A bit of foreground: my task required converting UTF-8 XML file to UTF-16 (with proper header, of course). And so I searched about usual ways of converting UTF-8 to UTF-16, and found out that one should use templates from <codecvt>.
But now when it is deprecated, I wonder what is the new common way of doing the same task?
(Don't mind using Boost at all, but other than that I prefer to stay as close to standard library as possible.)
std::codecvt template from <locale> itself isn't deprecated. For UTF-8 to UTF-16, there is still std::codecvt<char16_t, char, std::mbstate_t> specialization.
However, since std::wstring_convert and std::wbuffer_convert are deprecated along with the standard conversion facets, there isn't any easy way to convert strings using the facets.
So, as Bolas already answered: Implement it yourself (or you can use a third party library, as always) or keep using the deprecated API.