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

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< cpp‎ | utility‎ | format

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)

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