Re: [Boost-Users] Spirit - rule generating functions

Hi, Spirit has its own mailing list. I'll reply to your questions but it would be best to contibue our discussion there 1) to save bandwidth, and 2) so that other Spirit users (and experts :-) can chime in.... More replies at the bottom... brockpeabody wrote:

Does anyone know the best way to return a rule from a function by value? I know that the following does not work, but what's the best workaround?

struct calculator : grammar<calculator> {

template <typename ScannerT> struct definition {

definition(const calculator&) {

r = make_complex_rule('A') | make_complex_rule('B'); }

typedef rule<ScannerT> rule_type;

rule_type const& start() { return r;}

private:

rule_type make_complex_rule(char c) {

return c >> real_p; //boom }

rule_type r; }; };

my current workaround is the following nastiness:

//inside struct calculator::definition<>

rule_type& anonymous_rule() {

anonymous_rules.push_back( boost::shared_ptr(new rule_type));

return *anonymous_rules.back(); }

rule_type& make_complex_rule(char c) {

return anonymous_rule() = c >> real_p; }

std::vector > anonymous_rules;

Is there a better way?

Short story: Rules *CANNOT* be stored in containers. Long story: Actually, Spirit v1.5.1 rules can be stored as you've tried to do. Unfortunately, the C++ copy/assignment semantics differ too much from EBNF that when I tried to merge a hybrid, people complained and it caused lots of confusion (see the Boost Spirit review sometime in October 2002). That said, there is *really* a pressing need to store rules in containers. Such an ability will open the possibility to do quite interesting dynamic parsing. The consensus was to leave the rule as it is and have another class "rule_holder" that has the behavior that you desire, i.e. storable in containers. Hmmm, maybe this reply is not too helpful. Well, before we have the rule_holder, you can probably rip-off Spirit 1.5.1's rule, rename it and use it as such: http://spirit.sourceforge.net/index.php?doc=download/v1_5.html << you have to massage it a bit, e.g. change the namespace to boost::spirit, etc. >> If you wish to go this route, tell me and I'd gladly help out.

Also, I've got an easy one... Why is a grammar passed by const reference to its definition? Does Spirit make copies of the grammars internally? I ask because otherwise the grammar class seems like the ideal place to store state information (like numbers on a calculator's stack).

Short story: Grammar definitions are created upon entering the grammar's parse function. Parser parse functions are const. Long story: Arghhhh.... do you really want to hear the long story? :-) Anyway, parsers, and thus grammars, are best be lightweight. The best way to store state information in the grammar is through a reference to external state. Example: struct state { /* some vars here */ }; struct my_grammar { my_grammar(state& compiler_) state(state_) {} state& compiler; }; Then when you instantiate your grammar: state my_compiler; my_grammar g(my_compiler);

Thanks ahead of time!

Welcome ahead of time :-) -- Joel de Guzman joel at boost-consulting.com http://www.boost-consulting.com http://spirit.sf.net