boost spirit qi numeric parsing of integer and floating points

Question

i am trying to make sense of the following result. The test case code is

#include 
#include 

        

Answer 1

For your specific example, I think it's actually described in the Boost Spirit documentation under RealPolicies Specialization. To make things a bit easier for you, I whipped out a quick "real" parser, that only parses real numbers and not integers(or at least it worked with your simplified examples):

template <typename T>
struct strict_real_policies : qi::real_policies<T>
{
    static bool const expect_dot = true;
};

qi::real_parser< double, strict_real_policies<double> > real;

And you can use this just like any other parser(like int_ and double_). You might have to add:

#include <boost/spirit/include/qi_numeric.hpp>

To get it to compile.

Answer 2

The problem with the double-not-int qi::double_ - qi::int_ is that an individual parser doesn't have to match the whole input to be a successful match. For "+12345.34", qi::double_ makes a successful match on the whole thing and qi::int makes a successful match on "+12345", so that qi::double_ - qi::int_ is a non-match. For difference operator, think about applying each parser separately and whether there is a valid match for each for even the first part of the input.

You can get the behavior you want by requiring some kind of boundary after qi::int_. What follows when an qi::int_ matches the first part of a float is a valid float (e.g. qi::int_ on "+12345.34" matches "+12345", leaving ".34" next on the stream). Therefore, you can do a negative look ahead for a float:

int_rule %= qi::int_ >> !qi::double_;

double_rule %= qi::double_ - int_rule; 

or

double_rule %= qi::double_ - (qi::int_ >> !qi::double_); 

!qi::double is also true for whitespace and eoi, so I think this should be pretty general for standard format. This won't work for scientific notation though.