3881. Incorrect formatting of container adapters backed by std::string

Section: 23.6.13 [container.adaptors.format] Status: C++23 Submitter: Victor Zverovich Opened: 2023-02-10 Last modified: 2023-11-22

Priority: Not Prioritized

View all issues with C++23 status.

Discussion:

According to 23.6.13 [container.adaptors.format] container adapters such as std::stack are formatted by forwarding to the underlying container:

template<class FormatContext>
  typename FormatContext::iterator
    format(maybe-const-adaptor& r, FormatContext& ctx) const;

Effects: Equivalent to: return underlying_.format(r.c, ctx);

This gives expected results for std::stack<T> and most types of underlying container:

auto s = std::format("{}", std::stack(std::deque{'a', 'b', 'c'}));
// s == "['a', 'b', 'c']"

However, when the underlying container is std::string the output is:

auto s = std::format("{}", std::stack{std::string{"abc"}});
// s == "abc"

This is clearly incorrect because std::stack itself is not a string (it is only backed by a string) and inconsistent with formatting of ranges where non-string range types are formatted as comma-separated values delimited by '[' and ']'. The correct output in this case would be ['a', 'b', 'c'].

Here is an illustration of this issue on godbolt using {fmt} and an implementation of the formatter for container adapters based on the one from the standard: https://godbolt.org/z/P1nrM1986.

A simple fix is to wrap the underlying container in std::views::all(_t) (https://godbolt.org/z/8MT1be838).

Previous resolution [SUPERSEDED]:

This wording is relative to N4928.

1. Modify 23.6.13 [container.adaptors.format] as indicated:

   -1- For each of queue, priority_queue, and stack, the library provides the following formatter specialization where adaptor-type is the name of the template:

   namespace std {
     template<class charT, class T, formattable<charT> Container, class... U>
     struct formatter<adaptor-type<T, Container, U...>, charT> {
     private:
       using maybe-const-adaptor =                       // exposition only
         fmt-maybe-const<adaptor-type<T, Container, U...>, charT>;
       formatter<views::all_t<const Container&>, charT> underlying_;          // exposition only
   
     public:
       template<class ParseContext>
         constexpr typename ParseContext::iterator
           parse(ParseContext& ctx);
   
       template<class FormatContext>
         typename FormatContext::iterator
           format(maybe-const-adaptor& r, FormatContext& ctx) const;
     };
   }
   

   […]

   template<class FormatContext>
     typename FormatContext::iterator
       format(maybe-const-adaptor& r, FormatContext& ctx) const;
   

   -3- Effects: Equivalent to: return underlying_.format(views::all(r.c), ctx);

[2023-02-10 Tim provides updated wording]

The container elements may not be const-formattable so we cannot use the const formatter unconditionally. Also the current wording is broken because an adaptor is not range and we cannot use fmt-maybe-const on the adaptor — only the underlying container.

[Issaquah 2023-02-10; LWG issue processing]

Move to Immediate for C++23

[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N4928.

1. Modify 23.6.13 [container.adaptors.format] as indicated:

   -1- For each of queue, priority_queue, and stack, the library provides the following formatter specialization where adaptor-type is the name of the template:

   namespace std {
     template<class charT, class T, formattable<charT> Container, class... U>
     struct formatter<adaptor-type<T, Container, U...>, charT> {
     private:
       using maybe-const-container =                     // exposition only
         fmt-maybe-const<Container, charT>;
       using maybe-const-adaptor =                       // exposition only
         fmt-maybe-const<is_const_v<maybe-const-container>, adaptor-type<T, Container, U...>, charT>; // see 25.2 [ranges.syn]
         
       formatter<ranges::ref_view<maybe-const-container>Container, charT> underlying_;          // exposition only
   
     public:
       template<class ParseContext>
         constexpr typename ParseContext::iterator
           parse(ParseContext& ctx);
   
       template<class FormatContext>
         typename FormatContext::iterator
           format(maybe-const-adaptor& r, FormatContext& ctx) const;
     };
   }
   

   […]

   template<class FormatContext>
     typename FormatContext::iterator
       format(maybe-const-adaptor& r, FormatContext& ctx) const;
   

   -3- Effects: Equivalent to: return underlying_.format(r.c, ctx);