I am trying to solve the relatively simple problem of being able to write a double to a file and then to read the file into a double again . Based on this answer I decided to use the human readable format.
I have successfully circumvented the problems some compilers have with nan and [-]infinity according to this question . With finite numbers I use the std::stod
function to convert the string representation of a number into the number itself. But from time to time the parsing fails with numbers close to zero, such as in the following example:
#include <cmath>
#include <iostream>
#include <sstream>
#include <limits>
const std::size_t maxPrecision = std::numeric_limits<double>::digits;
const double small = std::exp(-730.0);
int main()
{
std::stringstream stream;
stream.precision(maxPrecision);
stream << small;
std::cout << "serialized: " << stream.str() << std::endl;
double out = std::stod(stream.str());
std::cout << "de-serialized: " << out << std::endl;
return 0;
}
On my machine the result is:
serialized: 9.2263152681638151025201733115952403273156653201666065e-318
terminate called after throwing an instance of 'std::out_of_range'
what(): stod
The program has unexpectedly finished.
That is, the number is too close to zero to be properly parsed. At first I thougth that the problem is that this number is denormal , but this doesn't seem to be the case, since the mantissa starts with a 9 and not a 0.
Qt on the other hand has no problems with this number:
#include <cmath>
#include <limits>
#include <QString>
#include <QTextStream>
const std::size_t maxPrecision = std::numeric_limits<double>::digits;
const double small = std::exp(-730.0);
int main()
{
QString string = QString::number(small, 'g', maxPrecision);
QTextStream stream(stdout);
stream.setRealNumberPrecision(maxPrecision);
stream << "serialized: " << string << '\n';
bool ok;
double out = string.toDouble(&ok);
stream << "de-serialized: " << out << '\n' << (ok?"ok":"not ok") << '\n';
return 0;
}
Outputs:
serialized: 9.2263152681638151025201733115952403273156653201666065e-318
de-serialized: 9.2263152681638151025201733115952403273156653201666065e-318
ok
Summary:
Answering question #2:
This is probably my "C-way" kind of thinking, but you could copy the double
into a uint64_t
(mem-copying, not type-casting), serialize the uint64_t
instead, and do the opposite on de-serialization.
Here is an example (without even having to copy from double
into uint64_t
and vice-versa):
uint64_t* pi = (uint64_t*)&small;
stringstream stream;
stream.precision(maxPrecision);
stream << *pi;
cout << "serialized: " << stream.str() << endl;
uint64_t out = stoull(stream.str());
double* pf = (double*)&out;
cout << "de-serialized: " << *pf << endl;
Please note that in order to avoid breaking strict-aliasing rule , you actually do need to copy it first, because the standard does not impose the allocation of double
and uint64_t
to the same address-alignment:
uint64_t ismall;
memcpy((void*)&ismall,(void*)&small,sizeof(small));
stringstream stream;
stream.precision(maxPrecision);
stream << ismall;
cout << "serialized: " << stream.str() << endl;
ismall = stoull(stream.str());
double fsmall;
memcpy((void*)&fsmall,(void*)&ismall,sizeof(small));
cout << "de-serialized: " << fsmall << endl;
If you're open to other recording methods you can use frexp
:
#include <cmath>
#include <iostream>
#include <sstream>
#include <limits>
const std::size_t maxPrecision = std::numeric_limits<double>::digits;
const double small = std::exp(-730.0);
int main()
{
std::stringstream stream;
stream.precision(maxPrecision);
int exp;
double x = frexp(small, &exp);
//std::cout << x << " * 2 ^ " << exp << std::endl;
stream << x << " * 2 ^ " << exp;
int outexp;
double outx;
stream.seekg(0);
stream >> outx;
stream.ignore(7); // >> " * 2 ^ "
stream >> outexp;
//std::cout << outx << " * 2 ^ " << outexp << std::endl;
std::cout << small << std::endl << outx * pow(2, outexp) << std::endl;
return 0;
}
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