Template:cpp/utility/format/format string

an object that represents the format string. The format string consists of

• ordinary characters (except { and }), which are copied unchanged to the output,
• escape sequences {{ and }}, which are replaced with { and } respectively in the output, and
• replacement fields.

Each replacement field has the following format:

{ arg-id (optional) }	(1)
{ arg-id (optional) : format-spec }	(2)

1) replacement field without a format specification

2) replacement field with a format specification

arg-id	-	specifies the index of the argument in args whose value is to be used for formatting; if it is omitted, the arguments are used in order. The arg-ids in a format string must all be present or all be omitted. Mixing manual and automatic indexing is an error.
format-spec	-	the format specification defined by the std::formatter specialization for the corresponding argument.

• For basic types and standard string types, the format specification is interpreted as standard format specification.
• For chrono types, the format specification is interpreted as chrono format specification.

For range types, the format specification is interpreted as range format specification. For std::pair and std::tuple, the format specification is interpreted as tuple format specification. For std::thread::id and std::stacktrace_entry, see thread id format specification and stacktrace entry format specification. For std::basic_stacktrace, no format specifier is allowed.

(since C++23)

• For other formattable types, the format specification is determined by user-defined formatter specializations.