std::endl
| Defined in header <ostream>
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| template< class CharT, class Traits > std::basic_ostream<CharT, Traits>& endl( std::basic_ostream<CharT, Traits>& os ); |
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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.
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.
Parameters
| os | - | reference to output stream |
Return value
os (reference to the stream after manipulation)
Example
With \n instead of endl, the output would be the same, but may 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 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
See also
| controls whether output is flushed after each operation (function) | |
| flushes the output stream (function template) | |
| synchronizes with the underlying storage device (public member function of std::basic_ostream) |