best way to output a full precision double into a text file

Question

I need to use an existing text file to store some very precise values. When read back in, the numbers essentially need to be exactly equivalent to the ones that were originally written. Now, a normal person would use a binary file... for a number of reasons, that's not possible in this case.

So... do any of you have a good way of encoding a double as a string of characters (aside from increasing the precision). My first thought was to cast the double to a char[] and write out the chars. I don't think that's going to work because some of the characters are not visible, produce sounds, and even terminate strings ('\0'... I'm talkin to you!)

Thoughts?

[Edit] - once I figure out which of the solutions proposed works best for me, I'll mark one as 'the' solution.

Answer 1

If you want to keep the format strictly human readable, you can write out the double thusly:

#include <iomanip>
#include <sstream>

std::string doubleToText(const double & d)
{
    std::stringstream ss;
    //ss << std::setprecision( std::numeric_limits<double>::digits10+2);
    ss << std::setprecision( std::numeric_limits<int>::max() );
    ss << d;
    return ss.str();
}

std::numeric_limits<int>::max() will output with the maximum possible decimal precision. This will preserve the value most precisely across differently floating point implementations. Swapping that line for the commented line using std::numeric_limits<double>::digits10+2 will give just enough precision to make the double precisely recoverable on the platform the code is compiled for. This gives a much shorter output and preserves as much information as the double can uniquely represent.

The C++ stream operators do not preserve denormalized numbers or the infinities and not-a-numbers when reading strings in. However, the POSIX strtod function does, and is defined to by the standard. Hence, the most precise way to read a decimal number back with a standard library call would be this function:

#include <stdlib.h>

double textToDouble(const std::string & str)
{
    return strtod( str.c_str(), NULL );
}