What's the proper way to use `NUMBERFMT` with `GetNumberFormatEx`? - c++

Question

I'm writing a locale-aware Node application for Windows, Mac, and Linux. Since JavaScript's handling of locales doesn't take into account a user's custom regional settings, I'm writing a native Node module to handle this for each platform.
I'm designing a function that takes a number to format and a precision (number of digits after the decimal) and returns a string. For example, if I pass in 1234.456 with precision 1, and my regional settings use "_" for groups and "/" for decimals, I would expect "1_234/5", using the appropriate locale info of the user's current language and region settings.
On the Windows side of things, I'm struggling to get the Windows API to give me exactly what I want. Here's the C++ function I'm writing:
std::string FormatNumber(const double number, const int precision)
{
std::wstringstream stream;
stream.setf(std::ios::fixed, std::ios::floatfield);
stream.precision(precision);
stream << number;
std::wstring roundedNumberStr = stream.str();
const wchar_t *numStr = roundedNumberStr.c_str();
wchar_t wBuffer[MAX_PATH];
GetNumberFormatEx(
LOCALE_NAME_USER_DEFAULT, // lpLocaleName,
0, // dwFlags,
numStr, // lpValue,
NULL, // lpFormat,
wBuffer, // lpNumberStr,
MAX_PATH // cchNumber
);
// Return a std::string using wBuffer
}
This almost works perfectly, except it always tacks on 2 decimal places regardless of the precision I use (something like "1_234/00"). To fix this, I suspect I need to pass in a parameter for lpFormat. The Windows API documentation suggests that I only need to provide values that I want to control and leave the rest to the user's settings, so I tried something like this:
NUMBERFMTW format{ precision };
GetNumberFormatEx(
LOCALE_NAME_USER_DEFAULT, // lpLocaleName,
0, // dwFlags,
numStr, // lpValue,
&format, // lpFormat,
wBuffer, // lpNumberStr,
MAX_PATH // cchNumber
);
But...that causes an ERROR_INVALID_PARAMETER error. Sadly, the documentation doesn't give me any more insight.
So how do I get this function to give me the strings I am expecting?

Answer 1

The two strings in NUMBERFMT are not optional. The other members are optional in the sense that 0 is valid but 0 does not mean they get their correct locale specific values!
Call GetLocaleInfoEx with LOCALE_SDECIMAL and LOCALE_STHOUSAND to load the "correct" strings and LOCALE_SGROUPING for the grouping. MSDN tells you which locale query to use for the other members.

Answer 2

Completely initialize your NUMBERFORMAT:
NUMBERFMTW format{NumDigits };
With what you have any members that aren't initialized have unspecified (and likely random) values.