# std::unitbuf, std::nounitbuf

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< cpp‎ | io‎ | manip

Defined in header `<ios>` |  |
std::ios_base & unitbuf( std::ios_base & str ); | (1) |
std::ios_base & nounitbuf( std::ios_base & str ); | (2) |

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.

### Notes

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 == true.

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

### Parameters

str | - | reference to I/O stream

### Return value

`str` (reference to the stream after manipulation)

### Example

Without std::unitbuf or another explicit flush, the output is the same, but does 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)..";
    }

    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';
    }

Output:

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

### See also

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

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