22 General utilities library [utilities]

22.14 Formatting [format]

22.14.7 Formatting of ranges [format.range]

22.14.7.2 Class template range_formatter [format.range.formatter]

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namespace std { template<class T, class charT = char> requires same_as<remove_cvref_t<T>, T> && formattable<T, charT> class range_formatter { formatter<T, 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; constexpr formatter<T, charT>& underlying() noexcept { return underlying_; } constexpr const formatter<T, charT>& underlying() const noexcept { return underlying_; } template<class ParseContext> constexpr typename ParseContext::iterator parse(ParseContext& ctx); template<ranges::input_range R, class FormatContext> requires formattable<ranges::range_reference_t<R>, charT> && same_as<remove_cvref_t<ranges::range_reference_t<R>>, T> typename FormatContext::iterator format(R&& r, FormatContext& ctx) const; }; }
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The class template range_formatter is a utility for implementing formatter specializations for range types.
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range_formatter interprets format-spec as a range-format-spec.
The syntax of format specifications is as follows:
range-format-spec:
range-fill-and-align width n range-type range-underlying-spec
range-fill-and-align:
range-fill align
range-fill:
any character other than { or } or :
range-type:
m
s
?s
range-underlying-spec:
: format-spec
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For range_formatter<T, charT>, the format-spec in a range-underlying-spec, if any, is interpreted by formatter<T, charT>.
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The range-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 n option causes the range to be formatted without the opening and closing brackets.
[Note 1: 
This is equivalent to invoking set_brackets({}, {}).
— end note]
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The range-type specifier changes the way a range is formatted, with certain options only valid with certain argument types.
The meaning of the various type options is as specified in Table 76.
Table 76: Meaning of range-type options [tab:formatter.range.type]
Option
Requirements
Meaning
m
T shall be either a specialization of pair or a specialization of tuple such that tuple_size_v<T> is 2.
Indicates that the opening bracket should be "{", the closing bracket should be "}", the separator should be ", ", and each range element should be formatted as if m were specified for its tuple-type.
[Note 2: 
If the n option is provided in addition to the m option, both the opening and closing brackets are still empty.
— end note]
s
T shall be charT.
Indicates that the range should be formatted as a string.
?s
T shall be charT.
Indicates that the range should be formatted as an escaped string ([format.string.escaped]).
If the range-type is s or ?s, then there shall be no n option and no range-underlying-spec.
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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 specifier as a range-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 range-type or the n option, if present.
If:
  • (9.1)
    the range-type is neither s nor ?s,
  • (9.2)
    underlying_.set_debug_format() is a valid expression, and
  • (9.3)
    there is no range-underlying-spec,
then calls underlying_.set_debug_format().
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Returns: An iterator past the end of the range-format-spec.
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template<ranges::input_range R, class FormatContext> requires formattable<ranges::range_reference_t<R>, charT> && same_as<remove_cvref_t<ranges::range_reference_t<R>>, T> typename FormatContext::iterator format(R&& r, FormatContext& ctx) const;
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Effects: Writes the following into ctx.out(), adjusted according to the range-format-spec:
  • (11.1)
    If the range-type was s, then as if by formatting basic_string<charT>(from_range, r).
  • (11.2)
    Otherwise, if the range-type was ?s, then as if by formatting basic_string<charT>(from_range, r) as an escaped string ([format.string.escaped]).
  • (11.3)
    Otherwise,
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Returns: An iterator past the end of the output range.