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Standard Error Stream Object - cerr in C++

Last Updated : 15 Jul, 2025

In C++, cerr is the standard error stream used to output the errors. It is an instance of the ostream class and is un-buffered, so it is used when we need to display the error message immediately and does not store the error message to display later. The 'c' in cerr refers to "character" and 'err' means "error", Hence cerr means "character error".

Example:

#include <iostream>
using namespace std;

int main() {
  
  	// Printing error
    cerr << "Welcome to GfG! :: cerr\n";
  
  	// Printing using cout
    cout << "Welcome to GfG! :: cout";
    return 0;
}

Error

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

Explanation: In this program, cerr is used to print an error message immediately to the console because it is unbuffered. The message "Welcome to GfG! :: cerr" is displayed without delay. On the other hand, cout is used for standard output, and the message "Welcome to GfG! :: cout" may be buffered and displayed after the program execution completes.

cerr is defined inside the <iostream> header file and is similar to the standard output stream.

cerr << "Error message";

The following are the key features of cerr object:

• Unbuffered Output: cerr outputs data immediately, which means it does not store the data in a buffer. This makes it ideal for error messages, as they need to be displayed right away.
• Separation of Error Messages: By using cerr, error messages can be distinguished from regular program output, which typically uses cout. This separation helps in logging and debugging.

#include <iostream>
using namespace std;

int main() {
    int n1, n2;
    
    cout << "Enter two numbers: ";
    cin >> n1 >> n2;
    
  	// Displaying error using cerr
    if (n2 == 0) {
        cerr << "Error: Division by zero!" << endl;
        return 1;
    }
    
    cout << "Result: " << n1 / n2 << endl;
    return 0;
}

Output

Enter two numbers: 10 0
Error: Division by zero!

Explanation: In this program, cerr is used to display an error message if the user tries to divide by zero. If n2 is zero, the message "Error: Division by zero!" is immediately printed to the console, signalling an error, and the program exits with a return value of 1.

Although cerr is typically used to output errors to the console, it can also be redirected to a file. This can be helpful when you need to log errors in a program.

#include <bits/stdc++.h>
using namespace std;

int main() {
  
  	// Creating ostream to a log file
    ofstream errorLog("error_log.txt");

  	// Redirect cerr to the file
    cerr.rdbuf(errorLog.rdbuf());
    cerr << "This is an error message written to a file." << endl;
    
    errorLog.close();
    return 0;
}

In the file error_log.txt:

This is an error message written to a file.

C++ cerr - FAQs

No, cerr is specifically for error output. Use cout for normal program output.

You can redirect cerr to a file using the rdbuf() method, similar to how you redirect cout.

No, cerr is unbuffered, ensuring immediate display of error messages.

If cerr fails to output an error message, it typically means there is an issue with the output stream or the system environment.

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