Initial implementation of ServiceWorkerCache.

net/base/io_buffer.h

 // Copyright (c) 2012 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 NET_BASE_IO_BUFFER_H_
 #define NET_BASE_IO_BUFFER_H_
 
 #include <string>
 
 #include "base/memory/ref_counted.h"
@@ skipping 105 matching lines @@
   explicit StringIOBuffer(const std::string& s);
 
   int size() const { return static_cast<int>(string_data_.size()); }
 
  private:
   virtual ~StringIOBuffer();
 
   std::string string_data_;
 };
 
+// This version allows a string to be used as the storage for a write-style
+// operation, avoiding an extra data copy.  The string passed to the constructor
+// is std::string::swap'd with an interal string.
+class NET_EXPORT ZeroCopyStringIOBuffer : public IOBuffer {

[gavinp, 2014/08/14 22:02:45]

Firstly, this is really cool. Well thought out and well done. However, our
current STL implementation on most platforms is C++98/C++03 compliant, which
means that it actually is, behind the scenes, doing refcounting. So in practice
this class won't have any advantage over the existing StringIOBuffer
constructor, since under the covers a refcount is being added.

However, in C++11 the iterator invalidation semantics make a refcounting
implementation of strings implausible. So this is more future proof. I like the
future.

However, and this is a big however, swap is a bit subtle, I'd like to see that
advertised a bit better to callers. The other thing is, since this is basically
the same as a StringIOBuffer, I'd like to see it unified.

There's already a lot of users of the StringIOBuffer, so let's leave that
constructor the same. However, why not add a static create method with a name
that really calls out the operation... Perhaps add another constructor:

   StringIOBuffer::StringIOBuffer(scoped_ptr<std::string> blah);

And document that constructor as being very low cost.

(Note: you might object that the string you want to use is on the heap so
dynamically allocating it is a waste; well, true, but the string object itself
is pretty small, most of its allocation is on the heap anyway, and the
scoped_ptr<> argument conveys the transfer of ownership better than any name we
could come up with)

[jkarlin, 2014/08/15 00:13:34]

Great suggestion.  Done.

+ public:
+  // The input |s| is swapped with string_.
+  ZeroCopyStringIOBuffer(std::string* s);
+
+  std::string* string() { return &string_; }
+
+ private:
+  virtual ~ZeroCopyStringIOBuffer();
+
+  std::string string_;
+};
+
 // This version wraps an existing IOBuffer and provides convenient functions
 // to progressively read all the data.
 //
 // DrainableIOBuffer is useful when you have an IOBuffer that contains data
 // to be written progressively, and Write() function takes an IOBuffer rather
 // than char*. DrainableIOBuffer can be used as follows:
 //
 // // payload is the IOBuffer containing the data to be written.
 // buf = new DrainableIOBuffer(payload, payload_size);
 //
@@ skipping 99 matching lines @@
  public:
   explicit WrappedIOBuffer(const char* data);
 
  protected:
   virtual ~WrappedIOBuffer();
 };
 
 }  // namespace net
 
 #endif  // NET_BASE_IO_BUFFER_H_