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Boost :: Spirit - on_error not printing

I am trying to use the on_error Boost :: Spirit :: qi mechanism to find out why the parsing failed.

I set a breakpoint in the on_error function, and the function is called, but there is no output (nada, nothing, void, ...).

Simple on_error :

 on_error<fail>(level1, boost::phoenix::ref(std::cout) << "I've failed.\n" );

Complex on_error (from different sites):

 on_error<fail> ( start, boost::phoenix::ref(std::cout) << val("Error! Expecting ") << _4 << val(" here: \"") << construct<std::string>(qi::_3, qi::_2) << val("\"") << std::endl );

Here is my class containing a simple on_error :

 template <typename Iterator, typename Skipper> struct Event_Compound : qi::grammar<Iterator, Skipper> { Event_Compound () : Event_Compound::base_type(start, "Compound-Event") { using qi::lexeme; using qi::lit; using namespace qi; using boost::spirit::ascii::char_; relational_operator = lit("&&")[Matched_Relational_AND] || lit("||")[Matched_Relational_OR] ; compound = level1[Matched_Nested_Level1_Begin] >> relational_operator[Matched_Relational_Operator] >> level1[Matched_Nested_Level1_End] ; compare_or_compound = compound[Matched_Compound] | grammar_comparison_event[Matched_Comparison_Event] ; level1 = grammar_boolean_event[Matched_Boolean_Event] | ( char_('(')[Matched_Open_Paren] >> compare_or_compound[Matched_Compare_Or] >> char_(')')[Matched_Close_Paren] ) ; start = level1[Matched_Level1_Begin] >> relational_operator[Matched_Relational_Operator] >> level1[Matched_Level1_End] ; on_error<fail>(level1, boost::phoenix::ref(std::cout) << "I've failed.\n" ); } Event_Boolean<Iterator, Skipper> grammar_boolean_event; Event_Comparison<Iterator, Skipper> grammar_comparison_event; qi::rule<Iterator, Skipper> level1; qi::rule<Iterator, Skipper> compound; qi::rule<Iterator, Skipper> compare_or_compound; qi::rule<Iterator, Skipper> relational_operator; qi::rule<Iterator, Skipper> start; };

Is there a simple method for tracking parser behavior or thinking patterns? (For example, specifying a preprocessor macro or some flag variable)

Why is there no output from on_error ?

Also, what do _1, _2, _3 and _4 link do?

I am trying to debug a grammar, and I have a conclusion about the rules that were mapped, but when the rule is not mapped, I want to know which rule and why.

I use:

  • Boost 1.57.0
  • Visual studio 2010
  • Windows 7

Investigated:

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1 answer

Error handling applies only to waiting points. You don't seem to have one.

To debug using grammar

  • #define BOOST_SPIRIT_DEBUG before turning on boost
  • BOOST_SPIRIT_DEBUG_NODE(node) or BOOST_SPIRIT_DEBUG_NODES((node1)(node2)...) for selecting nodes for debugging

This will show you backtracking (if any) and the distribution of the attribute in action. If you use them, local and inherited attributes will also be shown.

Please note that your rule attributes must be adapted or adapted for use in real time for debugging to work.

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Source: https://habr.com/ru/post/984256/

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