从重定向的标准输入中获取输入时使用seekg（） [英] Using seekg() when taking input from redirected stdin

问题描述

因此，我尝试使用cin.get（）两次读取一个字符串。输入被重定向为程序<输入。因此，使用seekg（）是有效的。

So i'm trying to read in a string of characters twice using cin.get(). The input is being redirected as "program < input". So it is valid to use seekg().

正如title所说，我认为我可以使用seekg（）保存字符串的开始位置，因此我可以再次使用同一字符串的起始位置。

As the titel says, I thought I would be able to use seekg() to save the beginning position of the string, so I could come back to use the starting position of the same string again.

这是我的尝试：

char c;
while (cin.get(c))
{
  //do stuff 
}

cin.seekg(0, ios::beg);

while (cin.get(c))
{
  //do stuff with the string a second time
}

第二个while循环什么也没做，所以我显然没有正确使用seekg。有人可以告诉我我做错了什么吗？

The second while loop isn't doing anything, so I'm obviously not using seekg correctly. Could someone tell me what I'm doing incorrectly?

感谢您的帮助！

推荐答案

您无法在流/管道上搜索。它们不再继续存在于内存中。假设键盘直接连接到您的程序。您只能使用键盘执行的操作是要求更多输入。它没有历史记录。

You can't seek on streams/pipes. They don't continue to exist in memory. Imagine the keyboard is directly connected to your program. The only operation you can do with a keyboard is ask for more input. It has no history.

如果只是键盘，您将找不到它，但是如果使用<在外壳程序中寻求效果很好：

If it's just a keyboard you can't seek, but if it's redirected with < in the shell seeking works fine:

#include <iostream>

int main() {
  std::cin.seekg(1, std::ios::beg);
  if (std::cin.fail()) 
    std::cout << "Failed to seek\n";
  std::cin.seekg(0, std::ios::beg);
  if (std::cin.fail()) 
    std::cout << "Failed to seek\n";

  if (!std::cin.fail()) 
    std::cout << "OK\n";
}

给定：

user @ host：/ tmp> ./a.out

找不到

找不到

user @ host ：/ tmp> ./a.out< test.cc

确定

user@host:/tmp > ./a.out
Failed to seek
Failed to seek
user@host:/tmp > ./a.out < test.cc
OK

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