std::print(std::ostream)

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

Formats args according to the format string fmt, and inserts the result into os stream.

If the ordinary literal encoding is UTF-8, equivalent to:

std::vprint_unicode(os, fmt.get(), std::make_format_args(args...));. Otherwise,
std::vprint_nonunicode(os, fmt.get(), std::make_format_args(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 }.

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

Return value

(none)

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

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

Run this code

#include <array>
#include <cctype>
#include <cstdio>
#include <format>
#include <numbers>
#include <ranges>
#include <sstream>
 
int main()
{
    std::array<char, 24> buf;
    std::format_to(buf.begin(), "{:.15f}", std::numbers::sqrt2);
 
    unsigned num{}, sum{};
 
    auto v = buf
           | std::views::filter(isdigit)
           | std::views::transform([](char x) { return x - '0'; })
           | std::views::take_while([&sum](char) { return sum < 42; });
 
    for (auto n : v)
        sum += n, ++num;
 
    std::stringstream stream;
 
#ifdef __cpp_lib_print
    std::print(stream,
#else
    stream << std::format(
#endif
        "√2 = {}...\n"
        "The sum of its first {} digits is {}{}",
        std::numbers::sqrt2, num, sum, '.'
    );
 
    std::puts(stream.str().data());
}

Output:

√2 = 1.4142135623730951...
The sum of its first 13 digits is 42.

Compiler support
Freestanding and hosted
Language
Standard library
Standard library headers
Named requirements
Feature test macros (C++20)
Language support library
Concepts library (C++20)
Metaprogramming library (C++11)
Diagnostics library
General utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
Localizations library
Input/output library
Filesystem library (C++17)
Regular expressions library (C++11)
Concurrency support library (C++11)
Execution support library (C++26)
Technical specifications
Symbols index
External libraries

I/O manipulators
Print functions (C++23)
C-style I/O
Buffers
basic_streambuf
basic_filebuf
basic_stringbuf
basic_spanbuf (C++23)
strstreambuf (C++98/26*)
basic_syncbuf (C++20)
Streams
Abstractions
ios_base
basic_ios
basic_istream
basic_ostream
basic_iostream
File I/O
basic_ifstream
basic_ofstream
basic_fstream
String I/O
basic_istringstream
basic_ostringstream
basic_stringstream
Array I/O
basic_ispanstream (C++23)
basic_ospanstream (C++23)
basic_spanstream (C++23)
istrstream (C++98/26*)
ostrstream (C++98/26*)
strstream (C++98/26*)
Synchronized Output
basic_osyncstream (C++20)
Types
streamoff
streamsize
fpos
Error category interface
iostream_category (C++11)
io_errc (C++11)

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