Is ther any built-in function that convert wstring or wchar_t* to UTF-8 in Linux?

The C++ language standard has no notion of explicit encodings. It only contains an opaque notion of a "system encoding", for which wchar_t is a "sufficiently large" type.

It's quite plausible that wcstombs will do what you need if what you actually want to do is convert from wide characters to the current locale.

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