std::vprint_unicode, std::vprint_unicode_buffered

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< cpp‎ | io
C++
Input/output library
Print functions
Print functions
(C++23)
(C++23)
vprint_unicodevprint_unicode_buffered
(C++23)(C++23)
(C++23)(C++23)
Defined in header <print>
void vprint_unicode( std::FILE * stream,
std::string_view fmt, std::format_args args ) ;
(1) (since C++23)
void vprint_unicode_buffered( std::FILE * stream,
std::string_view fmt, std::format_args args ) ;
(2) (since C++23)
void vprint_unicode_buffered( std::string_view fmt, std::format_args args ) ;
(3) (since C++23)

Format args according to the format string fmt, and writes the result to the output stream.

1) Performs the following operations in order:
  1. Locks stream.
  2. Let out denote the character representation of formatting arguments provided by args formatted according to specifications given in fmt
  3. Writes out to stream:
  • If stream refers to a terminal that is only capable of displaying Unicode via a native Unicode API, flushes stream and writes out
  • Otherwise, writes unmodified out to the stream.
Unconditionally unlocks stream on function exit.
If any of the following conditions is satisfied, the behavior is undefined:
  • stream is not a valid pointer to an output C stream.
  • out contains invalid Unicode code units
2) Equivalent to std::string out = std::vformat (fmt, args) ;
std:: vprint_unicode (stream, "{}", std::make_format_args (out) ) ;
3) Equivalent to std::vprint_unicode_buffered(stdout, fmt, args)

Parameters

stream - output file stream to write to
fmt -
an object that represents the format string. The format string consists of
  • ordinary characters (except { and }
  • escape sequences {{ and }}, which are replaced with { 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 }

(since C++23)
(since C++26)
  • For other formattable types, the format specification is determined by user-defined formatter specializations.
args - arguments to be formatted

Exceptions

Notes

The C++ standard encourages the implementers to produce a diagnostic message if out contains invalid Unicode code units.

On POSIX, writing to a terminal is done using the usual standard I/O functions, so there is no need to treat a terminal differently to any other file stream.

On Windows, the stream refers to a terminal if GetConsoleMode(_get_osfhandle(_fileno(stream))) returns nonzero (see Windows documentation for GetConsoleMode, _get_osfhandle, and _fileno). The native Unicode API on Windows is WriteConsoleW

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

Feature-test macro Value Std Feature
__cpp_lib_print 202207L (C++23) Formatted output
202403L (C++26)
(DR23)		 Unbuffered formatted output
202406L (C++26)
(DR23)		 Enabling unbuffered formatted output for more formattable types
__cpp_lib_format 202207L (C++23) Exposing std::basic_format_string

Example

This section is incomplete
Reason: no example

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 stream can display Unicode 		only used if the terminal can only use
the native Unicode API to display Unicode
P3107R5 C++23 printing operations were always buffered provides unbuffered printing operations
P3235R3 C++23 the names of the functions added
by P3107R5 were misleading 		changed the function names

See also

(C++23)
 prints to stdout or a file stream using type-erased argument representation
(function)
(C++23)
 performs Unicode aware output using type-erased argument representation
(function)
(C++23)
 prints to stdout or a file stream using formatted representation of the arguments
(function template)
(C++20)
 stores formatted representation of the arguments in a new string
(function template)

External links

1.  Unicode
2.  The Unicode Standard Version 14.0 - Core Specification
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