const char* to double translation issue with C++

Question

I have two sample applications using the same library and the main difference between them is that one uses qt and the other application is a console application.

In the common library, I have this test code:

double test = 0.1;
double test2 = atof("2.13134");
double test3 = atof("1,12345");

The values if I use the non-qt application are:

test = 0.10000000000001
test2 = 2.1323399999999999998
test3 = 1   // This is the expected result using a ',' as delimitation character

But with the qt application:

test = 0.10000000000001
test2 = 2     // This is not expected!!!
test3 = 1.1234500000000000001

Is there any case where the behaviour of the 'atof' changes because qt?

Answer 1

std::atof depends on the currently set locale to tell it which character is the decimal point. In the default case ("C locale"), that is the period character ' . '.

It's likely that Qt is setting the locale to something else. You can revert that using the standard C[++] mechanism :

std::setlocale(LC_ALL, "C");

Answer 2

The problem you are noticing is most likely caused by Qt's notion of locale. You can use:

QLocale::setDefault(QLocale::C);

to make it work like atof .

Update

It seems QLocale::setDefault does not set the default locale used by Qt. It merely sets the default locale that will be created when you construct a QLocale . See Changing locale in Qt and the accepted answer for more info.