Character access

• #include <string>
  using std::string;
  void main() {
      string s1 = "Hello";
      char ch = s1[4];  // uses const version
      s1[0] = 'J';      // uses non-const version
      char ch2 = s1[5]; // Bad error - unchecked access!
      // For checked access, use corresponding
      // function named at( pos ):
      char ch3 = s1.at( 5 ); // Bad error - unchecked access!
  }

  The s1.at(pos) behaves just as s1[pos], except that it also performs a range check, throwing an exception of type out_of_range in case that pos is not an actual position in the string.