Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!iuvax!rutgers!bellcore!u1100a!krohn
From: krohn@u1100a.UUCP (Eric Krohn)
Newsgroups: comp.lang.c++
Subject: Re: Using OO Languages for Compilers/Interpreters
Message-ID: <2145@u1100a.UUCP>
Date: 14 Apr 89 15:07:03 GMT
References: <16915@cup.portal.com> <3690007@wdl1.UUCP>
Reply-To: krohn@u1100a.UUCP (Eric Krohn)
Organization: Bell Communications Research, Piscataway, NJ
Lines: 52

In article <3690007@wdl1.UUCP> rme@wdl1.UUCP (Richard M Emberson) writes:
] 
] 	The yacc++ I produced from yacc yields instances of a class
] 	(called yacc) defined in a c++ header file. The grammar tables
] 	are passed in at instance creation. Thus multiple instances can
] 	exist in the same program and at the same time.

At least in System V yaccs, you also need different versions of yyparse
since the rule actions get interpolated into a giant switch statement
at the bottom of yyparse.  A fine use for virtual functions.
(I seem to recall that the first UNIX machine I used (v6 or v7)
had a separate function for the actions; so I believe that yyparse
could be inherited but the auxiliary function would have to be virtual.)

I have an abstract Yacc class that looks like:

class	Yacc	{
  public:
	int yydebug;			/* set to 1 to get debugging */
	int yychar;			/* current input token number */
	YYSTYPE yylval;			/* current token value */
	FILE	*yyin;			/* input file */
	LexLocation current;		/* current file name and line number */

	Yacc (FILE *input, long linenumber, const char *filename);

	virtual	int	yyparse ();
	virtual	int	yylex ();
	virtual	int	yyerror (const char *msg);

	long	yylineno ()	{ return (current.linenumber()); }
	};

This class depends on a straightforward sed script to make all the
yacc tables static, rename yyparse to be a member function, and omit
the global definitions of yychar and yydebug.  C++ automagically
takes care of the rest.

Multiple instances of a class derived from Yacc will run the same
parser and actions.  Different derived classes can run completely
different parsers and actions.

I originally wrote this class with the expectation of having one or
more yacc sub-parsers to handle some ambiguities in the topmost
parser, but I have not yet resorted to this because I've been
able to patch things up in the lexer.

-- 
--
Eric J. Krohn
krohn@ctt.ctt.bellcore.com  or  {bcr,bellcore}!u1100a!krohn
Bell Communications Research,	444 Hoes Ln,    Piscataway, NJ 08854