A type F meets the BasicFormatter requirements if it meets the

• (1.1)
  Cpp17DefaultConstructible (Table 30),
• (1.2)
  Cpp17CopyConstructible (Table 32),
• (1.3)
  Cpp17CopyAssignable (Table 34),
• (1.4)
  Cpp17Swappable ([swappable.requirements]), and
• (1.5)
  Cpp17Destructible (Table 35)

requirements, and the expressions shown in Table 112 are valid and have the indicated semantics.

A type F meets the Formatter requirements if it meets the BasicFormatter requirements and the expressions shown in Table 113 are valid and have the indicated semantics.

Given character type charT, output iterator type Out, and formatting argument type T, in Table 112 and Table 113:

• (3.1)
  f is a value of type (possibly const) F,
• (3.2)
  g is an lvalue of type F,
• (3.3)
  u is an lvalue of type T,
• (3.4)
  t is a value of a type convertible to (possibly const) T,
• (3.5)
  PC is basic_format_parse_context<charT>,
• (3.6)
  FC is basic_format_context<Out, charT>,
• (3.7)
  pc is an lvalue of type PC, and
• (3.8)
  fc is an lvalue of type FC.

pc.begin() points to the beginning of the format-spec ([format.string]) of the replacement field being formatted in the format string.

If format-spec is not present or empty then either pc.begin() == pc.end() or *pc.begin() == '}'.

Table 112 — BasicFormatter requirements [tab:formatter.basic]

🔗 Expression	Return type	Requirement
🔗 g.parse(pc)	PC​::​iterator	Parses format-spec ([format.string]) for type T in the range [pc.begin(), pc.end()) until the first unmatched character. Throws format_error unless the whole range is parsed or the unmatched character is }. [Note 1:  This allows formatters to emit meaningful error messages. — end note] Stores the parsed format specifiers in *this and returns an iterator past the end of the parsed range.
🔗 f.format(u, fc)	FC​::​iterator	Formats u according to the specifiers stored in *this, writes the output to fc.out(), and returns an iterator past the end of the output range. The output shall only depend on u, fc.locale(), fc.arg(n) for any value n of type size_t, and the range [pc.begin(), pc.end()) from the last call to f.parse(pc).

Table 113 — Formatter requirements [tab:formatter]

🔗 Expression	Return type	Requirement
🔗 f.format(t, fc)	FC​::​iterator	Formats t according to the specifiers stored in *this, writes the output to fc.out(), and returns an iterator past the end of the output range. The output shall only depend on t, fc.locale(), fc.arg(n) for any value n of type size_t, and the range [pc.begin(), pc.end()) from the last call to f.parse(pc).
🔗 f.format(u, fc)	FC​::​iterator	As above, but does not modify u.