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C++ named requirements: BasicFormatter (since C++20)

BasicFormatter is a type that abstracts formatting operations for a given formatting argument type and character type. Specializations of std::formatter are required to meet the requirements of BasicFormatter.

A BasicFormatter is a Formatter if it is able to format both const and non-const arguments.

Requirements

A type satisfies BasicFormatter if it is semiregular, meaning it satisfies:

And, given

  • Arg, a formatting argument type
  • CharT, a character type
  • BasicFormatter, a BasicFormatter type for types Arg and CharT
  • OutputIt, a LegacyOutputIterator type
  • f, a value of type (possibly const) BasicFormatter
  • g, a value of type BasicFormatter
  • arg, an lvalue of type Arg
  • ParseContext, an alias of std::basic_format_parse_context<CharT>
  • FormatContext, an alias of std::basic_format_context<OutputIt, CharT>
  • parse_ctx, an lvalue of type ParseContext
  • format_ctx, an lvalue of type FormatContext
Expression Return type Semantics
g.parse(parse_ctx) ParseContext::iterator
  • Parses the format-spec [parse_ctx.begin(), parse_ctx.end()) for type Arg until the first unmatched character.
  • Throws std::format_error unless the whole range is parsed or the unmatched character is }. [note 1]
  • Stores the parsed format specifiers in g and returns an end iterator of the parsed range.
f.format(arg, format_ctx) FormatContext::iterator
  • Formats arg according to the specifiers stored in f, writes the output to format_ctx.out() and returns an end iterator of the output range.
  • The output shall only depend on
    • arg,
    • format_ctx.locale(),
    • the range [parse_ctx.begin(), parse_ctx.end()) from the last call to f.parse(parse_ctx), and
    • format_ctx.arg(n) for any value n of type std::size_t.
  1. This allows formatters to emit meaningful error messages.

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