< cpp‎ | io‎ | manip
Input/output manipulators
Floating-point formatting
Integer formatting
Boolean formatting
Field width and fill control
Other formatting
Whitespace processing
Output flushing

Status flags manipulation
Time and money I/O
Quoted manipulator
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.


[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 <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';

Possible output:

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

[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<CharT,Traits>) [edit]