When to use cerr and when cout in C++?

Question

I am looking for an example that differentiates between cerr and cout in C++?

When do I need to use cerr?

Drosophila

Have you seen https://mcmap.net/q/967854/-shall-i-use-cerr? – Ibeam

The intended output of the program should go to std::cout. Unintended warnings or error messages that would spoil the program's output should go to std::cerr. You usually don't want to mix the output the program is producing with information reports. – Dispel

Possible duplicate of What is the difference between cout, cerr, clog of iostream header in c++? When to use which one? – Anfractuosity

Answer 1

Quite often the user of your program is interested only in results, because these are printed to stdout, for instance, if you use a unix command cat, eg:

$ cat file.txt

You're expecting file.txt contents to appear on stdout. However, if anything happens during the execution of cat (strictly theoretically speaking, nothing has ever happenned to me), you'd expect it to go to stderr, thus, as a user, you're still able to separate the two, eg:

$ cat file.txt 1>result.txt 2>stderr.txt

Suppose I want to collect contents of multiple files, I do the following

$ cat *.java 1>all_files_conent.java 2>errors.txt

In case any of the files isn't accessible (eg. because of permissions), errors.txt will have appropriate message:

cat: Controller.java: Permission denied

But the contents of all_files_content.java are as correct as they can be.

Therefore, if the message is an actual product of your program, you should use cout, if it's just a status message, use cerr. Of course, all this doesn't matter that much if what goes to the console is just a byproduct. However, you might still want to allow user to separate the two, as in the example above.

Chalcis

Answer 2

Many operating systems let you redirect input and output from/to files. When end-users redirect your output to a file, end-users do not see anything that you write to cout; if you want your output to be seen by end-users, you need a separate stream to which you print messages for them.

Suppose you are writing a program that reads from the standard input line-by-line, and writes these lines to the standard output in sorted order. Let's say that your program takes a command-line parameter that says if the output needs to be sorted in ascending or descending order. If end-users pass an invalid value for this parameter, you want to print a message "Invalid flag" to the console. Printing it to cout would be incorrect, because cout could be redirected to a file, so the users would not see it. The correct solution in this situation is to write this message to cerr.

Levorotation

Answer 3

std::cout : Regular output (console output)

std::cerr : Error output (console error)

Google is your friend :)

Blunderbuss

Answer 4

You can refer to this answer under another question here. The major difference is std::cerr is not buffered and is usually used to display an error. But I've also seen many people use std::cerr to output anything you want to immediately see (you don't want it to be buffered), even if it's not an error message.

Yestreen