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std::println(std::ostream) - cppreference.com

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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...));

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 }.
(since C++23)
  • 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 std::ios_base::badbit in the error state of os.
  • May throw std::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

outputs formatted representation of the arguments
(function template) [edit]
same as std::print except that each print is terminated by additional new line
(function template) [edit]
stores formatted representation of the arguments in a new string
(function template) [edit]