Whatever platform an application runs on (windows, linux, mac, etc), the way to visit file system is through OS API (e.g: windows ＡＰＩ).
Take "get a file name" for example on windows, finally the name will be returned by a calling to windows API, which will return char* or wchar* for sure. If it's wchar*, it's UTF16 encoding. So if running a unicode application, it will get utf16 file name always on any routine.
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