NAME¶

std::endl - std::endl

Synopsis¶

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.

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.

// 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);
}
}

Possible output:¶

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

See also¶

unitbuf controls whether output is flushed after each operation
nounitbuf (function)
flush flushes the output stream
(function template)
flush synchronizes with the underlying storage device
(public member function of std::basic_ostream<CharT,Traits>)