Why does endl get used as a synonym for “\n” even though it incurs significant performance penalties?

问题

This program:

#include <iostream>
#include <cstdlib>
#include <string>

int main(int argc, const char *argv[])
{
   using ::std::cerr;
   using ::std::cout;
   using ::std::endl;

   if (argc < 2 || argc > 3) {
      cerr << "Usage: " << argv[0] << " [<count>] <message>\n";
      return 1;
   }
   unsigned long count = 10000;
   if (argc > 2) {
      char *endptr = 0;
      count = ::std::strtoul(argv[1], &endptr, 10);
      if ((argv[1][0] == '\0') || (*endptr != '\0')) {
         cerr << "Usage: " << argv[0] << " [<count>] <message>\n";
         return 1;
      }
   }
   const ::std::string msg((argc < 3) ? argv[1] : argv[2]);
   for (unsigned long i = 0; i < count; ++i) {
      cout << i << ": " << msg << '\n';
   }
   return 0;
}

when timed like so:

$ time ./joe 10000000 fred >/dev/null

real  0m15.410s
user  0m10.551s
sys   0m0.166s

takes 15.4 seconds of real time to execute. Replace the output line with this: cout << i << ": " << msg << endl; and you end up with something like this:

$ time ./joe 10000000 fred >/dev/null

real  0m39.115s
user  0m16.482s
sys   0m15.803s

As you can see, the time to run more than doubles, and the program goes from spending minimal time in the OS to spending nearly half of it's time in the OS.

Both versions of the program have identical output, and are guaranteed by the standard to have identical output on every platform.

Given this, why do people persist in using endl as a synonym for '\n'?

Edit: In case it isn't obvious, this question is intended to be a leading question and is here for instructional purposes. I know why the performance penalty exists.

回答1:

I'm not certain. Inserting std::endl into the output stream is defined as being equivalent to inserting .widen('\n') and then calling flush() and yet many programmers persist in using std::endl even when there is no cause to flush, for example they go on to immediately output something else.

My assumption is that it comes from an incorrect belief that it is somehow a more portable because it doesn't explicitly use a specific newline character. This is incorrect as \n must always be mapped to the system's correct newline sequence for non-binary files by the stream library.

回答2:

Afaik, endl also flushes the stream, which may be the cause of the performance penalty.

回答3:

Not everyone cares so much about performance. For some applications, guaranteeing the stream is flushed is much more important.

Edit: Also, I find endl easier to type than '\n' :-)

回答4:

I tend to use endl with on stringstreams as it makes it easy to spot missing linebreaks.

回答5:

My guess is that instructional texts use std::endl with the belief that it's simpler and less confusing for beginners, and afterward people got accustomed to using it.

回答6:

The real question is, why did the compiler make such a dogs breakfast of compiling the endl version? If they're guaranteed to have the same semantics, then they should also have the same runtime.

Edit: obviously, I wasn't aware that endl flushed the stream... that's what you get for not looking it up.

来源：https://stackoverflow.com/questions/2122986/why-does-endl-get-used-as-a-synonym-for-n-even-though-it-incurs-significant-p

标签

c++

performance

iostream