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From "Cliff Jansen (Interop Systems Inc)" <v-clj...@microsoft.com>
Subject RE: c++ messaging api: a map message example
Date Tue, 04 Aug 2009 20:39:51 GMT
Hi Gordon,

> Jonathan previously requested support for the stream operator and I've
> had a tentative stab at exploring what that might look like as well.

In addition, I would like to see the some stream-ish functions that
are friendly to char* blobs that are not null terminated.  Something
along the lines of:

  MessageContent& MessageContent::write (const char* s, int n);
  MessageContent& MessageContent::read (const char* s, int n);  // + rewind() or seek

As well as something like

  int MessageContent::size(); // (or getSize(), getLength()...)

  Message::Message(const char* s, int n);

Note that std::string is not copy-on-write in all C++ implementations.
On Windows, forcing an application to use strings for its raw data (from
a char* perspective) results in needless copies of the content:

  string tmpdata(myBlobPtr, myBlobSize);   // memory copy #1
  Message m(tmpdata); // m.impl->bytes gets copy #2 when constructed
  // tmpdata never used again, freed when out of scope

The programmer can take evasive action:

  Message m;  // m.impl->bytes starts out as a sacrificial empty string
  m.getBytes().assign(myBlobPtr, myBlobSize);  // just one copy

but

  Message m(myBlobPtr, myBlobSize);

might create the FrameSet directly, and cut out another middleman.

Since MessageContent already has abstract content (string, MAP, and
LIST), extentending the set to include char* could allow performance
optimizations:

  For incoming messages, the actual content could be copied from the
  FrameSet directly to an application buffer.  (This can be a win even
  on Linux when messages are larger than the connection's
  maxFrameSize.)  The string form can be lazilly created when needed.

  For outgoing messages, the content may be placed directly in special
  OS memory in preparation for network transfer.


Cliff


-----Original Message-----
From: Gordon Sim [mailto:gsim@redhat.com]
Sent: Friday, July 31, 2009 10:11 AM
To: Qpid Dev; users@qpid.apache.org
Subject: c++ messaging api: a map message example

Attached is a patch for a higher level, protocol independent messaging api from c++ as last
discussed some months ago. I have not had much time to spend on this since then and so there
is not a huge amount of change (mainly moving to a better namespace and adjusting for the
revised include directory layout).

There is however now an example of a sending a map message using the amqp0-10 encoding (list
messages are also supported) and the message headers are now correctly sent. Jonathan previously
requested support for the stream operator and I've had a tentative stab at exploring what
that might look like as well.

As always keen to get any feedback. I'm hoping to have more time to spend on this for the
next few weeks. I'm aiming to align the c++ api with the similar work on python (at least
conceptually) and to flesh out the functionality (determining how best to expose asynchronous
completion of sends and acknowledgements will be a likely next step).

--Gordon.

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