6. Using existing classes

• 
  #include <iostream>
  #include <string>
  using std::string;
  int main()
  {
      string s1 = "This is a string";
      std::cout
          << "There are "
          << s1.length()
          << " chars in "
          << s1
          << std::endl
          ;
      return 0;
  }