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1.. title:: clang-tidy - performance-avoid-endl2 3performance-avoid-endl4============================5 6Checks for uses of ``std::endl`` on streams and suggests using the newline7character ``'\n'`` instead.8 9Rationale:10Using ``std::endl`` on streams can be less efficient than using the newline11character ``'\n'`` because ``std::endl`` performs two operations: it writes a12newline character to the output stream and then flushes the stream buffer.13Writing a single newline character using ``'\n'`` does not trigger a flush,14which can improve performance. In addition, flushing the stream buffer can15cause additional overhead when working with streams that are buffered.16 17Example:18 19Consider the following code:20 21.. code-block:: c++22 23 #include <iostream>24 25 int main() {26 std::cout << "Hello" << std::endl;27 }28 29Which gets transformed into:30 31.. code-block:: c++32 33 #include <iostream>34 35 int main() {36 std::cout << "Hello" << '\n';37 }38 39This code writes a single newline character to the ``std::cout`` stream without40flushing the stream buffer.41 42Additionally, it is important to note that the standard C++ streams (like43``std::cerr``, ``std::wcerr``, ``std::clog`` and ``std::wclog``)44always flush after a write operation, unless ``std::ios_base::sync_with_stdio``45is set to ``false``. regardless of whether ``std::endl`` or ``'\n'`` is used.46Therefore, using ``'\n'`` with these streams will not47result in any performance gain, but it is still recommended to use48``'\n'`` for consistency and readability.49 50If you do need to flush the stream buffer, you can use ``std::flush``51explicitly like this:52 53.. code-block:: c++54 55 #include <iostream>56 57 int main() {58 std::cout << "Hello\n" << std::flush;59 }60