22 General utilities library [utilities]

22.14 Formatting [format]

22.14.6 Formatter [format.formatter] Concept formattable [format.formattable]

Let fmt-iter-for<charT> be an unspecified type that models output_iterator<const charT&> ([iterator.concept.output]).
template<class T, class Context, class Formatter = typename Context::template formatter_type<remove_const_t<T>>> concept formattable-with = // exposition only semiregular<Formatter> && requires(Formatter& f, const Formatter& cf, T&& t, Context fc, basic_format_parse_context<typename Context::char_type> pc) { { f.parse(pc) } -> same_as<typename decltype(pc)::iterator>; { cf.format(t, fc) } -> same_as<typename Context::iterator>; }; template<class T, class charT> concept formattable = formattable-with<remove_reference_t<T>, basic_format_context<fmt-iter-for<charT>>>;
A type T and a character type charT model formattable if formatter<remove_cvref_t<T>, charT> meets the BasicFormatter requirements ([formatter.requirements]) and, if remove_reference_t<T> is const-qualified, the Formatter requirements.