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