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4627. Encoding of duration unit suffixes is incorrect for wide streams

Section: 30.5.11 [time.duration.io] Status: New Submitter: Victor Zverovich Opened: 2026-09-27 Last modified: 2026-09-27

Priority: Not Prioritized

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Discussion:

30.5.11 [time.duration.io] specifies operator<< for duration as if by:

basic_ostringstream<charT, traits> s;
s.flags(os.flags());
s.imbue(os.getloc());
s.precision(os.precision());
s << d.count() << units-suffix;
return os << s.str();

For Period::type equal to micro, units-suffix is implementation-defined to be either "μs" ("\u00b5\u0073") or "us".

The type and encoding of units-suffix are not specified in terms of charT. In particular, if "μs" denotes an ordinary string literal, then for a wide stream the expression above inserts a const char* into a basic_ostream<wchar_t>. 31.7.6.3.4 [ostream.inserters.character] specifies this operation by applying out.widen() to each individual char.

This does not perform transcoding. For example, when the ordinary literal encoding is UTF-8, the micro sign is represented by multiple char code units. Widening those code units individually does not produce the corresponding micro sign in the wide literal encoding.

For example:

std::wostringstream os;
os << std::chrono::microseconds{1};

should be able to produce:

1μs

when the implementation chooses the μs representation, but the current specification does not provide semantics that produce that result.

The same problem affects %q in 30.12 [time.format], whose replacement is specified as "the duration's unit suffix as specified in 30.5.11 [time.duration.io]".

This is similar to LWG 4522⁽ⁱ⁾, which clarified that formatting to wchar_t requires transcoding rather than widening narrow code units.

The unit suffix should therefore be specified directly as a sequence of charT, using the literal encoding associated with charT.

Proposed resolution:

This wording is relative to N5054.

1. Modify 30.5.11 [time.duration.io] as indicated:

   template<class charT, class traits, class Rep, class Period>
     basic_ostream<charT, traits>&
       operator<<(basic_ostream<charT, traits>& os, const duration<Rep, Period>& d);
   

   -1- Effects: Inserts the duration d onto the stream os as if it were implemented as follows:

   basic_ostringstream<charT, traits> s;
   s.flags(os.flags());
   s.imbue(os.getloc());
   s.precision(os.precision());
   s << d.count() << units-suffix;
   return os << s.str();
   

   where units-suffix depends onis a character sequence of charT determined by the type Period::type as follows. In the following list, each quoted character sequence denotes the corresponding STATICALLY-WIDEN<charT> character sequence: […]

   [Drafting note:

   (1) The existing list is unchanged, including:

   Otherwise, if Period::type is micro, it is implementation-defined whether units-suffix is "μs" ("\u00b5\u0073") or "us".

   (2) Since %q in 30.12 [time.format] refers to the duration's unit suffix as specified in 30.5.11 [time.duration.io], this change also specifies the appropriate charT representation for %q. ]