std::range_formatter

 

C++

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Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Metaprogramming library (C++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (C++17)

Regular expressions library (C++11)

Concurrency support library (C++11)

Technical specifications

Symbols index

External libraries

 

Utilities library

Language support Type support (basic types, RTTI) Library feature-test macros (C++20) Dynamic memory management Program utilities Coroutine support (C++20) Variadic functions is_constant_evaluated (C++20) is_within_lifetime (C++26) initializer_list (C++11) source_location (C++20) Debugging support is_debugger_present (C++26) breakpoint_if_debugging (C++26) breakpoint (C++26) Three-way comparison three_way_comparablethree_way_comparable_with (C++20)(C++20) strong_ordering (C++20) weak_ordering (C++20) partial_ordering (C++20) common_comparison_category (C++20) compare_three_way_result (C++20) compare_three_way (C++20) strong_order (C++20) weak_order (C++20) partial_order (C++20) compare_strong_order_fallback (C++20) compare_weak_order_fallback (C++20) compare_partial_order_fallback (C++20)    is_eqis_ltis_lteq (C++20)(C++20)(C++20) is_neqis_gtis_gteq (C++20)(C++20)(C++20)

General utilities Date and time Function objects Formatting library (C++20) bitset hash (C++11) Relational operators (deprecated in C++20) rel_ops::operator!=rel_ops::operator>     rel_ops::operator<=rel_ops::operator>= Integer comparison functions cmp_equalcmp_lesscmp_less_than (C++20)(C++20)(C++20)    cmp_not_equalcmp_greatercmp_greater_than (C++20)(C++20)(C++20) in_range (C++20) Swap and type operations swap ranges::swap (C++20) exchange (C++14) declval (C++11) to_underlying (C++23) forward (C++11) forward_like (C++23) move (C++11) move_if_noexcept (C++11) as_const (C++17) Common vocabulary types pair tuple (C++11) optional (C++17) any (C++17) variant (C++17) tuple_size (C++11) tuple_element (C++11) apply (C++17) make_from_tuple (C++17) expected (C++23) Elementary string conversions to_chars (C++17) from_chars (C++17) chars_format (C++17) to_chars_result (C++17) from_chars_result (C++17)

 

Formatting library

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_stringformat_stringwformat_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_contextformat_parse_contextwformat_parse_context (C++20)(C++20)(C++20)

basic_format_contextformat_contextwformat_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_argsformat_argswformat_args (C++20)(C++20)(C++20)

visit_format_arg (C++20) (deprecated in C++26)

make_format_argsmake_wformat_args (C++20)(C++20)

Format error

format_error (C++20)

 

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.

auto s0 = std::format("{}", std::views::iota(1, 5));   // value of s0 is "[1, 2, 3, 4]"
auto s1 = std::format("{:n}", std::views::iota(1, 5)); // value of s1 is "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};
auto s0 = std::format("{}", ints);       // value of s0 is "[12, 10, 15, 14]"
auto s1 = std::format("{::X}", ints);    // value of s1 is "[C, A, F, E]"
auto s2 = std::format("{:n:_^4}", ints); // value of s2 is "_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}
};
auto s0 = std::format("{}", char_pairs);
// value of s0 is "[('A', 5), ('B', 10), ('C', 12)]"
 
auto s1 = std::format("{:m}", char_pairs);
// value of s1 is "{'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'};
auto s0 = std::format("{}", star);    // value of s0 is "['S', 'T', 'A', 'R']"
auto s1 = std::format("{:s}", star);  // value of s1 is "STAR"
auto s2 = std::format("{:?s}", star); // value of s1 is "\"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

See also

formatter (C++20)

defines formatting rules for a given type (class template)