NAME

std::flush - std::flush

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.

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';
}

Possible output:

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