[xsd-users] no reaction on XSD_CXX_TREE_DECIMAL_PRECISION

[Boris Kolpackov]

Hi Ray,

Rizzuto, Raymond <Raymond.Rizzuto at sig.com> writes:

> One thing, however, that I don't like about the macros is that they
> apply globally.  I would really like to be able to specify the
> precision dynamically based on the input precision I read in.
> I.e., if I read in 25.99 as a string, and note that there are
> 2 digits of precision, I would want to somehow specify that
> detail for the XML double that I create from it.

You can do this with type customization. That is, you would define
a type in XML Schema that derives from the built-in double type.
You can then customize it in the generated code and override it's
parsing and serialization code to implement this logic (you will
also probably want to allow the user to specify precision manually
in case an instance of this type is not read from XML).

In case the precision for your type is fixed then there is even an
easier method which is supported in 3.2.0. You can use the
fractionDigits XML Schema facet which will then be honored during
serialization:

  <simpleType name="Timestamp">
    <restriction base="double">
      <fractionDigits value="6"/>
    </restriction>
  </simpleType>


> Also, I recall seeing an earlier posting where a small float, 
> say 0.357E-30 is truncated to 0 on output.

This will happen if the format is set to fixed and the precision
is not enough to represent this value. In 3.2.0 float and double
are saved in the automatic mode (fixed or scientific representation
if automatically selected depending on which one is more efficient)
so this rounding won't happen. The decimal type, on the other hand,
is always in the fixed format per the spec.


> I think the issue here is the way precision is defined. I would prefer
> somethin more akin to the engineering definition of significant digits.
> I.e. 0.357E-30 has 3 significant digits. If I request the precision to
> be 2 digits, then I'd expect 0.36E-30 on output.

That's exactly what you will get if you set the precision to 2:

#include <iostream>

using namespace std;

int main ()
{
  double d = 0.357E-30;
  cerr << d << endl;
  cerr.precision (2);
  cerr << d << endl;
}

Output:

3.57e-31
3.6e-31

Boris