https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66464
Jonathan Wakely <redi at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|ASSIGNED |RESOLVED
Resolution|--- |FIXED
Target Milestone|--- |5.2
--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Leo Carreon from comment #2)
> Just clarifying that my comments are to do with codecvt_utf16<char32_t> and
> codecvt_utf8<char32_t>.
>
> The way I understand it, codecvt_utf16<char32_t> should be converting
> between UTF-16 and UCS-4. UTF-16 uses 2 bytes for characters in the BMP
> (characters in the range 0x0000 to 0xFFFF) and 4 bytes (surrogate pairs) for
> characters above the BMP (0x010000 to 0x10FFFF). UCS-4 uses 4 byte values.
> Therefore, codecvt_utf16<char32_t>::max_length() should be returning 4 if
> the BOM is not taken into account.
Yes, that's now fixed.
> codecvt_utf8<char32_t> converts between UTF-8 and UCS-4. UTF-8 can use up
> to 4 bytes for characters up to the range 0x10FFFF. Therefore,
> codecvt_utf8<char32_t>::max_length() should be returning 4 if the BOM is not
> taken into account.
>
> As I said in my previous post, I'm not sure if the BOM should be accounted
> for in max_length().
I've raised that question with the C++ committee.
> If I'm not mistaken, the purpose of this function is
> to allow a user to estimate how many bytes are required to fit a UCS-4
> string when converted to either UTF-16 or UTF-8. And my guess, the BOM can
> be taken into account separately when doing the estimation. For example,
> when wstring_convert estimates the length of the std::string to be generated
> by wstring_convert::to_bytes(). It should be the number of UCS-4 characters
> multiplied by max_length() and then add the size of the BOM if required.
> The resulting std::string can be resized after the conversion to eliminate
> the unused bytes.
I believe that's the usual use case for max_length, and agree it's better to
calculate N * max_length() + length(BOM), rather than have max_length() include
the BOM, however the way max_length() is specified in the standard does suggest
it should be including the BOM. We'll discuss it in the committee and process
it as a defect report against the standard if necessary.
> Note that the comment you mentioned in your reply probably only applies to
> codecvt_utf8_utf16 which converts between UTF-8 and UTF-16 directly without
> going thru the UCS-4 conversion.
Agreed.
