All XML data in the File Geodatabase API is in the form of a UTF-8 encoded string, therefore using std::wstring is not appropriate. UTF-8 is a variable length Unicode character encoding, but wstring is a wide character, or UTF-16, Unicode character encoding.
Unfortunately, C++ does not have built in support for UTF-8 encoded strings. Because of this, if you need to read or print a UTF-8 encoded string, you must first convert it to another character encoding, such as wchar_t (also known as UTF-16). The Windows run time library provides a function for this purpose: MultiByteToWideChar. Be sure to pass �??CP_UTF8�?� as the value of the first parameter.