std:: println (std::ostream)

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

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 ).

Contents 1 Parameters 2 Exceptions 3 Notes 4 Example 5 Defect reports 6 See also

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

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 4088	C++23	( 1 ) ignored the locale imbued in os in formatting arguments	such locale is used

See also

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