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’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
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
qi::hold is one way about it, as correctly mentioned by @Andrzej I think I have a few observations that might help, as well as a better solution. The point is that Spirit will not require ‘temp’ storage for attributes by design. In fact, it can’t really assume the attribute be copyable in the first place. … Read more
This a really good question (and also a can of worms) because it gets at the interface of qi and phoenix. I haven’t seen an example either, so I’ll extend the article a little in this direction. As you say, functions for semantic actions can take up to three parameters Matched attribute – covered in … Read more
Spirit Parsers are not designed to be used with auto in Spirit V2. This is because the underlying Proto expression templates hold references to the temporaries. You can use qi::copy() (existing in the trunk after boost_1_55_0, not in any released version at this time) boost::proto::deep_copy or BOOST_SPIRIT_AUTO (first coined here) I’ve written about these things … Read more
In general the following directives are helpful for inhibiting/switching skippers mid-grammar: qi::lexeme [ p ]which inhibits a skipper, e.g. if you want to be sure you parse an identifier without internal skips) – see also no_skip for comparison qi::raw [ p ]which parses like always, including skips, but returns the raw iterator range of the … Read more
I’m sure Hartmut will answer in a second. Till then, this is my take: No that is not an official point. Semantic actions have some drawbacks The simplest disadvantage of semantic actions is the stylistic notion of separation of concerns. You want to express syntax in one place, and semantics in another. This helps maintainability … Read more