NAME¶

std::println - std::println

Synopsis¶

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

void println( std::FILE* stream, (2) (since C++23)

std::format_string<Args...> fmt, Args&&... args );
void println(); (3) (since C++26)
void println( std::FILE* stream ); (4) (since C++26)

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

1) Equivalent to std::println(stdout, fmt, std::forward<Args>(args)...).
2) Equivalent to std::print(stream, std::runtime_format(std::string(fmt.get()) +
'\n'),
std::forward<Args>(args)...).
3) Equivalent to std::println(stdout).
4) Equivalent to std::print(stream, "\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¶

stream - output file stream to write to

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

Exceptions¶

* std::bad_alloc on allocation failure.
* std::system_error, if writing to the stream fails.
* Propagates any exception thrown by used formatters, e.g. std::format_error.

Notes¶

Although overloads (3,4) are added in C++26, all known implementations make them
available in C++23 mode.

Feature-test macro Value Std Feature
202207L (C++23) Formatted output
__cpp_lib_print 202403L (C++26) Formatted output with stream locking
(DR23)
__cpp_lib_format 202207L (C++23) Exposing std::basic_format_string

Example¶

// Run this code

#include <print>

int main()
{
// Each call to std::println ends with new-line
std::println("Please"); // overload (1)
std::println("enter"); // (1)

std::print("pass");
std::print("word");

std::println(); // (3); valid since C++26; same effect as std::print("\n");
}

Output:¶

Please
enter
password

See also¶

print prints to stdout or a file stream using formatted
(C++23) representation of the arguments
(function template)
println(std::ostream) outputs formatted representation of the arguments with
(C++23) appended '\n'
(function template)
format stores formatted representation of the arguments in a new
(C++20) string
(function template)
printf
fprintf prints formatted output to stdout, a file stream or a buffer
sprintf (function)
snprintf
(C++11)