Why use endl when I can use a newline character?

Question

Is there a reason to use endl with cout when I can just use \n? My C++ book says to use endl, but I don't see why. Is \n not supported as widely as endl, or am I missing something?

Solution 1

endl appends '\n' to the stream and calls flush() on the stream. So

cout << x << endl;

is equivalent to

cout << x << '\n';
cout.flush();

A stream may use an internal buffer which gets actually streamed when the stream is flushed. In case of cout you may not notice the difference since it's somehow synchronized (tied) with cin, but for an arbitrary stream, such as file stream, you'll notice a difference in a multithreaded program, for example.

Here's an interesting discussion on why flushing may be necessary.

Solution 2

endl is more than just an alias for the \n character. When you send something to cout (or any other output stream), it does not process and output the data immediately. For example:

cout << "Hello, world!";
someFunction();

In the above example, there's is some chance that the function call will start to execute before the output is flushed. Using endl you force the flush to take place before the second instruction is executed. You can also ensure that with the ostream::flush function.

Comments

[André Puel]

Also, isn't endl converted to the proper plataform EndLine symbol("\r\n" on windows, "\n" on Linux)?

[Chad La Guardia]

The C standard specifies that '\n' is transparently converted to the appropriate character. So, yes, since endl attaches a '\n', this is happening. But its not specific to endl.

[Nawaz]

Or, more precisely, you could write (cout << x << '\n').flush();. That is, all in just one statement now as cout << x << endl; is one statement.

[Armen Tsirunyan]

@Nawaz: Eschew obfuscation and surplusage. Espouse elucidation

[Keith Thompson]

@Chad: Yes -- and so does the C++ standard, which is what we're discussing here.

[ildjarn]

You mean 'or any other buffered output stream`.

[Chad La Guardia]

@Keith Thompson The fact that the '\n' is transparently converted is not explicitly stated in the "C++" standard; it is something that carried over from the C standard.

[Keith Thompson]

@Chad: Hmm, I didn't know that. But in that case, it's important to note that C++ inherits that rule from C. (I'm trying to figure out where the C++ standard says that, but I find the C++ standard harder to read than the C standard.)

[Armen Tsirunyan]

@Keith: There was a question around SO: "What can you do with C that you cannot do with C++". One of my favorite answers read: "I can lift the printed version of the annotated C standard 1000 times with one hand"

[DirichletIsaPartyPooper]

Why would someFunction() execute before the first line? what could cause such a thing?