std::flush(3)

🌸 std::flush — the C++ Standard Library's "send it NOW" button 🌸

Hey bestie, welcome to std::flush(3) from the C++ Standard Library. It's a tiny manipulator with major main-character energy: it makes your output actually show up. Let's get into it. 🫶

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💖 NAME 💖

std::flush — std::flush (yes, literally, that's the name, periodt)

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✨ Synopsis ✨

Defined in header <ostream>.

template< class CharT, class Traits >
std::basic_ostream<CharT, Traits>& flush( std::basic_ostream<CharT,
Traits>& os );

Flushes the output sequence os as if by calling os.flush().

This is an output-only I/O manipulator, it may be called with an expression such as out << std::flush for any out of type std::basic_ostream.

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🧠 Notes 🧠

This manipulator may be used to produce an incomplete line of output immediately, e.g. when:

• displaying output from a long-running process
• logging activity of multiple threads
• logging activity of a program that may crash unexpectedly

An explicit flush of std::cout is also necessary before a call to std::system, if the spawned process performs any screen I/O (a common example is std::system("pause") on Windows). Very "why is my prompt invisible" energy. 😭

In most other usual interactive I/O scenarios, std::endl is redundant when used with std::cout because any input from std::cin, output to std::cerr, or program termination forces a call to std::cout.flush().

When a complete line of output needs to be flushed, the std::endl manipulator may be used.

When every output operation needs to be flushed, the std::unitbuf manipulator may be used.

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📦 Parameters 📦

• os — reference to output stream.

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🎁 Return value 🎁

os (reference to the stream after manipulation).

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💻 Example 💻

Without std::flush, the output would be the same, but may not appear in real time.

// Run this code

#include <chrono>
#include <iostream>

template<typename Diff>
void log_progress(Diff d)
{
    std::cout <<
std::chrono::duration_cast<std::chrono::milliseconds>(d) << " ... "
              << std::flush;
}

int main()
{
    volatile int sink = 0;

    auto t1 = std::chrono::high_resolution_clock::now();
    for (int j = 0; j < 5; ++j)
    {
        for (int n = 0; n < 10000; ++n)
            for (int m = 0; m < 20000; ++m)
                sink += m * n; // do some work
        auto now = std::chrono::high_resolution_clock::now();
        log_progress(now - t1);
    }
    std::cout << '\n';
}

Possible output:

567ms ... 1137ms ... 1707ms ... 2269ms ... 2842ms ...

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👀 See also 👀

• unitbuf — controls whether output is flushed after each operation
• nounitbuf — (function)
• endl — outputs '\n' and flushes the output stream (function template)
• flush — synchronizes with the underlying storage device (public member function of std::basic_ostream<CharT,Traits>)

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source: http://cppreference.com · version/date: 2024.06.10 · page: std::flush(3) — byeee bestie, go flush that output 💅✨🫶