Using scanf() in C++ programs is faster than using cin?

Question

I don't know if this is true, but when I was reading FAQ on one of the problem providing sites, I found something, that poke my attention:

Check your input/output methods. In C++, using cin and cout is too slow. Use these, and you will guarantee not being able to solve any problem with a decent amount of input or output. Use printf and scanf instead.

Can someone please clarify this? Is really using scanf() in C++ programs faster than using cin >> something ? If yes, that is it a good practice to use it in C++ programs? I thought that it was C specific, though I am just learning C++...

Answer 1

Here's a quick test of a simple case: a program to read a list of numbers from standard input and XOR all of the numbers.

iostream version:

#include <iostream>

int main(int argc, char **argv) {

  int parity = 0;
  int x;

  while (std::cin >> x)
    parity ^= x;
  std::cout << parity << std::endl;

  return 0;
}

scanf version:

#include <stdio.h>

int main(int argc, char **argv) {

  int parity = 0;
  int x;

  while (1 == scanf("%d", &x))
    parity ^= x;
  printf("%d\n", parity);

  return 0;
}

Results

Using a third program, I generated a text file containing 33,280,276 random numbers. The execution times are:

iostream version:  24.3 seconds
scanf version:      6.4 seconds

Changing the compiler's optimization settings didn't seem to change the results much at all.

Thus: there really is a speed difference.

---

EDIT: User clyfish points out below that the speed difference is largely due to the iostream I/O functions maintaining synchronization with the C I/O functions. We can turn this off with a call to std::ios::sync_with_stdio(false);:

#include <iostream>

int main(int argc, char **argv) {

  int parity = 0;
  int x;

  std::ios::sync_with_stdio(false);

  while (std::cin >> x)
    parity ^= x;
  std::cout << parity << std::endl;

  return 0;
}

New results:

iostream version:                       21.9 seconds
scanf version:                           6.8 seconds
iostream with sync_with_stdio(false):    5.5 seconds

C++ iostream wins! It turns out that this internal syncing / flushing is what normally slows down iostream i/o. If we're not mixing stdio and iostream, we can turn it off, and then iostream is fastest.

The code: https://gist.github.com/3845568