std:: range_formatter

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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.

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.

assert(std::format("{}", views::iota(1, 5)) == "[1, 2, 3, 4]");
assert(std::format("{:n}", views::iota(1, 5)) == "1, 2, 3, 4");

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> .

std::array ints{12, 10, 15, 14};
 
assert(std::format("{}", ints) == "[12, 10, 15, 14]");
assert(std::format("{::X}", ints) == "[C, A, F, E]");
assert(std::format("{:n:_^4}", ints) == "_12_, _10_, _15_, _14_");

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:

std::pair , or
std::tuple such that std:: tuple_size_v < T > == 2 is true .

std::array char_pairs
{
    std::pair{'A', 5}, std::pair{'B', 10}, std::pair{'C', 12}
};
 
assert(std::format("{}", char_pairs) == "[('A', 5), ('B', 10), ('C', 12)]");
assert(std::format("{:m}", char_pairs) == "{'A': 5, 'B': 10, 'C': 12}");

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 .

std::array star{'S', 'T', 'A', 'R'};
 
assert(std::format("{}", star) == "['S', 'T', 'A', 'R']");
assert(std::format("{:s}", star) == "STAR");
assert(std::format("{:?s}", star) == "\"STAR\"");

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* )

Member functions

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

std::range_formatter:: set_separator

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

Assigns sep to separator_ .

std::range_formatter:: 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::range_formatter:: underlying

constexpr std:: formatter < T, CharT > & underlying ( ) ;	(1)
constexpr const std:: formatter < T, CharT > & underlying ( ) const ;	(2)

Returns underlying_ (the underlying formatter).

std::range_formatter:: parse

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 .

std::range_formatter:: format

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.

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

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Standard format specification
Formatting functions
format (C++20)
format_to (C++20)
format_to_n (C++20)
formatted_size (C++20)
vformat (C++20)
vformat_to (C++20)
Format strings
basic_format_string format_string wformat_string (C++20) (C++20) (C++20)
runtime_format (C++26)
Formatting concepts
formattable (C++23)
Formatter
formatter (C++20)
formatter < pair-or-tuple > (C++23)
formatter < range > (C++23)
range_formatter (C++23)
enable_nonlocking_formatter_optimization (C++23)
basic_format_parse_context format_parse_context wformat_parse_context (C++20) (C++20) (C++20)
basic_format_context format_context wformat_context (C++20) (C++20) (C++20)
range_format (C++23)
format_kind (C++23)
Formatting arguments
basic_format_arg (C++20)
basic_format_arg::handle (C++20)
basic_format_args format_args wformat_args (C++20) (C++20) (C++20)
visit_format_arg (C++20) (deprecated in C++26)
make_format_args make_wformat_args (C++20) (C++20)
Format error
format_error (C++20)