Flushing std::cerr

std::cerr is used for outputting error messages and is unbuffered by default, which means it should display messages immediately.

However, in some environments or specific cases, you might still need to flush it manually to ensure error messages are visible right away.

Using std::flush

The simplest way to flush std::cerr is to use std::flush:

1#include <iostream>
2
3int main() {
4  std::cerr << "Error: Something went wrong!"
5            << std::flush;  
6}

1Error: Something went wrong!

This ensures that the message is output immediately, regardless of any buffering that might be occurring.

Using std::endl

Another way to flush std::cerr is to use std::endl, which inserts a newline character and flushes the stream:

1#include <iostream>
2
3int main() {
4  std::cerr << "Error: Something went wrong!"
5            << std::endl;  
6}

1Error: Something went wrong!

This method is useful when you want to both end the line and flush the stream.

Explicit Flush

In more complex applications, you might want to flush std::cerr at specific points, especially after critical errors:

1#include <iostream>
2
3int main() {
4  if (/* some error condition */) {
5    std::cerr << "Critical error occurred!"
6              << std::flush;  
7
8    // Perform some recovery or exit
9    // ...
10  }
11}

By doing this, you ensure that the error message is output immediately, helping with debugging and error tracking.

Flushing in Multithreaded Applications

In multithreaded applications, managing output can be tricky. Using std::cerr with explicit flushes helps in making sure that error messages from different threads don't mix and are displayed promptly:

1#include <iostream>
2#include <thread>
3
4void logError() {
5  std::cerr
6    << "Thread error message\n"
7    << std::flush;  
8}
9
10int main() {
11  std::thread t1(logError);
12  std::thread t2(logError);
13  t1.join();
14  t2.join();
15}

1Thread error message
2Thread error message

Flushing std::cerr helps to ensure that error messages are seen immediately, aiding in faster debugging and clearer error reporting.