Title
Specification of vformat_to contains ill-formed formatted_size calls
Status
new
Section
[format.functions]
Submitter
Tim Song

Created on 2021-10-17.00:00:00 last changed 1 month ago

Messages

Date: 2021-10-17.11:39:01

Proposed resolution:

This wording is relative to N4892.

  1. Modify [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- […]

Date: 2021-10-17.00:00:00

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.

History
Date User Action Args
2021-10-17 11:39:01adminsetmessages: + msg12176
2021-10-17 00:00:00admincreate