You have not defined the variable input_line.
Add this:
string input_line;
And add this include.
#include <string>
Here is the full example. I also removed the semi-colon after the while loop, and you should have getline inside the while to properly detect the end of the stream.
#include <iostream>
#include <string>
int main() {
for (std::string line; std::getline(std::cin, line);) {
std::cout << line << std::endl;
}
return 0;
}