Using seekg() when taking input from redirected stdin

Question

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().

As th

Answer 1

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.

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

#include 

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";
}

Gave:

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