Why does qjsonvalue todouble conversion cause data loss?

Question

QJsonValue toDouble() conversion generates wrong values causing data loss.

QJsonObject obj;
obj.insert("dbl",double(73183493944884961));


double d = obj.value("dbl").toDouble();
qulonglong ul = d;
qulonglong ul2 = 73183493944884961;

qDebug ()  << ul << ul2;

Output:

73183493944884960 73183493944884961

Answer 1

toDouble() means it returns a double (that you cast to qulonglong then) which is inherently a precision loss. You can easily figure this out without involving json into this, by printing out this:

main.cpp

#include <QDebug>

    int main()
    {
        qulonglong ul2 = double(73183493944884961);
        qDebug() << ul2;
        return 0;
    }

Building (something similar)

g++ -fPIC -I/usr/include/qt -I/usr/include/qt/QtCore -lQt5Core main.cpp && ./a.out

Output

73183493944884960