Recent changes to 20: double to unsigned long long precision error

#20 double to unsigned long long precision error

Artyom Beilis — Tue, 26 Feb 2013 08:44:28 -0000

milestone: 1.2 --> 2.0

#20 double to unsigned long long precision error

Artyom Beilis — Tue, 26 Feb 2013 08:42:47 -0000

Ticket moved from /p/cppcms/bugs/94/

#94 double to unsigned long long precision error

Artyom Beilis — Tue, 26 Feb 2013 08:42:25 -0000

status: open --> pending

double to unsigned long long precision error

Anonymous — Mon, 25 Feb 2013 13:15:09 -0000

Hi, there is a bug with unsigned long long values. At least on my system where sizeof(unsigned long long) = sizeof(double) = 8

The problem is that internally you use double for all intergral types, which might cause
loss of precision for big unsigned long long numbers.

this fails on my system:

unsigned long long ullValue = 63660062001020978;
 double doubleValue = 63660062001020978;

 double unsignedlonglong_to_double = static_cast<double>(ullValue);
 unsigned long long double_to_unsignedlonglong = static_cast<unsigned long             long>(doubleValue);

qDebug() << "unsigned long long: " << ullValue;
qDebug() << "double: " << doubleValue;
qDebug() << "unsigned long long to double: " << unsignedlonglong_to_double;
qDebug() << "double to unsigned long long: " << double_to_unsignedlonglong;

output is:

unsigned long long: 63660062001020978 <-- compare this
double: 6.36601e+16
unsigned long long to double: 6.36601e+16
double to unsigned long long: 63660062001020976 <-- with this

cast from double <--> unsigned long long might cause problems both ways.

double to unsigned long long precision error

Anonymous — Mon, 25 Feb 2013 13:15:09 -0000

Ticket 20 has been modified: double to unsigned long long precision error
Edited By: Artyom Beilis (artyom-beilis)
_milestone updated: u'1.2' => u'2.0'

double to unsigned long long precision error

Anonymous — Mon, 25 Feb 2013 13:15:09 -0000

Ticket 94 has been modified: double to unsigned long long precision error
Edited By: Artyom Beilis (artyom-beilis)
Status updated: u'open' => u'pending'