Why is it not recommended to use std::endl?

When deciding between std::endl and '\n' in C++, it really comes down to flushing behavior and performance:

🧠 What they do

• '\n'
  Inserts a newline into the output buffer, but does not flush it immediately. The buffer is flushed automatically when it's full, on program exit, or before input operations.

• std::endl
  Equivalent to '\n', PLUS an immediate flush() of the stream.

⚙️ Performance Implications

Frequent flushing slows things down, especially for file I/O or heavy output:

• Test cases show printing millions of lines can slow from 0.65 s with '\n' to over 2 s or more with std::endl.
• A Reddit user sums it up:

  “std::endl is often implemented as … os << '\n'; os.flush(); …”
  “std::endl can make your program unbearably slow. Use it only if you really need to flush”.

👍 Best Practices

1. Use '\n' by default—it's fast and sufficient in most cases.
2. Use std::flush explicitly if you need to flush—but only when necessary.
3. Reserve std::endl for interactive use (like showing progress or prompting users) or when you're absolutely sure you need an immediate flush.

From C++ Core Guidelines: “avoid std::endl” – it conflates two actions (newline + flush).

✅ Summary

🎤 Real-world Advice

Reddit users recommend:

🎯 Conclusion

• For most scenarios—like logging, file writes, competitive programming—go with '\n'.
• Only use std::flush or std::endl when you really need to ensure the output is written immediately, such as when prompting the user or reporting runtime errors interactively.