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# std::endl

From cppreference.com
< cpp‎ | io‎ | manip

Defined in header `<ostream>` |  |
template< class CharT, class Traits > std::basic_ostream<CharT, Traits>& endl( std::basic_ostream<CharT, Traits>& os ); |  |

Inserts a newline character into the output sequence `os` and flushes it as if by calling os.put(os.widen('\n')) followed by os.flush().

This is an output-only I/O manipulator, it may be called with an expression such as out << std::endl for any `out` of type std::basic_ostream.

### [edit] Notes

This manipulator may be used to produce a 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. 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(). Use of `std::endl` in place of '\n', encouraged by some sources, may significantly degrade output performance.

In many implementations, standard output is line-buffered, and writing '\n' causes a flush anyway, unless std::ios::sync_with_stdio(false) was executed. In those situations, unnecessary `endl` only degrades the performance of file output, not standard output.

The code samples on this wiki follow Bjarne Stroustrup and The C++ Core Guidelines in flushing the standard output only where necessary.

When an incomplete line of output needs to be flushed, the std::flush manipulator may be used.

When every character of output 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

With \n instead of endl, 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 passed" << std::endl;
    }

    int main()
    {
        std::cout.sync_with_stdio(false); // on some platforms, stdout flushes on \n
        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);
        }
    }

Output:

    487 ms passed
    974 ms passed
    1470 ms passed
    1965 ms passed
    2455 ms passed

### [edit] See also

unitbufnounitbuf | controls whether output is flushed after each operation (function) [edit]
flush | flushes the output stream (function template) [edit]
flush | synchronizes with the underlying storage device (public member function of `std::basic_ostream`) [edit]

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