std::unitbuf, std::nounitbuf

Defined in header <ios>
std::ios_base& unitbuf( std::ios_base& str );
std::ios_base& nounitbuf( std::ios_base& str );

Enables or disables automatic flushing of the output stream after any output operation. Has no effect on input.

1) enables the unitbuf flag in the stream str as if by calling str.setf(std::ios_base::unitbuf).

2) disables the unitbuf flag in the stream str as if by calling str.unsetf(std::ios_base::unitbuf).

This is an I/O manipulator, it may be called with an expression such as out << std::unitbuf for any out of type std::basic_ostream or with an expression such as in >> std::unitbuf for any in of type std::basic_istream.


Flushing is performed in the destructor of the std::basic_ostream::sentry object, which calls str.rdbuf()->pubsync() if str.flags() & std::ios_base::unitbuf is true.

The standard output objects std::cerr and std::wcerr have their unitbuf bit set by default.


str - reference to I/O stream

Return value

str (reference to the stream after manipulation).


Without std::unitbuf or another explicit flush, the output is the same, but does 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)..";
int main()
    volatile int sink = 0;
    std::cout << std::unitbuf; // enable automatic flushing
    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';


..(450 ms)....(902 ms)....(1352 ms)....(1802 ms)....(2252 ms)..

See also

flushes the output stream
(function template)
outputs '\n' and flushes the output stream
(function template)

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