std::print(std::ostream)(3)

🌸 NAME 🌸

Okay so bestie, welcome to the std::print(std::ostream)(3) page from the C++ Standard Libary manual 💖 — this one is literally all about std::print(std::ostream), the glow-up that formats your args with fmt and drops the result into an std::ostream. No cap, it's giving formatted-output main character energy ✨.

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🦋 Synopsis 🦋

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

So this is the whole vibe: it formats args according to the format string fmt, and then inserts the result into the os stream 🫶.

If the ordinary literal encoding is UTF-8, it's 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...));.

🚨 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). So like… don't do that, bestie 😭.

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🎀 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. (since C++23)
  • For std::basic_stacktrace, no format specifier is allowed.
  • 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.

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✨ Return value ✨

(none) — that's it, that's the whole return vibe 💅.

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💀 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.

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🧚 Notes 🧚

Feature-test macro tea ☕:

• __cpp_lib_print — value 202207L, std (C++23), feature: Formatted output
• __cpp_lib_format — value 202207L, std (C++23), feature: Exposing std::basic_format_string

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🍓 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.

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👑 See also 👑

• 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).

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mwah 💋 that's the whole page, bestie — it's from http://cppreference.com, stamped 2024.06.10, and it's the std::print(std::ostream)(3) page. periodt 💅