Parsing a Message

The ParseMessage method of the application Engine should be invoked to parse a message. If it successfully recognizes and parses a message, it will invoke one of the message virtual functions. If the parse fails, it will throw a CHMerror exception object. The following code shows an example of how this API can be invoked:

void CMyApp::testMessage()
{
   try
   {
      cout << "Testing..." << endl;
   
      // CHMstring is a string class which ships with the CHM library.  You can
      // use any string class you like, but we provide this one.
      CHMstring Message;
      
      // Load an HL7 message into the string Message.
      // Note the use of a double \\ to escape the \ character at the start of the message 
      // and the use of \r at the end of each segment.
      Message = "MSH|^~\\&amp;|REGADT|MCM|RSP1P8|MCM|199601051530|SEC|ADT^A44|00000007|P|2.3\r"; 
      Message = Message + "EVN|A44|199601051530\r"; 
      Message = Message + "PID|||MR2^^^XYZ||JONES^WILLIAM^A^JR||19501010|M|||123 EAST\r"; 
      Message = Message + "STREET^^NY^NY^10021||(212)111-3333|||S||ACCT1\r";
      Message = Message + "MRG|MR1^^^XYZ||ACCT1\r" ;
      
      m_Engine.ParseMessage(Message, 0); 
   }
   catch (CHMerror& Error) 
   {
      cout << "Error during parsing:" << Error;
   }
}

This is a call sequence:

An object invokes ParseMessage method on the HL7appEngine.
The HL7appEngine may have an implementation of this method if the user has chosen to override it for error handling and/or logging purposes (see below). After this, the call passes up the chain to the parent stubcode class HL7Engine.
This method invokes the core Chameleon engine. If a parsing error is found, an exception is thrown. If not, the class calls the virtual function for the appropriate message.
At this point, the HL7appEngine object now takes over. The user is responsible for creating these functions.

The following code fragment gives an idea of what a typical message virtual method would look like when the user overrides the ServerEngine class:

void HL7appEngine::msgPatientUpdate
(
   const HL7MsgPatientUpdate Message,
   size_t                    TransportIndex
) 
{
   strstream Output;
   Output << Message.Patient(0).FirstName() << NewLine();
   Output << Message.Patient(0).LastName() << NewLine();
   :
   // We send the contents of the strstream buffer to the log
   // which is implemented in the CHMoutputLog object assigned to this
   // engine object.
   OutputStream(Output);   
)

Example engine message virtual function

Overriding the ParseMessage Method

There are many times where it's helpful to override the ParseMessage method in an application. Some good reasons are:

Implementing standard ACKnowledgement for messages using a technique like auto-acknowledgement.
Logging each incoming message before parsing it.
Error handling - exceptions can be caught in a central place for uniform handling.

The following code fragment shows an example of how you might go about overriding the ParseMessage method in an application engine object:

void HL7appEngine::ParseMessage
(
   const char* pFlatWire, 
   size_t TransportIndex
)
{
   try
   {
      // Additional logging could be performed here.
      HL7Engine::ParseMessage(pFlatwire, TransportIndex);
      // We could also implement auto ACKing here.
   }
   catch (CHMerror& Error)
   {
      cout << Error;
      try
      {
          makeReturnACK(TransportIndex, "UNKNOWN");
      }
      catch (CHMerror& Error2)
      {
         cout << "Couldn't make NACK message." <<  Error2;
      }
   }
}

Overriding ParseMessage

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Parsing a Message

Overriding the ParseMessage Method