18 Apr 2020

Avoid using std::endl to insert line breaks

Sean Murthy

To insert a line break into an output stream, just insert the newline character ('\n') instead of inserting std::endl. For example, write std::cout << "hello world" << '\n'; instead of writing std::cout << "hello world" << std::endl;.

Inserting std::endl inserts a line break and also “flushes” the output stream (ensures that data is actually written to the final destination), which blocks the program until flushing completes. However, flushing is meaningful only with files and other kinds of streams where the final destination is some place other than main memory (a simplification that suffices for this post). Thus, flushing is unnecessary for destinations such as console (std::cout) and string streams.

Even when flushing is required, endeavor to use the flush member function to make flushing apparent and to potentially improve performance as shown in function useStream below. Obviously, the call to flush in function useStream would be pointless if the output stream is std::cout or a string stream, but that call is made only once and thus may be acceptable.

By the way, explicit flushing is unnecessary for any kind of stream because the stream is automatically flushed when it is closed or when the associated memory buffer becomes full. Thus, call the flush function only if you really need to ensure data reaches its final destination at that particular point in the program.

#include <iostream>
#include <fstream>

void useStream(std::ostream& out);

int main() {
   char c;
   std::cout << "Choose destination: f for file; anything else for console: ";
   std::cin >> c;

   if (c == 'f') {
      std::ofstream f("sample.txt");
      useStream(f);
   } else
      useStream(std::cout);
}


//write some numbers to an output stream
//insert a line break after each number, but don't flush
//flush once at end
void useStream(std::ostream& out) {
   for (int i = 0; i < 6; ++i)
      out << i << '\n';

   out.flush(); //even this flush is unnecessary in most cases
}

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