3776. Avoid parsing format-spec if it is not present or empty

Section: 28.5.6.1 [formatter.requirements] Status: NAD Submitter: Mark de Wever Opened: 2022-08-28 Last modified: 2023-06-16

Priority: 3

View all other issues in [formatter.requirements].

View all issues with NAD status.

Discussion:

A format string 28.5.2.1 [format.string.general]/1 has an optional format-specifier replacement field. If this field is not present, the current wording makes it clear the intention is to call parse when a formatting argument use a handle class 28.5.8.1 [format.arg]/19

Effects: Initializes ptr_ with addressof(val) and format_ with

   [](basic_format_parse_context<char_type>& parse_ctx,
      Context& format_ctx, const void* ptr) {
     typename Context::template formatter_type<TD> f;
     parse_ctx.advance_to(f.parse(parse_ctx));
     format_ctx.advance_to(f.format(*const_cast<TQ*>(static_cast<const TD*>(ptr)),
                                    format_ctx));
  }

For other types it is implied by [tab:formatter.basic]

Formats u according to the specifiers stored in *this, writes the output to fc.out(), and returns an iterator past the end of the output range. The output shall only depend on u, fc.locale(), fc.arg(n) for any value n of type size_t, and the range [pc.begin(), pc.end()) from the last call to f.parse(pc).

(Similar wording is used in [tab:formatter])

Before the parse function is called it is known whether or not a format-spec is present. It seems wasteful to call a function that is known not to parse anything. Therefore I propose to make the call optional.

This change is only observable for formatter specializations using the handle class, for the formatter specializations in 28.5.6.4 [format.formatter.spec] the change has no observable effect.

[2022-10-12; Reflector poll]

Set status to "LEWG" and priority to 3 after reflector poll.

Several votes for NAD as this would be a design change requiring a paper. Dissenting opinions: "Different handling of empty and default specs is entirely incidental and not a design feature. Moreover, built-in formatters treat these cases identically so it's a good idea to make this explicit." "Don't need a full paper to clarify the meaning of "don't call member parse if there's no format spec". If LEWG agree the direction PR can be polished."

[Issaquah 2023-02-07; LWG]

Will be resolved differently by P2733.

[Varna 2023-06-16; Status changed: LEWG → NAD.]

Resolved differently by LWG 3892.

Proposed resolution:

This wording is relative to N4917.

1. Modify 28.5.6.1 [formatter.requirements] as indicated:

   -3- Given character type charT, output iterator type Out, and formatting argument type T, in Table 74 and Table 75:

   […]

   pc.begin() points to the beginning of the format-spec (28.5.2 [format.string]) of the replacement field being formatted in the format string. If format-spec is not present or empty then either pc.begin() == pc.end() or *pc.begin() == '}'.

2. Modify BasicFormatter requirements [tab:formatter.basic] as indicated:

   Table 74: BasicFormatter requirements [tab:formatter.basic]

   Expression	Return type	Requirement
   …
   f.format(u, fc)	FC::iterator	Formats u according to the specifiers stored in *this, writes the output to fc.out(), and returns an iterator past the end of the output range. The output shall only depend on u, fc.locale(), fc.arg(n) for any value n of type size_t, and the range [pc.begin(), pc.end()) from the last call to f.parse(pc). When the format-spec (28.5.2 [format.string]) is not present or empty the call to f.parse(pc) is omitted.

3. Modify Formatter requirements [tab:formatter] as indicated:

   Table 75: Formatter requirements [tab:formatter]

   Expression	Return type	Requirement
   f.format(t, fc)	FC::iterator	Formats t according to the specifiers stored in *this, writes the output to fc.out(), and returns an iterator past the end of the output range. The output shall only depend on t, fc.locale(), fc.arg(n) for any value n of type size_t, and the range [pc.begin(), pc.end()) from the last call to f.parse(pc). When the format-spec (28.5.2 [format.string]) is not present or empty the call to f.parse(pc) is omitted.
   …

4. Modify 28.5.8.1 [format.arg] as indicated:

   […]

   -16- The class handle allows formatting an object of a user-defined type.

   namespace std {
     template<class Context>
     class basic_format_arg<Context>::handle {
       const void* ptr_;                                          // exposition only
       void (*format_)(basic_format_parse_context<char_type>&*,
                       Context&, const void*);                    // exposition only
   
       template<class T> explicit handle(T&& val) noexcept;       // exposition only
   
       friend class basic_format_arg<Context>;                    // exposition only
   
     public:
       void format(basic_format_parse_context<char_type>&, Context& ctx) const;
     };
   }
   

   template<class T> explicit handle(T&& val) noexcept;
   

   -17- […]

   -18- […]

   -19- Effects: Initializes ptr_ with addressof(val) and format_ with

   [](basic_format_parse_context<char_type>&* parse_ctx,
      Context& format_ctx, const void* ptr) {
     typename Context::template formatter_type<TD> f;
     if (parse_ctx) parse_ctx.->advance_to(f.parse(*parse_ctx));
     format_ctx.advance_to(f.format(*const_cast<TQ*>(static_cast<const TD*>(ptr)),
                                    format_ctx));
   }
   

   void format(basic_format_parse_context<char_type>& parse_ctx, Context& format_ctx) const;
   

   -20- Effects: If the format-spec (28.5.2 [format.string]) is not present or empty, equivalent to:

   format_(nullptr, format_ctx, ptr_);
   

   otherwise, eEquivalent to:

   format_(addressof(parse_ctx), format_ctx, ptr_);