完成下面的题目： （1）完成并测试下面的String类

String f(String a,String b)
{
  a[2] = 'x';
  char c = b[3];
  cout << "in f: " << a << ' ' << b << ' ' << c << '\n';
  return b;
}
int main()
{
  String x, y;
  cout << "Please enter two strings\n";
  cin >> x >> y;
  cout << "input: " << x << ' ' << y << '\n';
  String z = x;
  y = f(x,y);
  if (x ! = z) cout << "x corrupted!\n";
  x[0] = '!';
  if (x == z) cout << "write failed!\n";
  cout << "exit: " << x << ' ' << y << ' ' << z << '\n';
}