Using Unicode in C++ source code

What is the standard encoding of C++ source code? Does the C++ standard even say something about this? Can I write C++ source in Unicode?

For example, can I use non-ASCII characters such as Chinese characters in comments? If so, is full Unicode allowed or just a subset of Unicode? (e.g., that 16-bit first page or whatever it's called.)

Furthermore, can I use Unicode for strings? For example:

Wstring str=L"Strange chars: â Țđ ě €€";

Encoding in C++ is quite a bit complicated. Here is my understanding of it.

Every implementation has to support characters from the basic source character set. These include common characters listed in §2.2/1 (§2.3/1 in C++11). These characters should all fit into one char. In addition implementations have to support a way to name other characters using a way called universal-character-names and look like \uffff or \Uffffffff and can be used to refer to Unicode characters. A subset of them are usable in identifiers (listed in Annex E).

This is all nice, but the mapping from characters in the file, to source characters (used at compile time) is implementation defined. This constitutes the encoding used. Here is what it says literally (C++98 version):

Physical source file characters are mapped, in an implementation-defined manner, to the basic source character set (introducing new-line characters for end-of-line indicators) if necessary. Trigraph sequences (2.3) are replaced by corresponding single-character internal representations. Any source file character not in the basic source character set (2.2) is replaced by the universal-character-name that des- ignates that character. (An implementation may use any internal encoding, so long as an actual extended character encountered in the source file, and the same extended character expressed in the source file as a universal-character-name (i.e. using the \uXXXX notation), are handled equivalently.)

For gcc, you can change it using the option -finput-charset=charset. Additionally, you can change the execution character used to represet values at runtime. The proper option for this is -fexec-charset=charset for char (it defaults to utf-8) and -fwide-exec-charset=charset (which defaults to either utf-16 or utf-32 depending on the size of wchar_t).

The C++ standard doesn't say anything about source-code file encoding, so far as I know.

The usual encoding is (or used to be) 7-bit ASCII -- some compilers (Borland's, for instance) would balk at ASCII characters that used the high-bit. There's no technical reason that Unicode characters can't be used, if your compiler and editor accept them -- most modern Linux-based tools, and many of the better Windows-based editors, handle UTF-8 encoding with no problem, though I'm not sure that Microsoft's compiler will.

EDIT: It looks like Microsoft's compilers will accept Unicode-encoded files, but will sometimes produce errors on 8-bit ASCII too:

warning C4819: The file contains a character that cannot be represented
in the current code page (932). Save the file in Unicode format to prevent
data loss.