it's supported by all
toolsets?)

+struct AtomicInt {
+  AtomicInt() = delete;
+  // TODO(pbos): When MSVC allows brace initialization (or we move to

[tommi, 2015/11/06 22:53:36]

ah... so, how can we use it?

+  // std::atomic), remove copy constructor (or have it implicitly removed by
+  // std::atomic).
+  void operator=(const AtomicInt&) = delete;
 
+  // value_ is public to permit brace initialization. Should not be accessed
+  // directly.

[tommi, 2015/11/06 22:53:36]

The documentation doesn't clearly explain why we actually need this beyond
issues with compilers.  Having the member variable public doesn't feel ideal.

+  volatile int value_;
 
+  // Atomically increments |i|, returns the resulting incremented value.
+  static int Increment(AtomicInt* i) {
+    return AtomicOps::Increment(&i->value_);
+  }
+
+  // Atomically decrements |i|, returns the resulting decremented value.
+  static int Decrement(AtomicInt* i) {
+    return AtomicOps::Decrement(&i->value_);
+  }
+
+  // Atomically loads |i|.
+  static int AcquireLoad(const AtomicInt* i) {
+    return AtomicOps::AcquireLoad(&i->value_);
+  }
+
+  // Atomically stores |value| in |i|.
+  static void ReleaseStore(AtomicInt* i, int value) {
+    AtomicOps::ReleaseStore(&i->value_, value);
+  }
+
+  // Attempts to compare-and-swaps |old_value| for |new_value| in |i| , returns
+  // |i|'s initial value. If equal to |old_value|, then the CAS succeeded,
+  // otherwise no operation is performed.
+  static int CompareAndSwap(AtomicInt* i, int old_value, int new_value) {
+    return AtomicOps::CompareAndSwap(&i->value_, old_value, new_value);
+  }
+};
 }
 
 #endif  // WEBRTC_BASE_ATOMICOPS_H_