在C ++程序使用scanf（）的是比使用CIN快？ [英] Using scanf() in C++ programs is faster than using cin?

问题描述

我不知道这是不是真的，但是，当我开始提供网站问题的一个常见问题阅读，我找到了，那捅了我的注意：

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:

检查输入/输出方式。在C ++中，使用cin和cout太慢。使用这些，你将保证不能够解决任何问题的输入或输出，像样的数目。用printf和scanf代替。

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.

有人能澄清这？真的是使用的 scanf（）的的在C ++程序的速度比使用的 CIN >>东西的？如果是的话，那就是它一个很好的做法在C ++程序使用它？我认为这是特定于C，虽然我刚学C ++ ...

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++...

推荐答案

下面是一个简单的情况下快速测试：一个程序来读取从标准输入和XOR所有号码的号码列表

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的版本：

#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函数的版本：

#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;
}

结果

使用的第三方案，我生成包含33280276随机数的文本文件。的执行时间是：

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

改变编译器的优化设置似乎并没有改变结果很多都没有。

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

这样：真的有速度差

的编辑：的用户clyfish 下面指出的是速度差主要是由于iostream的I / O功能，保持与CI / O功能的同步。我们可以通过调用关闭此功能，以的std :: IOS :: sync_with_stdio（假）; ：

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;
}

新的结果：

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

C ++的iostream胜！原来，该内部同步/冲洗是正常减慢iostream的I / O。如果我们不混合cstdio和iostream的，我们可以将其关闭，然后iostream都是最快的。

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

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

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