Small example of redirecting std::cin to read from a string instead from the console.
#include <iostream>
#include <sstream>
int main(int, char **)
{
std::string s = "123";
std::istringstream is(s);
std::streambuf * cin_old = std::cin.rdbuf(is.rdbuf());
int a;
std::cin >> a;
std::cout << "a=" << a << std::endl;
return 0;
}std::cin >> a on line 11 will now read from the provided string instead from the console.