std::formatter<pair-or-tuple>

std::formatter<pair-or-tuple> - cppreference.com

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`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

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