“std::endl” vs “\n”

Question

Many C++ books contain example code like this...

std::cout << \"Test line\" << std::endl;

...so I\'ve always done that too. But

Answer 1

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.