# std::flush

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< cpp‎ | io‎ | manip

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.

### 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 or 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). 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.

### Parameters

os | - | reference to output stream

### Return value

`os` (reference to the stream after manipulation)

### Example

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

Run this code

    #include <iostream>
    #include <chrono>
    template<typename Diff>
    void log_progress(Diff d)
    {
        std::cout << "..("
                  << std::chrono::duration_cast<std::chrono::milliseconds>(d).count()
                  << " ms).." << 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';
    }

Output:

    ..(450 ms)....(901 ms)....(1350 ms)....(1800 ms)....(2250 ms)..

### See also

unitbuf nounitbuf | controls whether output is flushed after each operation (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`)

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