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28 Text processing library [text]

28.5 Formatting [format]

28.5.9 Tuple formatter [format.tuple]

For each of pair and tuple, the library provides the following formatter specialization where pair-or-tuple is the name of the template:

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namespace std { template<class charT, formattable<charT>... Ts> struct formatter<pair-or-tuple<Ts...>, charT> { private: tuple<formatter<remove_cvref_t<Ts>, charT>...> underlying_; // exposition only basic_string_view<charT> separator_ = STATICALLY-WIDEN<charT>(", "); // exposition only basic_string_view<charT> opening-bracket_ = STATICALLY-WIDEN<charT>("("); // exposition only basic_string_view<charT> closing-bracket_ = STATICALLY-WIDEN<charT>(")"); // exposition only public: constexpr void set_separator(basic_string_view<charT> sep) noexcept; constexpr void set_brackets(basic_string_view<charT> opening, basic_string_view<charT> closing) noexcept; template<class ParseContext> constexpr typename ParseContext::iterator parse(ParseContext& ctx); template<class FormatContext> constexpr typename FormatContext::iterator format(see below& elems, FormatContext& ctx) const; }; template<class... Ts> constexpr bool enable_nonlocking_formatter_optimization<pair-or-tuple<Ts...>> = (enable_nonlocking_formatter_optimization<remove_cvref_t<Ts>> && ...); }

The parse member functions of these formatters interpret the format specification as a tuple-format-spec according to the following syntax:

tuple-format-spec :
tuple-fill-and-alignopt widthopt tuple-typeopt

tuple-fill-and-align :
tuple-fillopt align

tuple-fill :
any character other than { or } or :

tuple-type :
m
n

The tuple-fill-and-align is interpreted the same way as a fill-and-align ([format.string.std]).

The productions align and width are described in [format.string].

The tuple-type specifier changes the way a pair or tuple is formatted, with certain options only valid with certain argument types.

The meaning of the various type options is as specified in Table 116 .

Table 116 — Meaning of tuple-type options [tab:formatter.tuple.type]

🔗 Option	Requirements	Meaning
🔗 m	sizeof...(Ts) == 2	Equivalent to: set_separator(STATICALLY-WIDEN<charT>(": ")); set_brackets({}, {});
🔗 n	none	Equivalent to: set_brackets({}, {});
🔗 none	none	No effects

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constexpr void set_separator(basic_string_view<charT> sep) noexcept;

Effects: Equivalent to: separator_ = sep;

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constexpr void set_brackets(basic_string_view<charT> opening,
                            basic_string_view<charT> closing) noexcept;

Effects: Equivalent to: opening-bracket_ = opening; closing-bracket_ = closing;

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template<class ParseContext>
  constexpr typename ParseContext::iterator
    parse(ParseContext& ctx);

Effects: Parses the format specifiers as a tuple-format-spec and stores the parsed specifiers in *this.

The values of opening-bracket_, closing-bracket_, and separator_ are modified if and only if required by the tuple-type, if present.

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.

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template<class FormatContext>
  constexpr typename FormatContext::iterator
    format(see below& elems, FormatContext& ctx) const;

The type of elems is:

(9.1)
If (formattable<const Ts, charT> && ...) is true, const pair-or-tuple<Ts...>&.
(9.2)
Otherwise pair-or-tuple<Ts...>&.

Effects: Writes the following into ctx.out(), adjusted according to the tuple-format-spec:

(10.1)
opening-bracket_,
(10.2)
for each index I in the [0, sizeof...(Ts)):
- (10.2.1)
  if I != 0, separator_,
- (10.2.2)
  the result of writing get<I>(elems) via get<I>(underlying_), and
(10.3)
closing-bracket_.

Returns: An iterator past the end of the output range.

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