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std::flush

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< cpp‎ | io‎ | manip
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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.

Contents

[edit] 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.

[edit] Parameters

os - reference to output stream

[edit] Return value

os (reference to the stream after manipulation)

[edit] Example

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

#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)..

[edit] See also

controls whether output is flushed after each operation
(function) [edit]
outputs '\n' and flushes the output stream
(function template) [edit]
synchronizes with the underlying storage device
(public member function of std::basic_ostream) [edit]