C++ named requirements: Formatter

Formatter is a type that abstracts formatting operations for a given formatting argument type and character type. Specializations of std::formatter provided by the standard library are required to meet the requirements of Formatter except as noted otherwise.

A Formatter is able to format both const and non-const arguments, typically by providing a format member function that takes a const reference.

Requirements

A type satisfies Formatter if it satisfies BasicFormatter and given.

• Arg, a formatting argument type
• CharT, a character type
• Formatter, a Formatter type for types Arg and CharT
• OutputIt, a LegacyOutputIterator type
• f, a value of type Formatter
• arg, an lvalue of type Arg
• t, a value of type convertible to (possibly const) 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
f.format(t, format_ctx)	FormatContext::iterator	Formats t 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 t, 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.
f.format(arg, format_ctx)	FormatContext::iterator	As above, but does not modify arg.

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

DR	Applied to	Behavior as published	Correct behavior
P2418R2	C++20	objects that are neither const-usable nor copyable (such as generator-like objects) are not formattable	allow formatting these objects (relaxed formatter requirements)