Re: semantic actions

[Hans Aberg]

On 27 Feb 2007, at 16:51, Bob Rossi wrote:

> I'm using bison along with it's semantic actions. In particular, I'm
> using the $$, $1, $2 constructs in order to help me build a parse  
> tree.
>
> In the past, I've place the top level AST type as a global object, and
> the top level rule would assign it's $$ to that global variable.
>
> However, I'm wondering if it's possible, after the parse is done,  
> to ask
> bison to give you back the top level's rule $$ variable. That way, I
> don't have to keep around an extra global variable.
>
> Is this question clear enough to be answerable? :)

Sure. In the past it was not possible. Perhaps if you try the pure  
parser. I tweaked my own skeleton file so that class parser contains:

#if !YYLSP_NEEDED
  int parse(parse_type& p, semantic_type& s);
  int parse(parse_type& p) {
                semantic_type s;
                return parse(p, s);
  }
#else
  int parse(parse_type& p, semantic_type& s = semantic_type(),
            location_type& l = location_type());
#endif

This way, one can get semantic and location values back as arguments.

In addition, I added some operators:
  std::istream& operator>>(parse_type& d);
  friend parser& operator>>(std::istream&, parser&);
  friend std::istream& operator>>(std::istream&, parse_type&);

This way, invocation can take place as:
  std::ifstream ifs(...);
  semantic_value sv;
  parser p(...);
  ...
  ifs >> p >> sv;  // Read and parse smenatic value
which I think is intuitive.

  Hans Aberg