Printing double without losing precision

Question

How do you print a double to a stream so that when it is read in you don\'t lose precision?

I tried:

std::stringstream ss;

double v = 0.1 * 0.1;
ss          


        

Answer 1

Don't print floating-point values in decimal if you don't want to lose precision. Even if you print enough digits to represent the number exactly, not all implementations have correctly-rounded conversions to/from decimal strings over the entire floating-point range, so you may still lose precision.

Use hexadecimal floating point instead. In C:

printf("%a\n", yourNumber);

C++0x provides the hexfloat manipulator for iostreams that does the same thing (on some platforms, using the std::hex modifier has the same result, but this is not a portable assumption).

Using hex floating point is preferred for several reasons.

First, the printed value is always exact. No rounding occurs in writing or reading a value formatted in this way. Beyond the accuracy benefits, this means that reading and writing such values can be faster with a well tuned I/O library. They also require fewer digits to represent values exactly.