C++ atof/_wtof, rounding error

Question

I have a CString variable which I need to convert into double

CString sVal(_T("    4.2"));
double dbl2 = _wtof(sVal);

And I got dbl2 = 4.0000 instead of 4.2. What could be the reason for the rounding?

Can you show a complete main program including this sample please (and check it shows the problem first). Ideally, you should convert it to use standard `std::wstring`, but if not, I would change the tags to include something MS specific. — Martin Bonner supports Monica, Feb 09 '17 at 13:51
Are you in continental Europe? (In other words, is your machine set up with a decimal separator of `,`?) It may be doing a locale specific transform. — Martin Bonner supports Monica, Feb 09 '17 at 13:51
Russia, setlocale(LC_ALL, ".1251"); You're right, replacing "." to "," eliminated the problem — Nika_Rika, Feb 09 '17 at 13:57
Sorry, Russia is indeed "Continental Europe", but I don't know what you use for a decimal separator. — Martin Bonner supports Monica, Feb 09 '17 at 13:59
*"replacing '.' to ',' eliminated the problem"* - No, it didn't. It just replaced it with another problem. If you run this on a US english system, you are back to solving a bug. — IInspectable, Feb 09 '17 at 14:00

score 1 · Accepted Answer · answered Feb 10 '17 at 11:20

decimal point is one of the "items" in localization

WARNING!!! below code is not optimized for many subsequent conversions

#include <locale.h> 
#include <string>

...

CString sVal(_T("    4.2"));

std::string currentLocale = setlocale(LC_NUMERIC, NULL); //retrive current locale
setlocale(LC_NUMERIC, "C"); //change numeric locale to C, now decimal separator is '.'

double dbl2 = _wtof(sVal);

setlocale(LC_NUMERIC, currentLocale.c_str()); //return to original locale

See http://www.cplusplus.com/reference/clocale/setlocale/

C++ atof/_wtof, rounding error

1 Answers1