std::flush

 

C++

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Input/output library

I/O manipulators

C-style I/O

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basic_streambuf

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Input/output manipulators

Floating-point formatting
showpoint noshowpoint setprecision	fixed scientific hexfloat defaultfloat (C++11) (C++11)
Integer formatting
setbase showbase noshowbase	dec hex oct
Boolean formatting
boolalpha noboolalpha
Field width and fill control
setfill setw	internal left right
Other formatting
showpos noshowpos	uppercase nouppercase
Whitespace processing
ws ends	skipws noskipws
Output flushing
flush endl	unitbuf nounitbuf
Status flags manipulation
resetiosflags	setiosflags
Time and money I/O
get_money (C++11) get_time (C++11)	put_money (C++11) put_time (C++11)

 

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)