C++ named requirements: BasicFormatter (since C++20)

BasicFormatter is a type that abstracts formatting operations for a given formatting argument type and character type. Specializations of std::formatter are required to meet the requirements of BasicFormatter .

A BasicFormatter is a Formatter if it is able to format both const and non-const arguments.

Requirements

A type satisfies BasicFormatter if it is semiregular, meaning it satisfies:

• DefaultConstructible
• CopyConstructible
• CopyAssignable
• Destructible
• Swappable

And, given the following types and values, the expressions shown in the table below are valid and have the indicated semantics:

Type	Definition
CharT	a character type
Arg	a formatting argument type
Formatter	a Formatter type for types Arg and CharT
OutputIt	a LegacyOutputIterator type
ParseCtx	std:: basic_format_parse_context < CharT >
FmtCtx	std:: basic_format_context < OutputIt, CharT >
Value	Definition
f	a value of type (possibly const-qualified) Formatter
g	a value of type Formatter
arg	an lvalue of type Arg
t	a value of type convertible to (possibly const-qualified) Arg
parse_ctx	an lvalue of type ParseCtx satisfying all following conditions: parse_ctx. begin ( ) points to the beginning of the format-spec of the replacement field being formatted in the format string . If format-spec is not present or empty, then either parse_ctx. begin ( ) == parse_ctx. end ( ) or * parse_ctx. begin ( ) == '}' .
fmt_ctx	an lvalue of type FmtCtx

Expression	Return type	Semantics
g. parse ( parse_ctx )	ParseCtx::iterator	In the range [ parse_ctx. begin ( ) , parse_ctx. end ( ) ) , parses the format-spec for type Arg until the first unmatched character. Throws std::format_error unless the whole range is parsed or the unmatched character is } . [note 1] Stores the parsed format specifiers in g and returns an end iterator of the parsed range.
f. format ( arg, fmt_ctx )	FmtCtx::iterator	Formats arg according to the specifiers stored in f , writes the output to fmt_ctx. out ( ) and returns an end iterator of the output range. The output shall only depend on arg , fmt_ctx. locale ( ) , the range [ parse_ctx. begin ( ) , parse_ctx. end ( ) ) from the last call to f. parse ( parse_ctx ) , and fmt_ctx. arg ( n ) for any value n of type std::size_t .

1. ↑ This allows formatters to emit meaningful error messages.

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 3892	C++20	the value of pc. begin ( ) was unclear if format-spec is not present	made clear