Writing UTF16 file with std::fstream

Question

Is it possible to imbue a std::fstream so that a std::string containing UTF-8 encoding can be streamed to an UTF-16 file?

I tried the following using the utf8-to-utf16 facet, but the result file is still UTF-8:

std::fstream utf16_stream("test.txt", std::ios_base::trunc | std::ios_base::out);
utf16_stream.imbue(std::locale(std::locale(), new codecvt_utf8_utf16<wchar_t, 
                               std::codecvt_mode(std::generate_header | std::little_endian)>);

std::string utf8_string = "\x54\\xE2\x83\xac\x73\x74";

utf16_stream << utf8_string;

References for the codecvt_utf8_utf16 facet seem to indicate it can be used to read and write UTF-8 files, not UTF-16 - is that correct, and if so, is there a simple way to do what I want to do?

Answer 1

file streams (by virtue of the requirements of std::basic_filebuf §22.4.1.4.2[locale.codecvt.virtuals]/3) do not support N:M character encoding conversions as would be the case with UTF8 internal / UTF16 external.

You'd have to build a UTF-16 string, e.g. by using wstring_convert, reinterpret it as a sequence of bytes, and output it using usual (non-converting) std::ofstream.

Or, alternatively, convert UTF-8 to wide first, and then use std::codecvt_utf16 which produces UTF-16 as a sequence of bytes, and therefore, can be used with file streams.