2011/4/7 Vincent Agnus <vincent.agnus@ircad.fr>

thanks for our answer !

Your solution work, but it is not exactly I want. Indeed this expression return the typename of a tag (for example literal-char) instead its value ( "A" , "B" , .... ).

Does my first suggestion solve your first attempt?
i.e. (ostream&)m_error << ...

for example if my start rule is
start = qi::char_("a-z") > qi::char_("!");

and I parse the string "a#" then I expect that _4 contain "!" , phx::bind(&boost::spirit::info::tag, qi::_4) return "literal-char"

2 questions :
a) I have a look in boost::spirit::info class is the boost::spirit::info::value member is containing the data I want ( ie "!")

True here, but it's a more subtle thing than I can explain...
Not every case can you get a utf8_string form boost::spirit::info::value.

For example, "... > qi::char_('a', 'z')" you can get the value as a utf8_string "a-z";
while "... > qi::char_("a-z")" you can only get a nil object from value.

b) perhaps it exists an alternative like
phx::construct<>( phx::val("") << _4 )

I don't know what you mean here.