std::basic_ostream::flush(3)

💖 std::basic_ostream::flush(3) 💖

Hey bestie!! Welcome to the C++ Standard Libary page for std::basic_ostream::flush — yes, that spelling is exactly what the header says, and we're keeping it, no cap 😭. This one is short and sweet: it's the function that writes uncommitted changes to the underlying output sequence. Let's get into it ✨

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✨ NAME ✨

std::basic_ostream::flush — std::basic_ostream::flush. That's the whole NAME section. It said the thing twice and honestly? iconic. Periodt 💅

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🌸 Synopsis 🌸

basic_ostream& flush();

Okay so what does it do? It writes uncommitted changes to the underlying output sequence. It behaves as an UnformattedOutputFunction — that's the vibe check 🦋.

Now the details, bestie: If rdbuf() is a null pointer, the sentry object is not constructed. Otherwise, after constructing and checking the sentry object, it calls rdbuf()->pubsync(). If that call returns -1, it calls setstate(badbit). That's the whole flow 💅.

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🎀 Parameters 🎀

(none) — literally no parameters. Not a single one. We love a no-argument queen 🫶.

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🦋 Return value 🦋

*this — it returns itself. Main character energy, fr fr 👑.

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🔥 Exceptions 🔥

May throw std::ios_base::failure if (exceptions() & badbit) != 0. That's the condition, copy-pasted with love 💖.

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🌈 Example 🌈

The page says // Run this code, so here is the code, exactly:

#include <chrono>
#include <iostream>
#include <thread>

using namespace std::chrono_literals;

void f()
{
    std::cout << "Output from thread... ";
    for (int i{1}; i != 10; ++i)
    {
        std::this_thread::sleep_for(250ms);
        std::cout << i << ' ';

        // output three numbers at once;
        // the effect is observable only in real-time
        if (0 == (i % 3))
            std::cout.flush();
    }
    std::cout << std::endl; // flushes as well
}

int main()
{
    std::thread tr{f};
    std::this_thread::sleep_for(150ms);
    std::clog << "Output from main\n";
    tr.join();
}

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⭐ Output ⭐

Here's what it prints:

Output from main
Output from thread... 1 2 3 4 5 6 7 8 9

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💀 Defect reports 💀

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

• DR: LWG 581
• Applied to: C++98
• Behavior as published: flush() did not behave as an UnformattedOutputFunction because of the resolution of LWG issue 60
• Correct behavior: behaves as an UnformattedOutputFunction

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💜 See also 💜

• pubsync — invokes sync() (public member function of std::basic_streambuf<CharT,Traits>)
• sync — synchronizes the buffers with the associated character sequence [virtual] (virtual protected member function of std::basic_streambuf<CharT,Traits>)
• flush — flushes the output stream (function template)
• endl — outputs '\n' and flushes the output stream (function template)
• sync — synchronizes with the underlying storage device (public member function of std::basic_istream<CharT,Traits>)

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That's the whole page, bestie. It came from http://cppreference.com, dated 2024.06.10 — mwah 💋✨