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

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C++
C++ named requirements

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.

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 .

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
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