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

 

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C++ named requirements

Basic DefaultConstructible MoveConstructible (C++11) CopyConstructible CopyAssignable MoveAssignable (C++11) Destructible Type properties ScalarType PODType TriviallyCopyable (C++11) TrivialType (C++11) StandardLayoutType (C++11)   ImplicitLifetimeType Library-wide BooleanTestable EqualityComparable LessThanComparable Swappable ValueSwappable (C++11) NullablePointer (C++11) Hash (C++11) Allocator FunctionObject Callable Predicate BinaryPredicate Compare

Container Container ReversibleContainer AllocatorAwareContainer SequenceContainer ContiguousContainer (C++17) AssociativeContainer UnorderedAssociativeContainer (C++11)   Container element DefaultInsertable (C++11) CopyInsertable (C++11) MoveInsertable (C++11) EmplaceConstructible (C++11) Erasable (C++11) Iterator LegacyIterator LegacyInputIterator LegacyOutputIterator LegacyForwardIterator LegacyBidirectionalIterator LegacyRandomAccessIterator LegacyContiguousIterator (C++17) ConstexprIterator (C++20) Stream I/O FormattedInputFunction UnformattedInputFunction FormattedOutputFunction UnformattedOutputFunction Formatters BasicFormatter (C++20) Formatter (C++20)

Random Numbers SeedSequence (C++11) RandomNumberEngine (C++11) RandomNumberDistribution (C++11) UniformRandomBitGenerator (C++11) RandomNumberEngineAdaptor (C++11) Concurrency BasicLockable (C++11) Lockable (C++11) TimedLockable (C++11) SharedLockable (C++14) SharedTimedLockable (C++14) Mutex (C++11) TimedMutex (C++11) SharedMutex (C++17) SharedTimedMutex (C++14) Ranges RangeAdaptorObject (C++20) RangeAdaptorClosureObject (C++20) Multidimensional View LayoutMapping (C++23) LayoutMappingPolicy (C++23) AccessorPolicy (C++23) Other CharTraits RegexTraits (C++11)   BitmaskType LiteralType (C++11) NumericType UnaryTypeTrait (C++11) BinaryTypeTrait (C++11) TransformationTrait (C++11) Clock (C++11) TrivialClock (C++11)

 

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.

Requirements

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

• DefaultConstructible
• CopyConstructible
• CopyAssignable
• Destructible
• Swappable

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 }. 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.

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