NAME¶

std::println(std::ostream) - std::println(std::ostream)

Synopsis¶

Defined in header <ostream>
template< class... Args >
void println( std::ostream& os, std::format_string<Args...> fmt, (1) (since C++23)
Args&&... args );
void println( std::ostream& os ); (2) (since C++26)

Formats args according to the format string fmt with appended '\n' (which means that
each output ends with a new-line), and inserts the result into os stream.

1) Equivalent to: std::print(os, "{}\n", std::format(fmt, args...));
2) Equivalent to: std::print(os, "\n");

The behavior is undefined if std::formatter<Ti, char> does not meet the
BasicFormatter requirements for any Ti in Args (as required by
std::make_format_args).

Parameters¶

os - output stream to insert data into

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

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.
arg-id -
The arg-id s in a format string must all be present or all
be omitted. Mixing manual and automatic indexing is an
error.
fmt - the format specification defined by the std::formatter
format-spec - 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 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 (since C++23)
thread id format specification and stacktrace entry
format specification.
* For std::basic_stacktrace, no format specifier is
allowed.

* For std::filesystem::path, see path format (since C++26)
specification.

* For other formattable types, the format specification is determined by
user-defined formatter specializations.
args... - arguments to be formatted

Return value¶

(none)

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.

Notes¶

Although overload (2) is added in C++26, all known implementations make it available
in C++23 mode.

Feature-test macro Value Std Feature
__cpp_lib_print 202207L (C++23) Formatted output
__cpp_lib_format 202207L (C++23) Exposing std::basic_format_string

Example¶

This section is incomplete
Reason: no example

See also¶

print(std::ostream) outputs formatted representation of the arguments
(C++23) (function template)
println same as std::print except that each print is terminated by
(C++23) additional new line
(function template)
format stores formatted representation of the arguments in a new string
(C++20) (function template)

Category:¶

* Todo no example