Your example I don’t see how your sample requires any of this. Just reorder your branches, then realize that the short branch is just a special case of the qualified case with n=1: Live On Coliru¹ (or using X3 version if you prefer). The generic case Now, having mentioned x3, it has the capacity to … Read more
It isn’t entirely clear to me what you are trying to achieve. Most importantly, are you not worried about operator associativity? I’ll just show simple answers based on using right-recursion – this leads to left-associative operators being parsed. The straight answer to your visible question would be to juggle a fusion::vector2<char, ast::expression> – which isn’t … Read more
From what I gather, reading the source and the mailing list, Phoenix is not integrated into X3 at all: the reason being that c++14 makes most of it obsolete. I agree that this leaves a few spots where Qi used to have elegant solutions, e.g. eps(DEFERRED_CONDITION), lazy(*RULE_PTR) (the Nabialek trick), and indeed, this case. Spirit … Read more
It’s a bug in your code, nothing wrong with the compiler or the optimization levels. The cinch is with expression templates (like the ones used by Boost Proto, and hence by Boost Spirit). They are only valid to the end of their enclosing full expression [1] The canonical workaound is: BOOST_SPIRIT_AUTO(ana, *~qi::char_(‘*’) > +qi::char_(‘*’)); You … Read more
Mmm. I feel that we have discussed a few more details in chat than have been reflected in the question as it is. Let me entertain you with my ‘toy’ implementation, complete with test cases, of a grammar that will recognize <<macros>> like this, including nested expansion of the same. Notable features: Expansion is done … Read more
You cannot use auto to store parser expressions¹ Either you need to evaluate from the temporary expression directly, or you need to assign to a rule/grammar: const qi::rule<std::string::const_iterator, qi::space_type> doubles_parser_local = qi::double_ >> *(‘,’ >> qi::double_); You can have your cake and eat it too on most recent BOost versions (possibly the dev branch) there … Read more
It’s not the string matcher per se. It’s [attribute propagation] + [backtracking] in action. A string attribute is a container attribute and many elements could be assigned into it by different parser subexpressions. Now for efficiency reasons, Spirit doesn’t rollback the values of emitted attributes on backtracking. Often this is no problem at all, but … Read more
The reference to qq becomes dangling after leaving the constructor, so that is indeed a problem. qi::locals is the canonical way to keep local state inside parser expressions. Your other option would be to extend the lifetime of qq (by making it a member of the grammar class, e.g.). Lastly, you might be interested in … Read more
Spirit is a lot friendlier to compiletime-polymorphism typedef variant<Command1, Command2, Command3> Command; But, let’s suppose you really want to do the old-fashioned polymorphism thing… Just newing-up the polymorphic objects on the fly during parsing, however, is a sure-fire way to make your parser bloated with semantic actions create lot of memory leaks on back-tracking in … Read more
I could solve the problem for this particular case (in fact we discussed options on the list), but really, this kind of ‘enigmatic’ error creeps up more often with Boost Spirit and it would be nice to get a handle on the general class of problems. Your first resource should be the excellent spirit documentation, … Read more