1.. title:: clang-tidy - performance-avoid-endl 2 3performance-avoid-endl 4============================ 5 6Checks for uses of ``std::endl`` on streams and suggests using the newline 7character ``'\n'`` instead. 8 9Rationale: 10Using ``std::endl`` on streams can be less efficient than using the newline 11character ``'\n'`` because ``std::endl`` performs two operations: it writes a 12newline 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 can 15cause 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 without 40flushing the stream buffer. 41 42Additionally, it is important to note that the standard C++ streams (like 43``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 not 47result in any performance gain, but it is still recommended to use 48``'\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