C++ named requirements: BasicFormatter (since C++20) (original) (raw)
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. |
- ↑ 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 |