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std::formattable

Defined in header <format> 
template< class T, class CharT >
concept formattable = /* formattable_with */<
  std::remove_reference_t<T>,
  std::basic_format_context</* fmt_iter_for */<CharT>, CharT>
>;
 (1) (since C++23)
Helper templates 
template< class CharT >
using /* fmt_iter_for */ = /* unspecified */;
 (2) (exposition only*) 
template< class T, class Context, 
          class Formatter =
              typename Context::template
                  formatter_type<std::remove_const_t<T>> >
concept /* formattable_with */ = 
  std::semiregular<Formatter> &&
  requires (Formatter& f, const Formatter& cf, T&& t, Context fc, 
            std::basic_format_parse_context<
                typename Context::char_type
            > pc) {
    { f.parse(pc) } -> std::same_as<typename decltype(pc)::iterator>;
    { cf.format(t, fc) } -> std::same_as<typename Context::iterator>;
  };
 (3) (exposition only*)

The concept formattable specifies that std::formatter<std::remove_cvref_t<T>, CharT> meets the requirements of BasicFormatter and Formatter (if std::remove_reference_t<T> is const-qualified).

The exposition-only alias template /* fmt_iter_for */ yields an unspecified type that satisfies std::output_iterator<const CharT&>.

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 3925 C++23 the second template argument of std::basic_format_context was not provided provided

See also

(C++20)
defines formatting rules for a given type
(class template)
(C++20)
abstracts formatting operations for a given formatting argument type and character type
(named requirement)
(C++20)
defines functions used by the formatting library
(named requirement)

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