Richer error return message for boost::spirit::qi parsing

Question

I'm using Boost spirit to parse what is essentially a mathematical expression (some text held in m_formula which is a std::string)

I set

double value;
auto first = m_formula.begin();
auto last = m_formula.end();

then, for a grammar grammar, I parse m_formula:

boost::spirit::qi::phrase_parse(first, last, grammar, ascii::space, value);

Currently I have

if (first != last){
    /*ToDo - display "invalid formula " + m_formula*/
}

Is there a way I can improve the error handling, such as telling me which bit of the formula caused the parser to fail?

Answer 1

You can use expectation points, which will raise qi::expectation_failure<It> exceptions.

They contain information telling what rule failed (and source iterators pointing to the start of matching for that parser expression as well as point of throw).

qi::on_error is a mechanism to handle these expectation failures inside your grammar (in case you don't want to catch them externally.

Now, if your input is multiline, you could want to keep track of the input line/column information. The line_pos_iterator does this. If you do, look at the repository::qi::iter_pos directive to get the line/column information exposed as attributes within your rules.

I'll leave it at this for now: you can search the boost-spirit tag with any of the above keywords for samples if you like.