22 General utilities library [utilities]

22.14 Formatting [format]

22.14.9 Tuple formatter [format.tuple]

1
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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> typename FormatContext::iterator format(see below& elems, FormatContext& ctx) const; }; }
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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-align width tuple-type
tuple-fill-and-align:
tuple-fill align
tuple-fill:
any character other than { or } or :
tuple-type:
m
n
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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].
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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 77.
Table 77: 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;
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Effects: Equivalent to: separator_ = sep;
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constexpr void set_brackets(basic_string_view<charT> opening, basic_string_view<charT> closing) noexcept;
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Effects: Equivalent to: opening-bracket_ = opening; closing-bracket_ = closing;
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template<class ParseContext> constexpr typename ParseContext::iterator parse(ParseContext& ctx);
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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().
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Returns: An iterator past the end of the tuple-format-spec.
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template<class FormatContext> typename FormatContext::iterator format(see below& elems, FormatContext& ctx) const;
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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...>&.
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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_.
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Returns: An iterator past the end of the output range.