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std::range_formatter - cppreference.com

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

The syntax of range-format-spec is:

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.

Assigns sep to separator_.

Assigns opening and closing to opening-bracket_ and closing-bracket_, respectively.

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.

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.

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

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