C++ named requirements: Formatter

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Formatter is a type that abstracts formatting operations for a given formatting argument type and character type. Specializations of std::formatter provided by the standard library are required to meet the requirements of Formatter except as noted otherwise.

A Formatter is able to format both const and non-const arguments, typically by providing a format member function that takes a const reference.

[edit] Requirements

A type satisfies Formatter if it satisfies BasicFormatter and given

  • Arg, a formatting argument type
  • CharT, a character type
  • Formatter, a Formatter type for types Arg and CharT
  • OutputIt, a LegacyOutputIterator type
  • f, a value of type Formatter
  • arg, an lvalue of type Arg
  • t, a value of type convertible to (possibly const) 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
f.format(t, format_ctx) FormatContext::iterator
  • Formats t 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
    • t,
    • 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.
f.format(arg, format_ctx) FormatContext::iterator

As above, but does not modify arg.

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

DR Applied to Behavior as published Correct behavior
P2418R2 C++20 objects that are neither const-usable nor copyable
(such as generator-like objects) are not formattable
allow formatting these objects
(relaxed formatter requirements)