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Showing content from https://en.cppreference.com/w/cpp/algorithm/../named_req/../io/basic_ostream/vprint_unicode.html below:

std::vprint_unicode(std::ostream) - cppreference.com

Format args according to the format string fmt, and writes the result to the output stream os. Behaves as FormattedOutputFunction of os, except that some details of error reporting differ.

Performs the following operations in order:

First, the function constructs and checks the sentry object.
Initializes an automatic variable as if by std::string out = std::vformat(os.getloc(), fmt, args);.
Writes out to os:

If os refers to a terminal that is only capable of displaying Unicode via a native Unicode API, flushes os and writes out to the terminal using the native Unicode API.
Otherwise, inserts the character sequence [out.begin(), out.end()) into os.

If writing to the terminal or inserting into os fails, calls os.setstate(std::ios_base::badbit).

If out contains invalid Unicode code units when the native Unicode API is used, the behavior is undefined.

[edit] Parameters os - output stream to insert data into fmt - 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-id s 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. Cannot start with }.

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

args - arguments to be formatted [edit] Exceptions

std::bad_alloc on allocation failure.
Propagate any exception thrown by any formatter, e.g. std::format_error, without regard to the value of os.exceptions() and without turning on ios_base::badbit in the error state of os.
May throw ios_base::failure caused by os.setstate(ios_base::badbit) which is called if an insertion into os fails.

[edit] Notes

If invoking the native Unicode API requires transcoding, the invalid code units are substituted with U+FFFD REPLACEMENT CHARACTER (see "The Unicode Standard - Core Specification", Chapter 3.9).

[edit] Example [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 4044 C++23 the native Unicode API was always used if the
terminal referred to by os can display Unicode only used if the terminal can only use
the native Unicode API to display Unicode [edit] See also [edit] External links

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