Movable support for VideoReceiveStream::Config and avoid copies

webrtc/video/receive_statistics_proxy.h

Index: webrtc/video/receive_statistics_proxy.h
diff --git a/webrtc/video/receive_statistics_proxy.h b/webrtc/video/receive_statistics_proxy.h
index f88bbf51a27bcff761950c715252c4f204086ce9..7d4166f878038fb6f4d1559007cb9735e85ef32a 100644
--- a/webrtc/video/receive_statistics_proxy.h
+++ b/webrtc/video/receive_statistics_proxy.h
@@ -37,7 +37,7 @@ class ReceiveStatisticsProxy : public VCMReceiveStatisticsCallback,
                                public RtcpPacketTypeCounterObserver,
                                public StreamDataCountersCallback {
  public:
-  ReceiveStatisticsProxy(const VideoReceiveStream::Config& config,
+  ReceiveStatisticsProxy(const VideoReceiveStream::Config* config,

[pbos-webrtc, 2016/06/06 16:15:34]

Shouldn't this still be a const reference?

[tommi, 2016/06/08 09:49:39]

It could.  What I want to do is to make a distinction between these two cases:

* how const& passing is used in general, which is that it's assumed that the
const& may be a temporary, a variable on the stack or a variable who's lifetime
does not have guarantees.
* pass a pointer to an object that will be shared between the caller and callee
for a known period of time.

The way the second case is usually implemented is by passing a non-const
pointer.  However, in our case here, it's still appropriate to have the object
const.

[pbos-webrtc, 2016/06/08 11:24:05]

I think we have quite a bit of video-related code that uses const references for
this, if we should start using pointers for these arguments I'd like for us to
do that as a cross-webrtc/video change instead of having this discrepancy. Does
that make sense?

E.g. in webrtc/video/ we usually handle the second case by having const
references, right now. I'm aboard with changing that, but we should do it across
the directory instead of having a style inconsistency.

[tommi, 2016/06/08 13:28:41]

I would prefer to keep this as is actually.  The semantics of how |config| and
|clock| are being passed, are comparable.

Furthermore, the other code is not following the style guide.  So I don't see a
reason for bulking up those sort of fixes when we can help with that work in
changelists like this one.

See: https://google.github.io/styleguide/cppguide.html#Reference_Arguments

                          Clock* clock);
   virtual ~ReceiveStatisticsProxy();
 
@@ -96,7 +96,9 @@ class ReceiveStatisticsProxy : public VCMReceiveStatisticsCallback,
   void UpdateHistograms() EXCLUSIVE_LOCKS_REQUIRED(crit_);
 
   Clock* const clock_;
-  const VideoReceiveStream::Config config_;
+  // Ownership of this object lies with the owner of the ReceiveStatisticsProxy
+  // instance.  Lifetime is guaranteed to outlive |this|.
+  const VideoReceiveStream::Config& config_;
 
   rtc::CriticalSection crit_;
   VideoReceiveStream::Stats stats_ GUARDED_BY(crit_);