Getting different values from std::stod() and QString::toDouble() in Qt5 - qt

Question

I started a Qt project and linked to it some C++ code I wrote to parse a file. This code is using std::stod() to parse double values, and works fine in a plain c++ project, but when used with a Qt application, std::stod() returns only the integer part of the number.
I wrote and ran some test code, one compiled with g++ 6.1, and the other with qmake 5.6 and the same g++. The results are the same as my projects results.
code compiled with g++ :
#include <iostream>
int main(int argc, char ** argv)
{
const std::string number("3.14");
double dbl = std::stod(number);
std::cout << dbl << '\n'; // 3.14
return 0;
}
It shows the good value : 3.14
code compiled with Qt :
#include <QCoreApplication>
#include <QDebug>
#include <iostream>
int main(int argc, char *argv[])
{
QCoreApplication a(argc, argv);
const std::string snumber("3.14");
const QString qnumber = QString::fromStdString("3.14");
double std_d = std::stod(snumber);
double qt_d = qnumber.toDouble();
qDebug() << std_d << qt_d; // 3 3.14
std::cout << std_d << ' ' << qt_d << '\n'; // 3 3.14
return a.exec();
}
Can you tell me why std::stod() behaves like this ?

Answer 1

Because std::stod is broken beyond repair and should burn in hell is implemented in terms of std::strtod, which interprets doubles according to the current locale. QString instead always uses the C locale (use QLocale for locale-specific conversions).
Creating the QCoreApplication instance causes a call to setlocale(LC_ALL, ""), which sets the process' locale according to the environment, thus changing std::stod behavior.