How to detect a double's significant digits

[mrstephengross]

Ok, that won't work. First of all, str() is not a function. If I want
to convert the float into a string, the conversion function will have
to use some kind of numeric precision, which will screw things up.
Consider this:

float f = 1.004;
ostringstream s;
s << f;
cout << s.str();

The above code may produce "1.004", or "1.0040", or "1.00400",
depending on the stream's precision setting. I need a way to detect the
number of digits to the right of decimal point *prior* to doing any
kind of string conversion.

--Steve