3619. Specification of vformat_to contains ill-formed formatted_size calls

Section: 28.5.5 [format.functions] Status: C++23 Submitter: Tim Song Opened: 2021-10-17 Last modified: 2023-11-22

Priority: Not Prioritized

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Discussion:

The specification of vformat_to says that it formats "into the range [out, out + N), where N is formatted_size(fmt, args...) for the functions without a loc parameter and formatted_size(loc, fmt, args...) for the functions with a loc parameter".

This is wrong in at least two ways:

• First, args is a (w)format_args, not a pack, so it doesn't make sense to use ... to expand it.

• Second, fmt is a (w)string_view parameter, and it is never a constant expression, and so the call is ill-formed after P2216 added compile-time format string checking.

[2022-01-29; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[2022-02-10 Approved at February 2022 virtual plenary. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4892.

1. Modify 28.5.5 [format.functions] as indicated:

   template<class Out>
     Out vformat_to(Out out, string_view fmt, format_args args);
   template<class Out>
     Out vformat_to(Out out, wstring_view fmt, wformat_args args);
   template<class Out>
     Out vformat_to(Out out, const locale& loc, string_view fmt, format_args args);
   template<class Out>
     Out vformat_to(Out out, const locale& loc, wstring_view fmt, wformat_args args);
   

   -12- Let charT be decltype(fmt)::value_type.

   -13- Constraints: Out satisfies output_iterator<const charT&>.

   -14- Preconditions: Out models output_iterator<const charT&>.

   -15- Effects: Places the character representation of formatting the arguments provided by args, formatted according to the specifications given in fmt, into the range [out, out + N), where N is formatted_size(fmt, args...) for the functions without a loc parameter and formatted_size(loc, fmt, args...) for the functions with a loc parameterthe number of characters in that character representation. If present, loc is used for locale-specific formatting.

   -16- Returns: out + N.

   -17- […]