std:: formattable

From cppreference.com

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 & > .

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

Compiler support
Freestanding and hosted
Language
Standard library
Standard library headers
Named requirements
Feature test macros (C++20)
Language support library
Concepts library (C++20)
Metaprogramming library (C++11)
Diagnostics library
General utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
Localizations library
Input/output library
Filesystem library (C++17)
Regular expressions library (C++11)
Concurrency support library (C++11)
Execution support library (C++26)
Technical specifications
Symbols index
External libraries

Standard format specification
Formatting functions
format (C++20)
format_to (C++20)
format_to_n (C++20)
formatted_size (C++20)
vformat (C++20)
vformat_to (C++20)
Format strings
basic_format_string format_string wformat_string (C++20) (C++20) (C++20)
runtime_format (C++26)
Formatting concepts
formattable (C++23)
Formatter
formatter (C++20)
formatter < pair-or-tuple > (C++23)
formatter < range > (C++23)
range_formatter (C++23)
enable_nonlocking_formatter_optimization (C++23)
basic_format_parse_context format_parse_context wformat_parse_context (C++20) (C++20) (C++20)
basic_format_context format_context wformat_context (C++20) (C++20) (C++20)
range_format (C++23)
format_kind (C++23)
Formatting arguments
basic_format_arg (C++20)
basic_format_arg::handle (C++20)
basic_format_args format_args wformat_args (C++20) (C++20) (C++20)
visit_format_arg (C++20) (deprecated in C++26)
make_format_args make_wformat_args (C++20) (C++20)
Format error
format_error (C++20)

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