Why c++ string object when constructed with default constructor behaves differently?

I tried the below program :

#include <iostream>
#include <string>

int main ()
{
  std::string str;
  str[0] = 'o';
  str[1] = 'k';
  std::cout << str.length();
  std::cout << "as a whole :";
  std::cout << str << std::endl;
  std::cout << "character by character :";
  std::cout << str[0] << str[1] << std::endl;

  return 0;
}

I don't understand why I can't print the string as a whole using the object variable and why the length is returning as 0 since clearly I have added the characters using subscript operator as that would return char reference so I know that is legal.

In addition, I didn't get any kind of exception. So there's that. Obviously, there is lot happening behind the scenes in the string class and I know I am missing something. Could someone help me on this?. Thanks in advance