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<H2><A NAME="s5">5. Making a Parser in C++</A></H2>
<P>Although Lex and YACC predate C++, it is possible to generate a C++ parser.
While Flex includes an option to generate a C++ lexer, we won't be using
that, as YACC doesn't know how to deal with it directly.
<P>My preferred way to make a C++ parser is to have Lex generate a plain C
file, and to let YACC generate C++ code. When you then link your
application, you may run into some problems because the C++ code by default
won't be able to find C functions, unless you've told it that those
functions are extern "C".
<P>To do so, make a C header in YACC like this:
<P>
<BLOCKQUOTE><CODE>
<PRE>
extern "C"
{
int yyparse(void);
int yylex(void);
int yywrap()
{
return 1;
}
}
</PRE>
</CODE></BLOCKQUOTE>
<P>If you want to declare or change yydebug, you must now do it like this:
<P>
<BLOCKQUOTE><CODE>
<PRE>
extern int yydebug;
main()
{
yydebug=1;
yyparse();
}
</PRE>
</CODE></BLOCKQUOTE>
<P>This is because C++'s One Definition Rule, which disallows multiple
definitions of yydebug.
<P>You may also find that you need to repeat the #define of YYSTYPE in your Lex
file, because of C++'s stricter type checking.
<P>To compile, do something like this:
<BLOCKQUOTE><CODE>
<PRE>
lex bindconfig2.l
yacc --verbose --debug -d bindconfig2.y -o bindconfig2.cc
cc -c lex.yy.c -o lex.yy.o
c++ lex.yy.o bindconfig2.cc -o bindconfig2
</PRE>
</CODE></BLOCKQUOTE>
<P>Because of the -o statement, y.tab.h is now called bindconfig2.cc.h, so take
that into account.
<P>To summarize: don't bother to compile your Lexer in C++, keep it in C. Make
your Parser in C++ and explain your compiler that some functions are C
functions with extern "C" statements.
<P>
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