C++: “std::endl” vs “\n”
-
03-07-2019 - |
Question
Many C++ books contain example code like this...
std::cout << "Test line" << std::endl;
...so I've always done that too. But I've seen a lot of code from working developers like this instead:
std::cout << "Test line\n";
Is there a technical reason to prefer one over the other, or is it just a matter of coding style?
Solution
The varying line-ending characters don't matter, assuming the file is open in text mode, which is what you get unless you ask for binary. The compiled program will write out the correct thing for the system compiled for.
The only difference is that std::endl
flushes the output buffer, and '\n'
doesn't. If you don't want the buffer flushed frequently, use '\n'
. If you do (for example, if you want to get all the output, and the program is unstable), use std::endl
.
OTHER TIPS
The difference can be illustrated by the following:
std::cout << std::endl;
is equivalent to
std::cout << '\n' << std::flush;
So,
- Use
std::endl
If you want to force an immediate flush to the output. - Use
\n
if you are worried about performance (which is probably not the case if you are using the<<
operator).
I use \n
on most lines.
Then use std::endl
at the end of a paragraph (but that is just a habit and not usually necessary).
Contrary to other claims, the \n
character is mapped to the correct platform end of line sequence only if the stream is going to a file (std::cin
and std::cout
being special but still files (or file-like)).
There might be performance issues, std::endl
forces a flush of the output stream.
I recalled reading about this in the standard, so here goes:
See C11 standard which defines how the standard streams behave, as C++ programs interface the CRT, the C11 standard should govern the flushing policy here.
ISO/IEC 9899:201x
7.21.3 §7
At program startup, three text streams are predefined and need not be opened explicitly — standard input (for reading conventional input), standard output (for writing conventional output), and standard error (for writing diagnostic output). As initially opened, the standard error stream is not fully buffered; the standard input and standard output streams are fully buffered if and only if the stream can be determined not to refer to an interactive device.
7.21.3 §3
When a stream is unbuffered, characters are intended to appear from the source or at the destination as soon as possible. Otherwise characters may be accumulated and transmitted to or from the host environment as a block. When a stream is fully buffered, characters are intended to be transmitted to or from the host environment as a block when a buffer is filled. When a stream is line buffered, characters are intended to be transmitted to or from the host environment as a block when a new-line character is encountered. Furthermore, characters are intended to be transmitted as a block to the host environment when a buffer is filled, when input is requested on an unbuffered stream, or when input is requested on a line buffered stream that requires the transmission of characters from the host environment. Support for these characteristics is implementation-defined, and may be affected via the setbuf and setvbuf functions.
This means that std::cout
and std::cin
are fully buffered if and only if they are referring to a non-interactive device. In other words, if stdout is attached to a terminal then there is no difference in behavior.
However, if std::cout.sync_with_stdio(false)
is called, then '\n'
will not cause a flush even to interactive devices. Otherwise '\n'
is equivalent to std::endl
unless piping to files: c++ ref on std::endl.
There's another function call implied in there if you're going to use std::endl
a) std::cout << "Hello\n";
b) std::cout << "Hello" << std::endl;
a) calls operator <<
once.
b) calls operator <<
twice.
They will both write the appropriate end-of-line character(s). In addition to that endl will cause the buffer to be committed. You usually don't want to use endl when doing file I/O because the unnecessary commits can impact performance.
Not a big deal, but endl won't work in boost::lambda.
(cout<<_1<<endl)(3); //error
(cout<<_1<<"\n")(3); //OK , prints 3
If you use Qt and endl, you could accidentally use the wrong endl
, happened to me today and i was like ..WTF ??
#include <iostream>
#include <QtCore/QtCore>
#include <QtGui/QtGui>
//notice that i dont have a "using namespace std;"
int main(int argc, char** argv)
{
QApplication qapp(argc,argv);
QMainWindow mw;
mw.show();
std::cout << "Finished Execution !" << endl << "...";
// Line above printed: "Finished Execution !67006AB4..."
return qapp.exec();
}
Of course that was my mistake, since i should have written std::endl
, but if you use *endl
, qt and using namespace std;
it depends on the order of the include files if the correct endl
will be used.
Of course you could recompile Qt to use a namespace, so you get a compilation error for the example above.
EDIT: Forgot to mention, Qt's endl
is declared in "qtextstream.h" which is part of QtCore
*EDIT2: C++ will pick the correct endl
if you have a using
for std::cout
or the namespace std
, since std::endl
is in the same namespace as std::cout
, C++'s ADL mechanism will pick std::endl
.
I've always had a habit of just using std::endl because it is easy for me to see.
With reference This is an output-only I/O manipulator.
std::endl
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()
.
When to use:
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.
Also
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.
If you intend to run your program on anything else than your own laptop, never ever use the endl
statement. Especially if you are writing a lot of short lines or as I have often seen single characters to a file. The use of endl
is know to kill networked file systems like NFS.
The endl
manipulator is equivalent to '\'
. But endl
always flushes the stream.
std::cout << "Test line" << std::endl; // with flush
std::cout << "Test line\n"; // no flush
If you didn't notice, endl
is like pressing the ENTER KEY while "\n"
is like pressing ENTER KEY + SPACE BAR.