Changed the semantics of Buffer::Clear to not alter the capacity

webrtc/base/buffer.h

Index: webrtc/base/buffer.h
diff --git a/webrtc/base/buffer.h b/webrtc/base/buffer.h
index 5c9380afee03a5e0825d92475e9ed578244314ae..234039607a1c862aa11763936ec236380d3252c5 100644
--- a/webrtc/base/buffer.h
+++ b/webrtc/base/buffer.h
@@ -180,12 +180,10 @@ class Buffer {
     return std::move(*this);
   }
 
-  // Resets the buffer to zero size and capacity. Works even if the buffer has
-  // been moved from.
+  // Resets the buffer to zero size without altering capacity. Works even if the

[henrika_webrtc, 2016/02/18 11:54:38]

"has been moved from"; what exactly does that mean?

[kwiberg-webrtc, 2016/02/18 12:05:35]

That the object has been used as the argument of a move constructor. (Move
constructors typically leave their argument in some valid but unusable state,
where the destructor is the only method that's guaranteed to work. Individual
classes can give stronger guarantees, which is what rtc::Buffer does here.)

[henrika_webrtc, 2016/02/18 12:28:19]

Fine, would be nice with a more clear (less "lingo-like") comment to explain
that. Thanks. 

[kwiberg-webrtc, 2016/02/18 13:12:04]

Yes, maybe. But I count 4 mentions of that concept in this file alone, so it
would have to be a pointer to an explanation elsewhere. However, a short
explanation is unlikely to suffice, since someone who doesn't know that values
that have been moved from shouldn't be used likely has more to learn about
rvalue references than just that one fact. At which point we'll just end up
pointing at something like https://www.chromium.org/rvalue-references (or
possibly straight at
"https://www.chromium.org/rvalue-references#TOC-5.-You-must-not-use-a-variable-after-calling-std::move-on-it.").

But do we want to sprinkle our code with pointers to C++ language tutorials? My
thinking is that rvalue references and move semantics are well-known enough by
now that we can count on people to at least recognize this as a specific
language feature, which they know how to look up if necessary.

+  // buffer has been moved from.

[kwiberg-webrtc, 2016/02/18 11:22:32]

Please unit test the "Works even if the buffer has been moved from" part too.

[ossu, 2016/02/18 12:00:10]

The TestMoveConstruct and TestMoveAssign tests already cover this.

[kwiberg-webrtc, 2016/02/18 12:05:35]

Acknowledged.

   void Clear() {
-    data_.reset();
     size_ = 0;
-    capacity_ = 0;
     assert(IsConsistent());
   }