Add CopyOnWriteBuffer class

webrtc/base/buffer.cc

 /*
  *  Copyright 2015 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.
  */
 
+#include <utility>
+
+#include "webrtc/base/checks.h"
 #include "webrtc/base/buffer.h"
 
-#include <cassert>
-#include <utility>
-
 namespace rtc {
 
-Buffer::Buffer() : size_(0), capacity_(0), data_(nullptr) {
-  assert(IsConsistent());
+Buffer::BufferData::BufferData(const void* data, size_t size, size_t capacity)
+    : size_(size),
+      capacity_(std::max(size, capacity)),
+      data_(new uint8_t[capacity_]) {
+  RTC_DCHECK(IsConsistent());
+  if (data && size) {
+    std::memcpy(data_.get(), data, size);
+  }
 }
 
-Buffer::Buffer(const Buffer& buf) : Buffer(buf.data(), buf.size()) {
+Buffer::Buffer() : data_(nullptr) {
 }
 
-Buffer::Buffer(Buffer&& buf)
-    : size_(buf.size()),
-      capacity_(buf.capacity()),
-      data_(std::move(buf.data_)) {
-  assert(IsConsistent());
+Buffer::Buffer(const Buffer& buf) : data_(buf.data_) {
+}
+
+Buffer::Buffer(Buffer&& buf) : data_(std::move(buf.data_)) {

[kwiberg-webrtc, 2016/02/15 12:33:16]

If I'm reading things right, scoped_refptr isn't movable, so this will be a
normal copy. But performance will be in the ballpark of what the caller expects
of a move, so it's probably OK to leave it like this.

[tommi, 2016/02/15 14:56:51]

scoped_refptr isn't movable, so this would leave two Buffer instances pointing
to the same data.  Instead, you could do:

Buffer::Buffer(Buffer&& buf) {
  data_.swap(buf.data_);
}

This also avoids extra AddRef/Release calls.

[kwiberg-webrtc, 2016/02/15 15:05:03]

Yeah, that's better. Introducing sharing is going to be more expensive than I
thought, since it can lead to an extra clone of the data if the new copy writes
to the data before the old copy is destroyed.

Maybe scoped_ref_ptr should support move? Or maybe we should use shared_ptr...

   buf.OnMovedFrom();
 }
 
 Buffer::Buffer(size_t size) : Buffer(size, size) {
 }
 
 Buffer::Buffer(size_t size, size_t capacity)
-    : size_(size),
-      capacity_(std::max(size, capacity)),
-      data_(new uint8_t[capacity_]) {
-  assert(IsConsistent());
+    : data_(new BufferData(nullptr, size, capacity)) {
+  RTC_DCHECK(IsConsistent());
 }
 
 // Note: The destructor works even if the buffer has been moved from.
 Buffer::~Buffer() = default;
 
 };  // namespace rtc