std::println(std::ostream)

 

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print(std::ostream) (C++23)
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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.

1) Equivalent to: std::print(os, "{}\n", std::format(os.getloc(), 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
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

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)

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