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

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C++ named requirements
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Formatters
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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 the following types and values, the expressions shown in the table below are valid and have the indicated semantics:

Type Definition
CharT a character type
Arg a formatting argument type
Formatter a Formatter type for types Arg and CharT
OutputIt a LegacyOutputIterator type
ParseCtx std::basic_format_parse_context<CharT>
FmtCtx std::basic_format_context<OutputIt, CharT>
Value Definition
f a value of type (possibly const-qualified) Formatter
arg an lvalue of type Arg
t a value of type convertible to (possibly const-qualified) Arg
parse_ctx an lvalue of type ParseCtx satisfying all following conditions:
  • parse_ctx.begin() points to the beginning of the format-spec of the replacement field being formatted in the format string.
  • If format-spec is not present or empty, then either parse_ctx.begin() == parse_ctx.end() or *parse_ctx.begin() == '}'.
fmt_ctx an lvalue of type FmtCtx
Expression Return type Semantics
f.format(t, fmt_ctx) FmtCtx::iterator 
  • Formats t according to the specifiers stored in f, writes the output to fmt_ctx.out() and returns an end iterator of the output range.
  • The output can only depend on
    • t,
    • fmt_ctx.locale(),
    • the range [parse_ctx.begin()parse_ctx.end()) from the last call to f.parse(parse_ctx), and
    • fmt_ctx.arg(n) for any value n of type std::size_t.
f.format(arg, fmt_ctx) FmtCtx::iterator As above, but does not modify arg.

[edit] 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++20 the value of pc.begin() was unclear if format-spec is not present made clear