std:: formatter < pair-or-tuple >

From cppreference.com

Defined in header `<format>`
template < class CharT, std:: formattable < CharT > ... Ts > struct formatter < /pair-or-tuple/ < Ts... > , CharT > ;		(since C++23)

The template specialization of std::formatter for the std::pair and std::tuple allows users to convert a pair or a tuple to its textual representation as a collection of elements using formatting functions .

The exposition-only name /*pair-or-tuple*/ denotes either class template std::pair or std::tuple .

This specialization meets the Formatter requirements if ( std:: formattable < const Ts, CharT > && ... ) is true . It always meets the BasicFormatter requirements.

Format specification

The syntax of tuple-format-spec is:


tuple-fill-and-align (optional) width (optional) tuple-type (optional)

The tuple-fill-and-align is interpreted the same way as a fill-and-align except that the fill in tuple-fill-and-align is any character other than { , } , or : .

The width is described in standard format width specification .

The tuple-type changes the way a tuple is formatted, with certain options only valid with certain argument types.

The available tuple presentation types are:

m : Indicates that both opening and closing brackets should be "" while the separator should be ": " .

If m is chosen as the tuple-type , the program is ill-formed unless sizeof... ( Ts ) == 2 is true .

n : Indicates that separator, opening and closing brackets should be "" .

Member objects

Member name	Definition
`underlying_` (private)	tuple of underlying formatters of type std:: tuple < std:: formatter < std:: remove_cvref_t < Ts > , CharT > ... > ( exposition-only member object* )
`separator_` (private)	a string representing the separator of the tuple formatted result (defaults to ", " ) ( exposition-only member object* )
`opening-bracket_` (private)	a string representing the opening bracket of the tuple formatted result (defaults to "(" ) ( exposition-only member object* )
`closing-bracket_` (private)	a string representing the closing bracket of the tuple formatted result (defaults to ")" ) ( exposition-only member object* )

Member functions

set_separator	sets a specified separator for the tuple formatted result (public member function)
set_brackets	sets a specified opening and closing brackets for the tuple formatted result (public member function)
parse	parses the format specifier as specified by tuple-format-spec (public member function)
format	writes the tuple formatted output as specified by tuple-format-spec (public member function)

std::formatter< pair-or-tuple >:: set_separator

constexpr void set_separator ( std:: basic_string_view < CharT > sep ) noexcept ;

Assigns sep to separator_ .

std::formatter< pair-or-tuple >:: set_brackets

constexpr void set_brackets ( std:: basic_string_view < CharT > opening, std:: basic_string_view < CharT > closing ) noexcept ;

Assigns opening and closing to opening-bracket_ and closing-bracket_ , respectively.

std::formatter< pair-or-tuple >:: parse

template < class ParseContext > constexpr auto parse ( ParseContext & ctx ) - > ParseContext :: iterator ;

Parses the format specifiers as a tuple-format-spec and stores the parsed specifiers in the current object.

If tuple-type or the n option is present, the values of opening-bracket , closing-bracket , and separator are modified as required.

For each element e in underlying_ , calls e. parse ( ctx ) to parse an empty format-spec and, if e. set_debug_format ( ) is a valid expression, calls e. set_debug_format ( ) .

Returns an iterator past the end of the tuple-format-spec .

std::formatter< pair-or-tuple >:: format

template < class FormatContext > FormatContext :: iterator format ( /maybe-const-pair-or-tuple/ < Ts... > & elems, FormatContext & ctx ) const ;

/*maybe-const-pair-or-tuple*/ denotes:

const /*pair-or-tuple*/ , if ( std:: formattable < const Ts, CharT > && ... ) is true ,
/*pair-or-tuple*/ otherwise.

Writes the following into ctx. out ( ) as specified by tuple-format-spec , in order:

opening-bracket_ ,
for each index I in [ 0 , sizeof... ( Ts ) ) :

if I ! = 0 , separator_ ,
the result of writing std :: get < I > ( elems ) via std :: get < I > ( underlying_ ) , and

closing-bracket_ .

Returns an iterator past the end of the output range.

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 3892	C++23	the formatting of nested tuples was incorrect	corrected

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)