Demonstration of apparent need for as_string when synthesizing a sequence of characters to a std::string

as_string.cpp

#include <string>
#include <iostream>
#include <sstream>

#define BOOST_SPIRIT_USE_PHOENIX_V3
#include <boost/spirit/include/qi.hpp>
#include <boost/spirit/include/phoenix.hpp>

int main() {

    using namespace boost::spirit;
    typedef istream_iterator iter_t;

    // a rule that looks for any string that sandwiches another string
    qi::rule<iter_t, locals<std::string>, qi::space_type> r;

    // Works
    r %= qi::as_string[lexeme[*(qi::char_ - ' ')]][qi::_a = _1] >> lexeme[*(qi::char_ - ' ')] >> ascii::string(qi::_a) ;
    // Does not work
    // r %= lexeme[*(qi::char_ - ' ')][qi::_a = _1] >> lexeme[*(qi::char_ - ' ')] >> ascii::string(qi::_a) ;

    std::string test("FOO BAR FOO");

    std::istringstream ss(test);
    ss.unsetf(std::ios::skipws);
    iter_t beg(ss), end;
    if (!qi::phrase_parse(beg, end, r, qi::space)) {
        std::cerr << "parse failed!\n";
        return 1;
    }
    test = ("FOO BAR BAZ");
    std::istringstream ss2(test);
    ss2.unsetf(std::ios::skipws);
    beg = iter_t(ss2);
    if (qi::phrase_parse(beg, end, r, qi::space)) {
        std::cerr << "parse succeeded, but should have failed!\n";
        return 1;
    }
    return 0;
}

Outcome: it's needed, but only because setting the local attribute in a semantic action as I do is just the same as performing an assignment, but lazy. Only attributes of an entire expression, being assigned into the rule's attribute type, get the special matching treatment that allows a sequence of characters to become either a string or a vector etc.