The Annoyance Filter.pdf - Fourmilab

80 APPLICATION STRING PARSERS ANNOYANCE-FILTER §98
98. Application string parsers.
An application string parser reads files in application-defined formats (for example, word processor
documents, spreadsheets, page description languages, etc.) and returns strings included in the file.
Unlike tokenParser in “byte stream” mode, there is nothing heuristic in the operation of an application
string parser—it must understand the structure of the application data file in order to identify and
extract strings within it.
The applicationStringParser class is the virtual parent of all specific application string parsers. It
provides common services to derived classes and defines the external interface. When initialising an
applicationStringParser , the caller must supply a pointer to the mailFolder from which it will be
invoked, through which the folder’s nextByte method will be called to return decoded binary bytes of
the application file. It would be much cleaner if we could simply supply an arbitrary function which
returned the next byte of the stream we’re decoding, but that runs afoul of C++’s rules for taking the
address of class members. Consequently, we’re forced to make applicationStringParser co-operate with
mailFolder to obtain decoded bytes.
〈 Class definitions 10 〉 +≡
class applicationStringParser {
protected:
bool error , eof ; /∗ Error and end of file indicators ∗/
mailFolder ∗mf ;
virtual unsigned char get8 (void);
virtual void get8n (unsigned char ∗buf , const int n)
{ /∗ Store next n bytes into buf ∗/
for (int i = 0; (¬eof ) ∧ (i < n); i++) {
buf [i] = get8 ( );
}
}
public:
applicationStringParser(mailFolder ∗f = Λ) : error (false ) , eof (false ), mf (Λ)
{
setMailFolder (f);
}
virtual ∼applicationStringParser( )
{ }
virtual string name (void) const = 0;
void setMailFolder (mailFolder ∗f)
{
mf = f;
}
virtual bool nextString (string &s) = 0;
virtual void close (void){ error = eof = false ; } bool isError (void) const { return error ; }
bool isEOF (void) const
{
}
return eof ;
bool isOK (void) const
{
return (¬isEOF ( )) ∧ (¬isError ( ));
}
} ;