karma real generator does not produce enough digits to guarantee round trip through spirit

[reallyasi9@…]

The real_inserter::call_n method in ​http://svn.boost.org/svn/boost/trunk/boost/spirit/home/karma/numeric/detail/real_utils.hpp limits the number of output characters representing a real (floating-point) number of type U to std::numeric_limits<U>::digits10. However, digits10 represents the number of decimal digits that are guaranteed to survive a round-trip conversion for the type. This is not the same as the number of decimal digits that a type can represent, which is std::numeric_limits<U>::max_digits10 (in c++11).

The following code describes the problem when attempting to round-trip using karma and spirit vs. using stringstreams, picking a particular value that fails:

// define a new real number formatting policy
struct precise_policy : real_policies<float>
{
  // Limited to digits10, no matter what I put here
  static unsigned int precision(float) {
    return std::numeric_limits<float>::max_digits10;
  }
};

float a = 135.477005;

std::stringstream ss;
ss.precision(std::numeric_limits<float>::max_digits10);
ss << a;
float b;
ss >> b;

assert(a == b); // succeeds

std::string st;
typedef std::back_insert_iterator<std::string> sink_type;
sink_type sink(st);
typedef real_generator<float, precise_policy> precise_type;
precise_type const precise = precise_type();
generate(sink, precise, a);

float c;
parse(st.begin(), st.end(), float_, c);

assert(a == c); // fails, even though enough digits were requested
                // with precise_policy!

One solution would be to change the maximum number of digits to std::numeric_limits<U>::max_digits10 (or the equivalent in non-c++11 code).

[Joel de Guzman]

fixed in #586 ​https://github.com/boostorg/spirit/pull/586