std::formatter<pair-or-tuple>

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

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_

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

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_ )

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
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Language
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Feature test macros (C++20)
Language support library
Concepts library (C++20)
Diagnostics library
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Metaprogramming library (C++11)
General utilities library
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Iterators library
Ranges library (C++20)
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Technical specifications
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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_stringformat_stringwformat_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_contextformat_parse_contextwformat_parse_context (C++20)(C++20)(C++20)
basic_format_contextformat_contextwformat_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_argsformat_argswformat_args (C++20)(C++20)(C++20)
visit_format_arg (C++20) (deprecated in C++26)
make_format_argsmake_wformat_args (C++20)(C++20)
Format error
format_error (C++20)