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

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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:

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

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

#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{};

    for (auto n : buf
                | std::views::filter(isdigit)
                | std::views::transform([](char x) { return x - '0'; })
                | std::views::take_while([&sum](char) { return sum < 42; }))
        sum += n, ++num;

    std::stringstream stream;

#ifdef __cpp_lib_print
    std::print(stream,
#else
    stream << std::format(
#endif
        "√2 \N{ALMOST EQUAL TO} {0}.\n"
        "The sum of its first {1} digits is {2}.",
        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

outputs formatted representation of the arguments with appended '\n'
(function template) [edit]
prints to stdout or a file stream using formatted representation of the arguments
(function template) [edit]
stores formatted representation of the arguments in a new string
(function template) [edit]