When injecting std::endl into an output stream, two things happen:
- An end of line character
'\n' is added to the stream
- The stream is flushed
In many situations, you don’t need the stream to be flushed: It takes some time, and additionally, the stream is also flushed automatically in
several circumstances:
- When the stream is closed
- In the case of
std::cout, each time an input is read on std::cin or an output is written on std::cerr
- In the case of
std::cerr, each output is immediately written, the is no need to flush
Therefore, if your only goal is to add an end of line, '\n' is usually more efficient than std::endl. If you do want to
flush, you can be explicit and inject std::flush into the stream, or call the flush member function on the stream.
Noncompliant code example
void f() {
cout << "Hello world!" << endl << endl << "How are you?" << endl; // Noncompliant, 3 useless flushes
string s;
cin >> s;
cout << "Starting long operation now..." << endl; // Noncompliant, flushing is useful, but not explicit enough
longOperation();
cout << "Long operation is done" << endl; // Noncompliant
}
Compliant solution
void f() {
cout << R"(Hello world!
How are you?
)" << endl;
// Or
cout << "Hello world!\n\nHow are you?\n";
string s;
cin >> s;
cout << "Starting long operation now...\n" << flush;
longOperation();
cout << "Long operation is done\n";
}
