< cpp‎ | utility‎ | format
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Formatting library (C++20)
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Defined in header <format>
template< class T, class CharT = char >

    requires std::same_as<std::remove_cvref_t<T>, T> && std::formattable<T, CharT>

class range_formatter;
(since C++23)

The std::range_formatter is a helper class template for implementing std::formatter specializations for range types.


[edit] Range format specification

The syntax of range-format-spec is:

range-fill-and-align (optional) width (optional) n(optional) range-type (optional) range-underlying-spec (optional)

The range-fill-and-align is interpreted the same way as a fill-and-align except that the fill in range-fill-and-align is any character other than {, }, or :.

The width is described in standard format width specification.

The n option causes the range to be formatted without the opening and closing brackets.

The format-spec in a range-underlying-spec (its syntax is equivalent to : format-spec), if any, is interpreted by the range element formatter std::formatter<T, CharT>.

The range-type changes the way a range is formatted, with certain options only valid with certain argument types.

The available range presentation types are:

  • m: 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 (in tuple-format-spec).
  • If m is chosen as the range-type, the program is ill-formed unless T is either a specialization of:
  • s: Indicates that the range should be formatted as a string.
  • ?s: Indicates that the range should be formatted as an escaped string.
  • If s or ?s is chosen as the range-type, both n option and range-underlying-spec should not be included in the format specifier, and
  • the program is ill-formed unless T is CharT.

[edit] Member objects

Member name Definition
underlying_ (private) the underlying formatter of type std::formatter<T, CharT>
(exposition-only member object*)
separator_ (private) a string representing the separator of the range formatted result. The default separator is ", ".
(exposition-only member object*)
opening-bracket_ (private) a string representing the opening bracket of the range formatted result. The default opening bracket is "[".
(exposition-only member object*)
closing-bracket_ (private) a string representing the closing bracket of the range formatted result. The default closing bracket is "]".
(exposition-only member object*)

[edit] Member functions

sets a specified separator for the range formatted result
(public member function)
sets a specified opening and closing brackets for the range formatted result
(public member function)
returns the underlying formatter
(public member function)
parses the format specifier as specified by range-format-spec
(public member function)
writes the range formatted output as specified by range-format-spec
(public member function)


constexpr void set_separator( std::basic_string_view<CharT> sep ) noexcept;

Assigns sep to separator_.


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.


constexpr std::formatter<T, CharT>& underlying();
constexpr const std::formatter<T, CharT>& underlying() const;

Returns underlying_ (the underlying formatter).


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

Parses the format specifiers as a range-format-spec and stores the parsed specifiers in the current object.

Calls underlying_.parse(ctx) to parse format-spec in range-format-spec or, if the latter is not present, an empty format-spec.

If range-type or the n option is present, the values of opening-bracket_, closing-bracket_, and separator_ are modified as required.

It calls underlying_.set_debug_format() if:

  • the range-type is neither s nor ?s,
  • underlying_.set_debug_format() is a valid expression, and
  • there is no range-underlying-spec.

Returns an iterator past the end of the range-format-spec.


template< ranges::input_range R, class FormatContext >

  requires std::formattable<ranges::range_reference_t<R>, CharT> &&
           std::same_as<std::remove_cvref_t<ranges::range_reference_t<R>>, T>

auto format( R&& r, FormatContext& ctx ) const -> FormatContext::iterator;

If the range-type was either s or ?s, it writes the formatted std::basic_string<CharT>(std::from_range, r) as a string or an escaped string, respectively, into ctx.out().

Otherwise, it writes the following into ctx.out() as specified by range-format-spec, in order:

  • opening-bracket_,
  • for each formattable element e of the range r:
  • the result of writing e via underlying_, and
  • separator_, unless e is the last element of r, and
  • closing-bracket_.

Returns an iterator past the end of the output range.

[edit] 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 ranges was incorrect corrected

[edit] See also

defines formatting rules for a given type
(class template) [edit]