Understanding Boost.spirit’s string parser

by

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 as you can see, the ‘a’ from the failed first branch of the alternative sticks around.

Either reword or employ the ‘big gun’ qi::hold[] directive:

(qi::hold [ string("a")  >> string("a") ] | string("a")),

Rewording could look like:

qi::string("a") >> -qi::string("a"),

Also, if you’re really just trying to match certain textual strings, consider:

(qi::raw [ qi::lit("aa") | "a" ]), 
// or even just
qi::string("aa") | qi::string("a"),

Now which one of these applies most, depends on your grammar.

More Related Contents:

  1. Parse quoted strings with boost::spirit
  2. Spirit-Qi: How can I write a nonterminal parser?
  3. Constraining the existing Boost.Spirit real_parser (with a policy)
  4. Boost spirit skipper issues
  5. boost spirit semantic action parameters
  6. boost::spirit::qi duplicate parsing on the output
  7. How can I use polymorphic attributes with boost::spirit::qi parsers?
  8. Understanding the List Operator (%) in Boost.Spirit
  9. How to parse csv using boost::spirit
  10. Boost Spirit: “Semantic actions are evil”?
  11. Boolean expression (grammar) parser in c++
  12. How to parse space-separated floats in C++ quickly?
  13. Assigning parsers to auto variables
  14. Cross-platform way to get line number of an INI file where given option was found
  15. How to parse ini file with Boost
  16. Spirit Qi attribute propagation issue with single-member struct
  17. How to make my split work only on one real line and be capable to skip quoted parts of string?
  18. Why does nvcc fails to compile a CUDA file with boost::spirit?
  19. How to provider user with autocomplete suggestions for given boost::spirit grammar?
  20. undefined behaviour somewhere in boost::spirit::qi::phrase_parse
  21. How to benchmark Boost Spirit Parser?
  22. Boost::Spirit::QI parser: index of parsed element
  23. parsing into several vector members
  24. What is the usefulness of `enable_shared_from_this`?
  25. How to store functional objects with different signatures in a container?
  26. Using vector as a buffer without initializing it on resize()
  27. Serializing and deserializing JSON with Boost
  28. Copy a streambuf’s contents to a string
  29. Is-braces-constructible type trait
  30. Creating a thread pool using boost

Leave a Comment