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std::println - cppreference.com

From cppreference.com

Defined in header <print> 

template< class... Args >
void println( std::format_string<Args...> fmt, Args&&... args );
(1) (since C++23) 
template< class... Args >
void println( std::FILE* stream,
              std::format_string<Args...> fmt, Args&&... args );
(2) (since C++23) 
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::dynamic_format(std::string(fmt.get()) + '\n'),
           std::forward<Args>(args)...)

3) Equivalent to std::println(stdout).

4) Equivalent to std::print(stream, "\n").

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

Parameters

stream - output file stream to write to
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.
  • 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
__cpp_lib_print 202207L (C++23) Formatted output
202403L (C++26)
(DR23)		 Unbuffered formatted output
__cpp_lib_format 202207L (C++23) Exposing std::basic_format_string

Example

#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:

See also