> I very enjoy playing with spirit, but I was looking for a 5-line code that
> only boost have the secret...
Well, not exactly five lines, but here is a simple calculator grammar written in Spirit:
namespace qi = boost::spirit::qi;
template <typename Iterator>
struct calculator : qi::grammar<Iterator, qi::space_type>
{
calculator() : calculator::base_type(expression)
{
expression = term >> *( ('+' >> term) | ('-' >> term) ) ;
term = factor >> *( ('*' >> factor) | ('/' >> factor) ) ;
factor = qi::uint_ | '(' >> expression >> ')' |
('-' >> factor) | ('+' >> factor) ;
}
qi::rule<Iterator, qi::space_type> expression, term, factor;
};
And here is how to use it:
std::string input("1+2/3");
calculator<std::string::const_iterator> calc;
if (phrase_parse(input.begin(), input.end(), calc, qi::space))
std::cout << "successfully parsed: " << input << std::endl;