std:: flush

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

Return value

os (reference to the stream after manipulation).

Example

#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';
}

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

See also