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

• Arg, a formatting argument type
• CharT, a character type
• BasicFormatter, a BasicFormatter type for types Arg and CharT
• OutputIt, a LegacyOutputIterator type
• f, a value of type (possibly const) BasicFormatter
• g, a value of type BasicFormatter
• arg, an lvalue of type Arg
• ParseContext, an alias of std::basic_format_parse_context<CharT>
• FormatContext, an alias of std::basic_format_context<OutputIt, CharT>
• parse_ctx, an lvalue of type ParseContext
• format_ctx, an lvalue of type FormatContext

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

1 ↑ This allows formatters to emit meaningful error messages.