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according to qt4.8 doc:

If you want to use the standard QIODevice functions read(), readLine(), write(), etc., you must first connect the socket directly to a peer by calling connectToHost().
but:

Qt Code: 
  1. udp.connectToHost(hostName, hostPort);   // connected
  2. udp.bind(hostName, hostPort);  // trigger Q_CHECK_STATE, require unconnected states
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or
Qt Code: 
  1. udp.bind(hostName, hostPort);  // bound
  2. udp.connectToHost(hostName, hostPort);   // cannot connect to host, leave in error states
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