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Showing content from http://en.cppreference.com/w/cpp/language/../numeric/../named_req/BasicFormatter.html below:

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.

[edit] Requirements

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

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 g a value of type 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 g.parse(parse_ctx) ParseCtx::iterator

In the range [parse_ctx.begin(), parse_ctx.end()), parses the format-spec 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, fmt_ctx) FmtCtx::iterator

Formats arg 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 shall only depend on
- arg,
- 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.

â This allows formatters to emit meaningful error messages.

[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

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