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

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.

[edit] 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 "".

[edit] 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*) [edit] 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

Assigns sep to separator_.

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

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 [â0â, 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.

[edit] 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 [edit] See also defines formatting rules for a given type
(class template) [edit]

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