Starting with Spirit X3

Question

I've just started using Spirit X3 and I have a little question related with my first test. Do you know why this function is returning "false"?

bool parse()
{
    std::string rc = "a 6 literal 8";

    auto iter_begin = rc.begin();
    auto iter_end = rc.end();

    bool bOK= phrase_parse( iter_begin, iter_end,
                            // ----- start parser -----

                            alpha >> *alnum >> "literal" >> *alnum

                            // ----- end parser -----
                            , space);

    return bOK && iter_begin == iter_end;
}

I've seen the problem is related with how I write the grammar. If I replace it with this one, it returns "true"

alpha >> -alnum >> "literal" >> *alnum

I'm using the Spirit version included in Boost 1.61.0.

Thanks in advance,

Sen

Answer 1

Your problem is a combination of the greediness of operator * and the use of a skipper. You need to keep in mind that alnum is a PrimitiveParser and that means that before every time this parser is tried, Spirit will pre-skip, and so the behaviour of your parser is:

• alpha parses a.
• The kleene operator starts.
• alnum skips the space and then parses 6.
• alnum skips the space and then parses l.
• alnum parses i.
• ...
• alnum parses l.
• alnum skips the space and then parses 8.
• alnum tries and fails to parse more. This completes the kleene operator with a parsed attribute of 6literal8.
• "literal" tries and fails to parse.
• The sequence operator fails and the invocation of phrase_parse returns false.

You can easily avoid this problem using the lexeme directive (barebones x3 docs, qi docs). Something like this should work:

alpha >> lexeme[*alnum] >> "literal" >> lexeme[*alnum];