1. A message processing device for sending and receiving a message composed of one or more elements between different kinds of devices, comprising: parse tree holding means having a function of holding a parse tree generated from syntax descriptions defining structure of a message sent or received; parse tree scanning means connected to said parse tree holding means and having a function of, as message data to be processed is applied, scanning said parse tree to encode and decode a value and outputting encoded and decoded values; preamble processing means connected to said parse tree scanning means and having a function of processing a preamble field of a message for use in indicating whether an element exists or not in message data sent or received; and index processing means connected to said parse tree scanning means and having a function of processing an index field of a message for use in indicating what number of element is selected in message data whose one of a plurality of elements is selected to be sent or received.
1. A message processing device for sending and receiving a message composed of one or more elements between different kinds of devices, comprising: parse tree holding means having a function of holding a parse tree generated from syntax descriptions defining structure of a message sent or received; parse tree scanning means connected to said parse tree holding means and having a function of, as message data to be processed is applied, scanning said parse tree to encode and decode a value and outputting encoded and decoded values; preamble processing means connected to said parse tree scanning means and having a function of processing a preamble field of a message for use in indicating whether an element exists or not in message data sent or received; and index processing means connected to said parse tree scanning means and having a function of processing an index field of a message for use in indicating what number of element is selected in message data whose one of a plurality of elements is selected to be sent or received. 11. The message processing device as set forth in claim 1, wherein in the decoding of a message, when message data is applied, first an index field is read and stored in said index processing means, the contents of the index field are decoded to obtain an index value, a variable indicative of a choice number is initialized to 0, said parse tree scanning means scans a subsequent node of the parse tree, examination is made whether said variable and the index value are equal and if the two values are not equal, 1 is added to said variable to again return to scanning of a subsequent node of the parse tree, and when said variable and the index value are equal, the element in question is decoded.