Standard Error Stream Object - cerr in C++

The standard error stream in C++ is a special output stream used to display error messages or warnings. It is represented by std::cerr and sends messages to the screen, just like std:: cout . However, unlike std:: cout, it is unbuffered, meaning it shows messages immediately. This helps in debugging by showing errors right away, even if the program crashes.

Example:

#include <iostream>
using namespace std;

int main() {
  
  	// Printing message immediately to the console- unbuffered
    cerr << "Welcome to GfG! :: cerr\n";
  
  	// Printing after the program execution completes- buffered
    cout << "Welcome to GfG! :: cout";
    return 0;
}

Error

Welcome to GfG! :: cerr
Welcome to GfG! :: cout

Buffered vs Unbuffered Output

A buffer is like a temporary storage area ( a "waiting room") for data before it is shown or written.

What is Redirection?

Redirection means sending output (like messages or errors) to a different place instead of the screen.
Normally, when we run a C++ program.

• std:: cout (normal output) and
• std:: cerr (error output)
  both print to the screen ( called the console or terminal).

But with redirection, we can tell our program:

"Send the output somewhere else, like a file."

Redirection of std::cerr (Error Messages)

#include <iostream>
#include <fstream>
using namespace std;

int main() {
    // This will go to the console
    cout << "This is normal output (cout).\n";   
    // This will be redirected to a file
    cerr << "This is an error message (cerr).\n"; 

    return 0;
}

In the file error_log.txt:

This is an error message written to a file.

How to Compile and Run with Redirection

g++ main.cpp -o myprogram # Compile the program

./myprogram 2> error_log.txt # Run the program and redirect std::cerr to a file

Output on Screen:

This is normal output (cout).

Why Redirect std::cerr?

• Saves Errors: So we can check them later, especially if our program crashes.
• Log files: Useful for programs that run often (like servers or scripts).
• Keep things separate: Separate normal results from error messages for clarity.
• Debugging: Helps track down what went wrong and when.