2-minute read: What is faster, std::endl or ‘\n’?

We at Johny’s Software Lab LLC are experts in performance. If performance is in any way concern in your software project, feel free to contact us.

A few days ago I wrote a small app to illustrate one of the articles I was preparing. Basically the program was loading a file from the hard disk, sorting it, and then outputting to another file only unique values (by omitting duplicates).

The function for writing unique values to a file looks like this:

void remove_duplicates_and_save(std::vector<std::string>& lines,
                                std::string file_name) {
    std::ofstream myfile(file_name);

    myfile << lines[0] << std::endl;
    for (int i = 1; i < lines.size(); i++) {
        if (lines[i] != lines[i - 1]) {
            myfile << lines[i] << std::endl;

As you can see, the function is simple enough and there is nothing special about it. The whole program took 2.3 seconds to complete on a file with 1 million lines. When I ran it through speedscope’s flamegraphs, I got the following output:

Flamegraphs clearly show that most of the runtime is eaten away by function remove_duplicates_and_save

As you can see, a lot of time is spent in remove_duplicates_and_save function, and if you look a little bit closer, a it involves a lot of flushing! For those who don’t know, flushing is moving data from your computer’s operating memory to the hard drive, and it is a very expensive operation if done often. So to increase performance, the C++ standard library performs flushing only when its internal data buffer is full.

I expected that remove_duplicates_and_save function would take a shorter time than sort_lines, however, this was not the case. Upon closer inspection, the culprit was found. According to C++ standard, outputting std::endl causes a buffer flush and degrades performance. Replacing std::endl with '\n' gave the following frame graph:

Flamegraph for whole program, where we replaced std::endl with ‘\n’

The overall program’s runtime went down from 2.3 seconds to 0.65 seconds. Function remove_duplicates_and_save almost disappeared from the flamegraph, which means its runtime is very short. Unlucky choice in the design of C++ standard library, but std::endl is a very inefficient way to write a new line to a file! So use '\n' instead!

Like what you are reading? Follow us on LinkedIn or Twitter and get notified as soon as new content becomes available.
Need help with software performance? Contact us!

Leave a Reply

Your email address will not be published.