Please provide us with a minimal, self-contained piece of code we can compile. We have no crystal ball to see what you're doing...
Sorry, I was sure it's so basic, that there's even no need for real source-code. Here it is: #include <string> #include <boost/spirit/include/qi.hpp> #include <boost/spirit/include/phoenix_core.hpp> #include <boost/spirit/include/phoenix_operator.hpp> namespace qi = boost::spirit::qi; namespace ascii = boost::spirit::ascii; namespace phoenix = boost::phoenix; int main() { using qi::int_; using qi::string; using qi::_1; using ascii::space; using phoenix::ref; int i1; std::string s1; std::string source; std::string::iterator first = source.begin(), last = source.end(); // compiles well: qi::phrase_parse(first, last, ( int_[ref(i1) = _1] ) , space); // doesn't compile: qi::phrase_parse(first, last, ( string[ref(s1) = _1] ) , space); }