NAME¶

std::print(std::ostream) - std::print(std::ostream)

Synopsis¶

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

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

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

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.
arg-id -
The arg-id s in a format string must all be present or all
be omitted. Mixing manual and automatic indexing is an
error.
fmt - the format specification defined by the std::formatter
format-spec - 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 (since C++23)
thread id format specification and stacktrace entry
format specification.
* For std::basic_stacktrace, no format specifier is
allowed.

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

* 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.

See also¶

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