std::formatter<pair-or-tuple>

 

C++

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

 

Utilities library

Language support Type support (basic types, RTTI) Library feature-test macros (C++20) Dynamic memory management Program utilities Coroutine support (C++20) Variadic functions is_constant_evaluated (C++20) is_within_lifetime (C++26) initializer_list (C++11) source_location (C++20) Three-way comparison three_way_comparablethree_way_comparable_with (C++20)(C++20) strong_ordering (C++20) weak_ordering (C++20) partial_ordering (C++20) common_comparison_category (C++20) compare_three_way_result (C++20) compare_three_way (C++20) strong_order (C++20) weak_order (C++20) partial_order (C++20) compare_strong_order_fallback (C++20) compare_weak_order_fallback (C++20) compare_partial_order_fallback (C++20)   is_eqis_ltis_lteq (C++20)(C++20)(C++20)   is_neqis_gtis_gteq (C++20)(C++20)(C++20)

General utilities Date and time Function objects Formatting library (C++20)   bitset hash (C++11) Relational operators (deprecated in C++20) rel_ops::operator!=rel_ops::operator>    rel_ops::operator<=rel_ops::operator>= Integer comparison functions cmp_equalcmp_lesscmp_less_than (C++20)(C++20)(C++20)   cmp_not_equalcmp_greatercmp_greater_than (C++20)(C++20)(C++20) in_range (C++20) Swap and type operations swap ranges::swap (C++20) exchange (C++14) declval (C++11) to_underlying (C++23) forward (C++11) forward_like (C++23) move (C++11) move_if_noexcept (C++11) as_const (C++17) Common vocabulary types pair tuple (C++11) optional (C++17) any (C++17) variant (C++17) tuple_size (C++11) tuple_element (C++11) apply (C++17) make_from_tuple (C++17) expected (C++23) Elementary string conversions to_chars (C++17) from_chars (C++17) chars_format (C++17) to_chars_result (C++17) from_chars_result (C++17)

 

Formatting library

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)

 

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

See also

formatter (C++20)

defines formatting rules for a given type (class template)