Is there an icu function to create a std::wstring from an icu UnicodeString ? I have been searching the ICU manual but haven't been able to find one.
(I know i can convert UnicodeString to UTF8 and then convert to platform dependent wchar_t* but i am looking for one function in UnicodeString which can do this conversion.
The C++ standard doesn't dictate any specific encoding for std::wstring. On Windows systems, wchar_t is 16-bit, and on Linux, macOS, and several other platforms, wchar_t is 32-bit. As far as C++'s std::wstring is concerned, it is just an arbitrary sequence of wchar_t in much the same way that std::string is just an arbitrary sequence of char.
It seems that icu::UnicodeString has no in-built way of creating a std::wstring, but if you really want to create a std::wstring anyway, you can use the C-based API u_strToWCS() like this:
icu::UnicodeString ustr = /* get from somewhere */;
std::wstring wstr;
int32_t requiredSize;
UErrorCode error = U_ZERO_ERROR;
// obtain the size of string we need
u_strToWCS(nullptr, 0, &requiredSize, ustr.getBuffer(), ustr.length(), &error);
// resize accordingly (this will not include any terminating null character, but it also doesn't need to either)
wstr.resize(requiredSize);
// copy the UnicodeString buffer to the std::wstring.
u_strToWCS(wstr.data(), wstr.size(), nullptr, ustr.getBuffer(), ustr.length(), &error);
Supposedly, u_strToWCS() will use the most efficient method for converting from UChar to wchar_t (if they are the same size, then it is just a straightfoward copy I suppose).