I have a program where I have a filename in a wchar_t string (it's unicode).

I want to open an ifstream to the file, but there is no wopen(), only
open(), which gets a char* string.

I'm perplexed.  Why doesn't ifstream have a wopen() method?

And, given that it doesn't, how to I know what conversion should be applied
to my unicode string to make it into a char string that will point to the
same file?
Obviously, I can convert to utf-8, but there is nothing that says open()
should treat the filename as utf-8.
It could treat it as EUC, iso-latin-1, etc.

Is there a portable way to do the "right" thing?

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