The consequences and pros/cons of flushing the stream in c++

Question

I have recently read an article which stated that using \\n is preferable to using std::endl because endl also flushes the stream.

Answer 1

When buffering occurs you will have no guarantees that the data is immediately received before a flush occurs. Under particular circumstances you might experience wrong output ordering and/or loss of information/debug data, e.g.

int main() {

    std::cout << "This text is quite nice and might as well be buffered";
    raise(SIGSEGV); // Oh dear.. segmentation violation
    std::cout << std::endl;
}

Live Example

Output:

bash: line 7: 22235 Segmentation fault      (core dumped) ./a.out

the above will not print any text since the buffering prevented the right output to be displayed.

Now if you simply add a flushing std::endl at the end of the buffer this is what you get

int main() {

    std::cout << "This text is quite nice and might as well be buffered" << std::endl;
    raise(SIGSEGV); // Oh dear.. segmentation violation
    std::cout << std::endl;
}

Live Example

Output:

This text is quite nice and might as well be buffered
bash: line 7: 22444 Segmentation fault      (core dumped) ./a.out

This time the output is visible before the program termination.

Implications of this fact are manifold. Purely speculative: if the data were related to a server log, your app could have been crashed before the actual logging.